JP3657497B2 - Image display method - Google Patents

Description

【００６１】
ここで、Ｉは出力輝度、Ｖは入力信号電圧、指数値γは、ガンマ値と呼ばれる非線形パラメータである。また、a およびｂはオフセット定数である。
【００６２】
入力信号が、米国映画テレビ技術者協会（ＳＭＴＰＥ）のテレビ規格であるＳＭＰＴＥ規格１７０Ｍまたは２４０Ｍ、もしくは、旧来の方式であった電波法／ＣＣＩＲ（国際無線通信諮問委員会）勧告６２４／ＲＳ１７０Ａにおいて規定される輝度−信号電圧変換式によって逆ガンマ補正された通常のＴＶ画像信号である場合について考える。このＴＶ画像信号は、γ＝２．２のＣＲＴ表示装置に入力することを想定して逆ガンマ補正されたものであるので、ＣＲＴ表示装置で表示すれば、一定条件においては特別な処理を施すことなく原画像がほぼ線形化されて出力される。入力信号電圧は、入力輝度信号レベルと概ね比例関係にあるので逆ガンマ処理された入力輝度信号レベルｇに対して、簡単化のため式（１）においてａ＝ｂ＝０とし、ｇが最大値の時に得られる最大輝度（最大出力輝度）ｉ_max を用いて式（１）を書き換えると、次の式（２）のように表現できる。
【００６３】
【数２】

【００６４】
このようにおくことで、入力と出力の関係を表示装置の制約を取り払って信号処理の部分で考えることができる。ここでは、概念を説明するため、式（１）の定数項を省略して考えているが、式（１）をそのまま用いて、ＣＲＴ表示装置の特性をより詳細に記述してもよい。これにより、以下に述べる式の精度がさらに向上する。
【００６５】
ＣＲＴ表示装置を例にとると、平均入力輝度信号レベルＧに応じてその系での最大出力輝度ｉ_max が構造上決定されている。そのため、式（２）における最大輝度ｉ_max を平均入力輝度信号レベルＧの関数ｉ_max （Ｇ）として書き改めると、Ｉは、ｇおよびＧの関数となり、次の式（３）のように表記することができる。
【００６６】
【数３】

【００６７】
ここで、平均入力輝度信号レベルＧは、表示装置の各画素Ｐ（ｘ，ｙ）に対応して入力される輝度信号のレベルｇ_xyの平均値であり次式として表現できる。
【００６８】
【数４】

【００６９】
なお、画素Ｐ（ｘ，ｙ）は、ｉ行（ｉ≧２）×ｊ列（ｊ≧２）のマトリックス状に配列された複数画素のうちのｘ行目（１≦ｘ≦ｉ）ｙ列目（１≦ｙ≦ｊ）の画素を表すものとする。
【００７０】
しかしながら、一般のＣＲＴ表示装置において、図８および図９で示されるように平均入力輝度信号レベルＧに対する最大出力輝度ｉ_max およびガンマ値γのの変化を調べたところ、図１０で示されるように、最大出力輝度ｉ_max だけでなく、ガンマ値γも平均入力輝度信号レベルＧの変化に伴って変化する。そのため、最大出力輝度ｉ_max と同様に、ガンマ値γも、平均入力輝度信号レベルＧの関数として書き換える必要がある。
【００７１】
つまり、図８の特性を元にすれば、式（３）は次の式（５）のように書き改めることができる。
【００７２】
【数５】

【００７３】
式（５）によれば、平均入力輝度信号レベルＧに応じた最大出力輝度ｉ_max （Ｇ）およびガンマ値γ（Ｇ）を設定すれば、入力輝度信号に対してＣＲＴ表示装置以外の表示装置を用いてＣＲＴ表示装置と同様の入出力特性で表示を行うことや、より再現性の良い入出力特性で画像再生を行うことができることが分かる。
【００７４】
そこで、任意の表示装置の規格化された入力信号−出力輝度特性を表す関数Ｄを、規格化された入力輝度信号レベルｇ_input および規格化された出力輝度レベルｇ_outputを用いて定義すると、
ｇ_output＝Ｄ（ｇ_input ）・・・・・（６）
の関係となる。
【００７５】
ここで、信号レベルや特性を規格化した理由は、表示装置の種類によって入力される信号や出力される信号の精度が異なっている場合や、表示装置の種類によって出力輝度のスケールが異なる場合が想定されるからである。すなわち、表示装置の調整によって、例えば、入力信号が８ビットであり出力信号が１０ビットである場合や、入力信号が８ビットであり出力信号が６ビットである場合、出力輝度のスケールが、ある設定値においては１０であったものが別の設定値においては２０になる場合等が起こりうるからである。
【００７６】
実際の信号処理においては、ｇ_input 、ｇ_output、およびｉ_max の値を予め０〜１の範囲内の値となるように規格化しておけば、最終的な信号形態への変換は、演算結果を信号の最大表現数（ｎビットのディジタル信号であれば、最大表現数２ⁿ −１）に乗じれば良いので、計算を容易に行うことができる。
【００７７】
以上のことから、最適な表示出力を得るためには、入力画像信号に対して種々の画像信号の補正が必要となることが分かった。
【００７８】
【発明の実施の形態】
〔実施の形態１〕
本発明の画像再生方法の実施の一形態について、図１に基づいて説明すれば以下の通りである。
【００７９】
本実施形態の画像再生方法に用いる画像表示装置は、図１に示すように、画像を表示するための複数の画素（図示しない）を有する表示装置８と画像信号補正装置７とを備え、各画素毎の情報を表す画素信号を含む画像信号ｇ₀ （輝度信号レベルｇ₀ ）が画像信号補正装置７を介して表示装置８に入力されるようになっている。
【００８０】
画像信号補正装置７は、全ての画素信号のレベルの平均を平均入力輝度信号レベルＧとして演算する平均信号レベル演算回路１と、画素信号のレベルに対する画素の輝度の変化を表す入力信号−出力輝度特性を、平均入力輝度信号レベルＧに応じて設定する入力信号−出力輝度特性設定回路２と、平均入力輝度信号レベルＧに応じて表示装置８の画素の最大出力輝度を調整する最大出力輝度調整回路３と、設定された入力信号−出力輝度特性を満たすように画像信号ｇ₀ を補正する信号補正部４とを備えている。
【００８１】
信号補正部４は、設定された入力信号−出力輝度特性に等しい入出力特性で画像信号ｇ₀ を補正するγ（Ｇ）補正回路５と、表示装置８の入出力特性（入力信号−出力輝度特性）に対して逆特性の入出力特性でさらに画像信号ｇ₁ を補正する逆特性補正回路６とを備えている。
【００８２】
次に、上記画像表示装置を用いた本発明の画像再生方法について説明する。
【００８３】
まず、平均信号レベル演算回路１にて、逆ガンマ補正された入力画像信号ｇ₀ の輝度信号レベルｇ₀ から平均入力輝度信号レベルＧを演算する。
【００８４】
次いで、入力信号−出力輝度特性設定回路２にて、画像表示装置の入力信号−出力輝度特性を指数関数で近似表現した場合の指数値（ガンマ値）γ（Ｇ）を平均入力輝度信号レベルＧから演算する。具体的には、画像表示装置の入力信号−出力輝度特性を指数関数で近似表現した場合の指数値（ガンマ値）γ（Ｇ）を、例えば前記の図１０の曲線γＤのように、平均入力輝度信号レベルＧが高くなるにつれて大きくなるように設定する。
【００８５】
さらに、信号補正部４にて、画像表示装置の入力信号−出力輝度特性を指数関数で近似表現した場合の指数値（ガンマ値）がその設定値γ（Ｇ）に一致するように入力画像信号ｇ₀ を補正し、補正後の画像信号ｇ₀ を表示装置８へ出力する。具体的には、まず、γ（Ｇ）補正回路５にて、設定値γ（Ｇ）を指数値とする指数関数を用いて、画像信号ｇ₀ の輝度信号レベルｇ₀ から画像信号ｇ₁ の輝度信号レベルｇ₁ を算出し、輝度信号レベルｇ₁ を持つ画像信号ｇ₁ を生成する。次いで、逆特性補正回路６にて、表示装置８の入力信号−出力輝度特性を表す関数の逆関数を用いて、画像信号ｇ₁ の輝度信号レベルｇ₁ から画像信号ｇ_out の輝度信号レベルｇ_out を算出し、輝度信号レベルｇ_out を持つ画像信号ｇ_out を表示装置８へ出力する。
【００８６】
また、最大出力輝度調整回路３にて、平均入力輝度信号レベルＧに応じて表示装置８の画素の最大出力輝度を調整する。具体的には、平均入力輝度信号レベルＧから表示装置８の最大出力輝度ｉ_max （Ｇ）を演算し、この演算結果を最大出力輝度ｉ_out として表示装置８へ出力する。最大出力輝度ｉ_max （Ｇ）の演算は、例えば前記の図１０の曲線ｉ_max Ｄのように、平均入力輝度信号レベルＧが高くなるにつれて最大出力輝度ｉ_max （Ｇ）が小さくなるような演算とする。
【００８７】
この場合、表示装置８の表示素子に入力される画像信号ｇ_out の輝度信号レベルｇ_out および表示装置８の最大出力輝度ｉ_out は、次の式（７）および式（８）で表現することができる。
【００８８】
【数６】

【００８９】
また、最終的な表示装置８の出力輝度Ｉは、次の式（９）のように表現することができる。
【００９０】
【数７】

【００９１】
なお、ｇ_out は前記の式（６）のｇ_input に対応する規格化された入力輝度信号レベルを表し、Ｄ^-1（）は、表示装置８の規格化された出力輝度レベルｇ_d （前記の式（６）のｇ_outputに対応する）を表す関数Ｄ（ｇ_out ）の逆関数である。
【００９２】
このように、本発明の画像再生方法では、逆ガンマ補正された画像信号ｇ₀ における平均入力輝度信号レベルＧを参照値としてガンマ値γ（Ｇ）と最大出力輝度ｉ_max （Ｇ）とを設定することによって、表示装置８の種類によらず再現性の良い高品位の画像が表示可能となる。
【００９３】
なお、前述の式（２）〜（９）においては、入力画像信号の輝度信号に対しての処理を示したが、実際の画像表示装置においては、輝度信号を表示素子駆動用の駆動電圧ｖ₁ に変換する処理、および最大出力輝度ｉ_out を表示装置８の発光素子の駆動電圧ｖ₂ に変換する処理が、一般的に含まれている。
【００９４】
このため、式（９）は、次の式（１０）のようにも表現できる。
【００９５】
【数８】

【００９６】
ここで、式中のＶは、輝度信号電圧の平均入力電圧値を示す。
【００９７】
また、輝度信号レベルｇ_out から表示装置８の表示素子の駆動電圧ｖ₁ への変換、および最大出力輝度ｉ_out から表示装置８の発光素子の駆動電圧ｖ₂ への変換は、線形関数Ｖ₁ およびＶ₂ を用いて、次の式（１１Ａ）および（１１Ｂ）のように表現できる。
【００９８】
ｖ₁ ＝Ｖ₁ （ｇ_out ）・・・（１１Ａ）
ｖ₂ ＝Ｖ₂ （ｉ_out ）・・・（１１Ｂ）
なお、式（１０）や式（１１Ａ）、（１１Ｂ）は、表示装置８の動作形態が電圧値の変化によって駆動される形態である場合のものであるが、表示装置８の動作形態が他の信号、例えば、熱、光、圧力、振動、音波等の信号によって駆動される形成である場合でも、式（１０）や式（１１Ａ）、（１１Ｂ）と同様の式が成り立つ。
【００９９】
また、図１は、表示装置８が、互いに独立に制御され得る表示素子および発光素子を備える場合の画像表示装置の構成を示している。この場合、式（７）により得られる画像信号ｇ_out （輝度信号レベルｇ_out ）および式（８）により得られる最大出力輝度ｉ_out を、それぞれ表示素子および発光素子に入力すれぱよい。
【０１００】
なお、表示装置８が、発光素子の機能を兼ね備えたスイッチング素子、すなわち、いわゆる発光型表示素子（自発光型スイッチング素子）を備える場合においては、最大出力輝度ｉ_out の調整は、画像信号の信号処理によって行う必要がある。
【０１０１】
したがって、逆特性補正回路６にて表示装置８の入力信号−出力輝度特性の逆特性の補正を行う前の画像信号ｇ₁ に対して最大出力輝度ｉ_out を乗じるようにすればよい。すなわち、この場合、画像信号補正装置７から出力される画像信号ｇ_out の輝度信号レベルｇ_out は、次の式（１２）で表される。
【０１０２】
【数９】

【０１０３】
また、最終的な出力輝度Ｉは、次の式（１３）で表現することができる。なお、信号の表現形態は、前述と同様である。
【０１０４】
【数１０】

【０１０５】
ここでは、テレビ放送用の画像等の逆ガンマ補正された画像信号ｇ₀ が入力された場合の画像信号の処理の概略について説明した。そのため、この場合には、特に前処理を行う必要がなかったが、例えば、コンピュータ等の画像信号を生成可能な装置から直接線形化された画像信号が入力される場合には、逆ガンマ補正を施してから、上記処理を行えばよい。
【０１０６】
また、上述した画像信号の処理については、アナログ信号系及びデジタル信号系でも同様に行うことができるが、デジタル信号系で行う方が数値の演算によって容易に行うことができ、パラメータの変更も容易である。デジタル信号系で、アナログ信号が入力される場合には、アナログ信号をデジタル信号に変換してから前述の処理を行えばよい。この処理におけるデジタルデータの階調表現ビット数としては少なくとも８ビット以上であれば精度良く処理が可能であるが、精度があまり必要でない場合や、構成をより簡略にする場合には表現ビット数を８ビットよりも小さくして処理を行ってもよい。
【０１０７】
上記の処理は、表示装置８は、例えば透過型液晶表示装置等のようなスイッチング素子と発光素子とが独立したものであってもよいし、ＦＥＤ(Field Emission Display)やＰＤＰのように発光素子がスイッチング素子と一体化しているものであっても、同様に適用ができ、容易に表示品位を設定することが可能となる。
【０１０８】
上記の処理において、画像表示装置の輝度パラメータであるガンマ値γ（Ｇ）と最大出力輝度ｉ_max （Ｇ）とは、自由な設定が可能である。そのため、ガンマ値γ（Ｇ）および最大出力輝度ｉ_max （Ｇ）の種々の設定パターンを記憶装置に記憶させておき、この記憶内容を必要に応じて読み出すようにすれば、異なる表示装置８を用いても、映像表示品位を統一することが容易となる。
【０１０９】
また、上記の画像信号補正装置７による信号処理において、入力画像信号が輝度信号ｇ₀ と色差信号とからなる場合には、出力輝度信号ｇ_out の信号レベルｇ_out が入力輝度信号ｇ₀ の信号レベルｇ₀ のα（０≦α≦１）倍であるとすると、色差信号に対しても同様の係数αを乗じ、得られた色差信号を出力すればよい。
【０１１０】
また、上記の画像信号補正装置７による信号処理において、入力画像信号が、各色成分信号からなり、各色成分信号の組み合わせにより輝度が表現されている場合にも、上述した演算により得られた出力輝度信号ｇ_out の信号レベルｇ_out が入力輝度信号ｇ₀ の信号レベルｇ₀ のβ（０≦β≦１）倍となったとすると、各色成分信号をβ倍した結果を出力信号として出力すればよい。
【０１１１】
また、上記の処理においては、入力された画像信号の線形化された輝度信号を基準にした処理について示したが、Ｒ・Ｇ・Ｂ三原色、あるいはより多原色の各色成分信号に対して独立に処理を行うように設定することもできる。その場合は、式中の輝度信号レベルを各色成分信号レベルに変更してそれぞれの処理をするだけで良い。この方法を用いた場合には、輝度信号のみによる補正を行う場合よりも高精度な補正が可能である一方、独立パラメータの個数が増加し、パラメータを記憶する記憶装置や演算処理回路等の装置コストが高くなる。そのため、装置コストよりも精度の向上を優先する場合にこの方法を適用すればよい。
【０１１２】
〔実施の形態２〕
次に、実施の形態１で説明した実施の形態の好ましい形態を図２および図３に基づいて説明する。なお、説明の便宜上、前記実施の形態１にて示した各部材と同一の機能を有する部材には、同一の符号を付記し、その説明を省略する。
【０１１３】
本実施形態の画像再生方法に用いる画像表示装置は、図２に示すように、画像信号補正装置７’と、発光素子と表示素子（スイッチング素子）とが独立に制御され得る表示装置８とを備えている。
【０１１４】
表示装置８は、図示しない複数の画素を有する液晶パネル等の非発光型の表示素子１６と、画像信号補正装置７から出力された画像信号ｇ_out を表示駆動用の信号（駆動信号）Ｓ_out に変換する液晶駆動回路等の表示素子駆動形態変換回路１５と、バックライト等の発光素子１８と、画像信号補正装置７から出力された最大出力輝度ｉ_out を発光素子１８の入力に応じた形態の信号Ｉ_out に変換する発光素子駆動形態変換回路１７、例えば、最大出力輝度ｉ_out に対応した電圧を発生させる可変電圧源とを備えている。
【０１１５】
画像信号補正装置７’は、実施の形態１と同様の平均信号レベル演算回路１、入力信号−出力輝度特性設定回路２、最大出力輝度調整回路３、および信号補正部４に加えて、スイッチ９、逆ガンマ補正回路１０、および遅延回路１１を備えている。
【０１１６】
スイッチ９は、逆ガンマ補正回路１０からの画像信号ｇ₀ と外部から入力された逆ガンマ補正された画像信号ｇ₀ とのいずれかを選択的に遅延回路１１および平均信号レベル演算回路１の両方へ出力するものである。また、逆ガンマ補正回路１０は、逆外部から入力された線形補正された画像信号ｇ₀ ’を逆ガンマ補正し、補正後の画像信号ｇ₀ をスイッチ９に出力するものである。
【０１１７】
また、遅延回路１１は、画像信号ｇ₀ が入力信号−出力輝度特性設定回路２に出力されるタイミングと、画像表示装置の入力信号−出力輝度特性の設定パラメータであるガンマ値γ（Ｇ）が信号補正部４に出力されるタイミングとが同期するように、平均信号レベル演算回路１における平均入力輝度信号レベルＧの演算および入力信号−出力輝度特性設定回路２におけるガンマ値γ（Ｇ）の演算に要する時間だけ画像信号ｇ₀ を遅延させるものである。
【０１１８】
本実施形態の最大出力輝度調整回路３は、平均入力輝度信号レベルＧに基づいて、規格化された最大出力輝度の設定値Ｉ_max を演算する最大出力輝度設定回路１２と、規格化された最大出力輝度の設定値Ｉ_max と外部から与えられた輝度基準値αとに基づいて最大出力輝度ｉ_max を演算し、演算結果を表示装置８の発光素子駆動形態変換回路１７に出力する輝度出力制御回路１３とを備えている。
【０１１９】
なお、図２では、逆ガンマ補正された画像信号ｇ₀ と、線形補正された入力画像信号ｇ₀ ’との両方を示しているが、これらは、必ずしも同時に入力されるものではない。また、ここでは、説明の便宜上、逆ガンマ補正された画像信号と、線形補正された入力画像信号ｇ₀ ’をさらに逆ガンマ補正した画像信号とを、同じ符号ｇ₀ で示すが、これらが同一の信号であることを示すものではない。
【０１２０】
また、本実施形態では、逆ガンマ補正された画像信号ｇ₀ および線形補正された画像信号（原画像信号）ｇ₀ ’のいずれが入力されても対応可能な構成としているが、いずれか一方の画像信号の入力にのみ対応する構成とすることも可能である。例えば、スイッチ９および逆ガンマ補正回路１０を省き、逆ガンマ補正された画像信号ｇ₀ の入力にのみ対応する構成としてもよい。
【０１２１】
次に、上記画像表示装置を用いた画像再生方法を図２および図３に基づいて説明する。なお、ここでは、輝度信号レベルｇ₀ ，ｇ₁ ，ｇ_out 、平均入力輝度信号レベルＧ、最大出力輝度（照明輝度調整レベル）Ｉ_max （Ｇ），ｉ_out 、および輝度基準値（外部輝度調整レベル）αは、全て０〜１の値となるように規格化されているものとする。
【０１２２】
表示装置８の入力端（信号入力ポート）に入力された画像信号ｇ₀ は、線形処理された画像信号ｇ₀ ’である場合と、ＣＲＴ表示装置向けに逆ガンマ補正された画像信号ｇ₀ である場合とがあり得る。そのため、まず、線形処理された画像信号ｇ₀ ’が入力された場合には、逆ガンマ補正回路１０にて、逆ガンマ補正を行う。一方、予め逆ガンマ補正された画像信号ｇ₀ が入力された場合には、逆ガンマ補正を行わない。
【０１２３】
次に、平均信号レベル演算回路１にて、逆ガンマ補正された画像信号ｇ₀ に対して一定信号量分の画像信号における画素信号レベルの平均ＡＶＥ（ｇ_{0 xy}）（＝ｆ₁ （ｇ₀ ））を平均入力輝度信号レベルＧとして演算する。平均化される一定信号量分の画像信号ｇ₀ は、１フィールドの画像分の画像信号ｇ₀ でも良いし、適当な間隔でサンプリングした画素信号の電圧値であってもよい。さらには、入力された画像信号ｇ₀ が輝度−色差信号（ＹＰｂＰｒあるいはＹＣｂＣｒ）である場合は、輝度信号Ｙの平均信号レベルを演算すればよい。また、入力された画像信号ｇ₀ が三原色信号（ＲＧＢ）である場合には、三原色信号（ＲＧＢ）の平均信号レベルを演算してもよく、三原色信号（ＲＧＢ）を輝度信号Ｙに変換してから輝度信号Ｙの平均信号レベルを演算してもよい。
【０１２４】
次に、入力信号−出力輝度特性設定回路２にて、平均入力輝度信号レベルＧに応じて、入力信号−出力輝度特性パラメータであるガンマ値γ（Ｇ）（＝ｆ₂ （Ｇ））を設定する。また、最大出力輝度調整回路３にて平均入力輝度信号レベルＧに対応する最大出力輝度ｉ_max （Ｇ）（＝ｉ_out ）を設定する。具体的には、まず、最大出力輝度設定回路１２にて、平均入力輝度信号レベルＧに基づいて、規格化された最大出力輝度の設定値Ｉ_max （Ｇ）を演算する。次いで、輝度出力制御回路１３にて、規格化された最大出力輝度の設定値Ｉ_max と外部から与えられた輝度基準値αとを乗算し、得られた値α・Ｉ_max を最大出力輝度ｉ_max （Ｇ）として発光素子駆動形態変換回路１７に出力する。
【０１２５】
その後、γ（Ｇ）補正回路５にて、設定ガンマ値γ（Ｇ）を指数値とする以下の式（１４）で表される指数関数を用いて、画像信号ｇ₀ の輝度信号レベルｇ₀ から画像信号ｇ₁ の輝度信号レベルｇ₁ を算出する。
【０１２６】
【数１１】

【０１２７】
このとき、入力画像信号ｇ₀ は、γ（Ｇ）補正回路５へ供給される前に、平均信号レベル演算回路１における平均入力輝度信号レベルＧの演算および入力信号−出力輝度特性設定回路２におけるガンマ値γ（Ｇ）の演算に要する時間だけ遅延され、平均入力輝度信号レベルＧが同時刻の入力画像信号ｇ₀ に反映するように図られている。
【０１２８】
次いで、逆特性補正回路６にて、表示装置８の入力信号−出力輝度特性を表す関数の逆関数ｇ_out ＝Ｄ^-1（ｇ₁ ）を用いて、画像信号ｇ₁ の輝度信号レベルｇ₁ から画像信号ｇ_out の輝度信号レベルｇ_out を算出し、輝度信号レベルｇ_out を持つ画像信号ｇ_out を表示素子駆動形態変換回路１５へ出力する。
【０１２９】
その後、画像信号ｇ_out は、表示素子駆動形態変換回路１５にて表示素子１６の入力形態の信号Ｓ_out へと変換される。具体的には、例えば、図３に示すように、関数ｖ₁ ＝Ｖ₁ （ｇ_out ) を用いて表示素子１６に対応した駆動電圧レベルｖ₁ （＝Ｓ_out ）に変換される。この際、最大輝度値ｇ_out がデータとして表示素子駆動形態変換回路１５に入力されるとすると、表示素子駆動形態変換回路１５では、最大輝度値ｇ_out のデータをｎビットのディジタル信号（０〜２ⁿ −１）に変換した後、ｎビットのディジタル信号を駆動電圧レベルｖ₁ （＝Ｓ_out ）に変換する。
【０１３０】
一方、最大輝度値ｉ_out は、発光素子駆動形態変換回路１７にて発光素子１８の駆動形態に対応した信号Ｉ_out に変換される。具体的には、例えば、図３に示すように、関数ｖ₂ ＝Ｖ₂ （ｉ_out ) を用いて表示素子１６に対応した駆動電圧レベルｖ₂ （＝Ｉ_out ）に変換される。この際、最大輝度値ｉ_out がデータとして発光素子駆動形態変換回路１７に入力されるとすると、発光素子駆動形態変換回路１７では、最大輝度値ｉ_out のデータをｎビットのディジタル信号（０〜２ⁿ −１）に変換した後、ｎビットのディジタル信号を駆動電圧レベルｖ₂ （＝Ｉ_out ）に変換する。
【０１３１】
本実施形態では、このような画像信号処理を行うことで、表示素子１６と発光素子１８とが独立に制御可能な表示装置８であれば、どのような輝度特性（平均入力輝度信号レベル−ガンマ値特性および平均入力輝度信号レベル−最大出力輝度特性）を持つ表示装置８を用いても、最適な輝度特性を持つ画像表示装置を実現することが可能となり、品質の高い表示を行うことができる。
【０１３２】
〔実施の形態３〕
本発明の他の実施の形態について図４および図５に基づいて説明すれば、以下の通りである。なお、説明の便宜上、前記実施の形態１または２にて示した各部材と同一の機能を有する部材には、同一の符号を付記し、その説明を省略する。
【０１３３】
本実施形態の画像再生方法に用いる画像表示装置は、図４に示すように、画像信号補正装置２７と、表示素子（スイッチング素子）自体が発光素子として機能する表示装置２８とを備えている。
【０１３４】
表示装置２８は、図示しない複数の画素を有するＣＲＴ等の発光型表示素子２３と、画像信号補正装置２７から出力された画像信号ｇ_out ’を表示駆動用の信号（駆動信号）Ｓ_out ’に変換する発光型表示素子駆動形態変換回路２２とを備えている。
【０１３５】
画像信号補正装置２７は、実施の形態２の画像信号補正装置７におけるγ（Ｇ）補正回路５と逆特性補正回路６との間に、信号変換回路２１が挿入され、輝度出力制御回路１３の出力が信号変換回路２１に供給されるようになっている以外は、実施の形態２の画像信号補正装置７と同様である。
【０１３６】
信号変換回路２１は、γ（Ｇ）補正回路５から出力された画像信号ｇ₁ の信号レベルｇ₁ と最大出力輝度ｉ₁ （＝Ｉ_max （Ｇ））とを乗算し、得られた画像信号ｇ₂ を逆特性補正回路６へ出力するものである。
【０１３７】
次に、上記画像表示装置を用いた画像再生方法を図４および図５に基づいて説明する。なお、ここでは、輝度信号レベルｇ₀ ，ｇ₁ ，ｇ₂ ，ｇ_out ’、平均入力輝度信号レベルＧ、最大出力輝度（照明輝度調整レベル）Ｉ_max （Ｇ），ｉ₁ 、および輝度基準値（外部輝度調整レベル）αは、全て０〜１の値となるように規格化されているものとする。
【０１３８】
まず、入力画像信号ｇ₀ から適正な最大出力輝度ｉ₁ （＝Ｉ_max （Ｇ））およびガンマ値γ（Ｇ）を算出し、γ（Ｇ）補正回路５にて入力画像信号ｇ₀ を画像信号ｇ₁ に補正するまでの各工程は、実施の形態２と同様である。
【０１３９】
ただし、最大出力輝度調整回路３では、この画像表示装置で予め設定されている、表示装置２８で出力可能な最大出力輝度レベルを１００％として、平均入力輝度信号レベルＧに応じて最大出力輝度ｉ_max （Ｇ）を変更する。
【０１４０】
ここで、表示装置２８で出力可能な最大出力輝度レベルをＹ₁₀₀ とし、平均入力輝度信号レベルＧの入力画像に対する最大出力輝度ｉ_max （Ｇ）とＹ₁₀₀ との比率をαとおくと、次式が成り立つ。
【０１４１】
α＝Ｉ_max （Ｇ）／Ｙ₁₀₀ ・・・（１５）
ここで、次式のように設定しておけばよい。
【０１４２】
Ｙ₁₀₀ ＝Ｉ_max （Ｇ₁₀₀ ）・・・（１６）
本実施形態の画像再生方法では、これらの各工程の後に、信号変換回路２１にて、γ（Ｇ）補正回路５から出力された画像信号ｇ₁ の信号レベルｇ₁ と最大出力輝度ｉ₁ （＝Ｉ_max （Ｇ））とを乗算する。すなわち、画像信号ｇ₁ に最大出力輝度ｉ₁ を乗じ、次式により、画像信号ｇ₂ を求める。
【０１４３】
ｇ₂ ＝ｉ₁ ・ｇ₁ ・・・（１７）
このように、画像信号（輝度信号）ｇ₁ に最大出力輝度ｉ₁ を乗算する信号変換を行っているのは、表示装置２８の最大出力輝度が発光型表示素子２３の入力信号レベルに依存するためである。
【０１４４】
その後は、得られた画像信号ｇ₂ を逆特性補正回路６へ出力し、逆特性補正回路６にて、表示装置２８の入力信号−出力輝度特性を表す関数の逆関数ｇ_out ’＝Ｄ^-1（ｇ₂ ）を用いて、画像信号ｇ₂ の輝度信号レベルｇ₂ から画像信号ｇ_out ’の輝度信号レベルｇ_out ’を算出する。
【０１４５】
さらに、輝度信号レベルｇ_out ’を持つ画像信号ｇ_out ’が、発光型表示素子駆動形態変換回路２２へ出力され、発光型表示素子駆動形態変換回路２２にて表示装置２８に対応した入力形態の信号Ｓ_out ’へと変換される。具体的には、例えば、図５に示すように、関数ｖ₃ ＝Ｖ₃ （ｇ_out ’) を用いて発光型表示素子２３に対応した駆動電圧レベルｖ₃ （＝Ｓ_out ’）に変換される。この際、最大輝度値ｇ_out ’がデータとして発光型表示素子駆動形態変換回路２２に入力されるとすると、発光型表示素子駆動形態変換回路２２では、最大輝度値ｇ_out ’のデータをｎビットのディジタル信号（０〜２ⁿ −１）、例えば、８ビットのディジタル信号（０〜２５５）に変換した後、ｎビットのディジタル信号を駆動電圧レベルｖ₃ （＝Ｓ_out ’）に変換する。
【０１４６】
本実施形態では、このような画像信号処理を行うことで、どのような輝度特性（平均入力輝度信号レベル−ガンマ値特性および平均入力輝度信号レベル−最大出力輝度特性）を持つ発光型表示素子を備える表示装置２８を用いても、最適な輝度特性を持つ画像表示装置を実現することが可能となり、品質の高い表示を行うことができる。
【０１４７】
なお、以上の各実施の形態では、平均入力輝度信号レベルに応じて、画像表示装置の入力信号−出力輝度特性と表示部の画素の最大出力輝度との両方を調整することで、品質の高い表示を得ていたが、平均入力輝度信号レベルに応じて、画像表示装置の入力信号−出力輝度特性と表示部の画素の最大出力輝度との一方だけを調整しても、ある程度品質の高い表示を得ることが可能である。
【０１４８】
【実施例】
〔実施例１〕
まず、高品位のＣＲＴ表示装置である市販のＨＤＴＶ（高品位テレビ）を用いて、平均入力輝度信号レベル（全画面での入力輝度信号の平均値）と、入力輝度信号データおよび最大出力輝度との関係を測定した。
【０１４９】
測定には、１９２０×１０３５ピクセルサイズのフレーム画像の中央に１５０×１５０ピクセルサイズのボックスを設けた画像を用い、ボックスの入力輝度信号レベルＢ（最大値を１００％として表した相対値）と、背景部（ボックス以外の部分）の入力輝度信号レベルＨ（最大値を１００％として表した相対値）とを変化させ、ボックスの出力輝度を色彩輝度計により測定した。また、入力輝度信号には、線形化されたデータを用いた。
【０１５０】
測定の結果、背景部の入力輝度信号レベルＨを固定してボックスの入力輝度信号レベルＢを変化させた場合の各ボックスの出力輝度は、図８に示す通りであった。また、背景部の入力輝度信号レベルＨを固定して画面の平均入力輝度信号レベルＧに対するボックスの出力輝度をプロットすると、図９に示す通りであった。
【０１５１】
この場合、ボックスの面積は、全画面の面積の１．１３％であり、十分に小さい。そのため、背景部の入力輝度信号レベルＨは、画像全体の平均入力輝度信号レベルＧに等しいとみなしても、差し支えない。
【０１５２】
そこで、本実施例では、前記の実施の形態１の方法において、ＣＲＴ表示装置Ｄ（またはＥ）の表示特性と同様な表示（再生）をするために、設定パラメータ、すなわち最大出力輝度ｉ_max （Ｇ）およびガンマ値γ（Ｇ）の設定値を、平均入力輝度信号レベルＧに対応して図１０に示す曲線ｉ_max Ｄ（またはｉ_max Ｅ）および曲線γＤ（またはγＥ）のように変化させた。これにより、ＣＲＴ表示装置以外の表示装置、例えば、液晶表示装置を用いて、ＣＲＴ表示装置Ｄ（またはＥ）と同様の、平均入力輝度信号レベル−最大出力輝度特性（平均入力輝度信号レベルＧに対する最大出力輝度ｉ_max （Ｇ）の変化）および平均入力輝度信号レベル−ガンマ値特性（平均入力輝度信号レベルＧに対するガンマ値γ（Ｇ）の変化）を有する画像表示装置が実現できる。
【０１５３】
そして、この画像表示装置では、図８〜図１０の結果から、平均入力輝度信号レベルＧが低い場合に、ガンマ値γ（Ｇ）が増加され暗部の出力輝度が相対的に増加されることによって暗部の視認性が向上することが期待される。また、平均入力輝度信号レベルＧが高い場合には、ガンマ値γ（Ｇ）が減少され明部の出力輝度が相対的に減少されることによって、明部の視認性が向上することが期待される。さらに、平均入力輝度信号レベルＧが高い場合には、最大出力輝度ｉ_max （Ｇ）が減少され画面のまぶしさが抑えられることによって、視認性が増すことも期待される。
【０１５４】
次に、一般的な液晶表示装置の入力信号−出力輝度特性を、ＣＲＴ表示装置と同様な手法で測定した。ただし、ここでは、液晶表示装置として、ＮＴＳＣ（全米テレビシステム委員会）方式表示用の液晶表示装置を用いたので、ＨＤＴＶとの表示画素数等の違いを考慮して、ボックスの面積を画面全体の面積の約１％になるように設定した。得られた測定結果を、最大値を基準に規格化して図１１に示す。なお、図１１に示す入力信号−出力輝度特性は、液晶表示装置において、液晶表示装置内の信号処理回路により、入力画像信号の逆ガンマ補正と、液晶の電圧−光学変換特性の補正（線形特性からのずれの補正）とを行った結果得られた入力信号−出力輝度特性を示している。この液晶表示装置の入力信号−出力輝度特性は、平均入力輝度信号レベルＧ（＝背景部の入力輝度信号レベルＨ）によらずほぼ一定であった。
【０１５５】
図１１の特性は、ＣＲＴ表示装置に対して用いられるような非線形な指数関数で近似表現すると輝度の高い部分で誤差が大きくなったため、６次多項式によって近似表現し、処理を行った。
【０１５６】
これらの入力信号−出力輝度特性をシミュレータにより図３に示される信号処理を通して液晶表示装置に適用した。平均入力輝度信号レベルＧがほぼ０％である場合の液晶表示装置の入力信号−出力輝度特性を図１２に、平均入力輝度信号レベルＧが７５％程度である場合の液晶表示装置の入力信号−出力輝度特性を図１３に示した。
【０１５７】
この処理形態では、図１１で示される液晶表示装置の入力信号−出力輝度特性は、式（９）で示される信号処理を通して線形特性に変換されるように処理される。そのため、実質的には図１０で示される入力信号−出力輝度特性が液晶表示装置にて実現されている。
【０１５８】
実際の画像で評価した結果、前述の期待される効果が得られた。また、図１０に対応するデータを異なる表示装置で測定しておき、得られた測定データを記憶装置等に記憶させておけば、図１で示されるような処理を経て任意の表示装置において画像を再現することが可能になる。
【０１５９】
〔比較例１〕
実施例１で用いた液晶表示装置に対して、本発明に係る画像再生方法を適用せず、入力画像信号をそのまま液晶表示装置へ入力した場合の表示特性を図１４に示した。逆ガンマ補正された画像信号が入力されるとすると、液晶表示装置での表示結果（出力輝度）が入力レベル（原画像の光強度）に対して線形特性を示せば、原画像（撮像された画像等）を忠実に再生したことになる。図１４の表示特性においては、４０％以上の輝度が高い部分で、相対的にレベルが増加しているため、霞のかかったような白っぽい画像として認知される可能性が高い。
【０１６０】
また、実際の画像で評価した結果、入力信号−出力輝度特性が線形である画像と比較して全体的に白っぽい表示として視認された。また、そのために、画像の色調はやや色があせたように感じられ、画像の質感はみずみずしさが損なわれているように感じられた。
【０１６１】
【発明の効果】
本発明の画像再生方法は、以上のように、全ての画素信号のレベルの平均を平均信号レベルとして演算した後、画素信号のレベルに対する画素の輝度の変化を表す入力信号−出力輝度特性を、上記平均信号レベルに応じて設定し、次いで、設定された入力信号−出力輝度特性を満たすように画像を再生する方法である。
【０１６２】
上記方法によれば、例えば、平均信号レベルに係わらず入力信号−出力輝度特性が一定であるような表示装置（液晶表示装置等）を用いても、全体的に暗い画像における暗部の視認性に優れ、かつ、全体的に明るい画像における明部の視認性に優れた画像を再生することが可能となる。したがって、上記方法は、表示装置の入力信号−出力輝度特性が平均信号レベルに応じて変化するか否かにかかわらず、高い表示品位で画像を再生することが可能な画像再生方法を提供することができるという効果を奏する。
【０１６３】
また、上記画像再生方法の好ましい形態は、さらに、上記平均信号レベルに応じて表示装置の画素の最大出力輝度が変化するように画像を再生する方法である。
【０１６４】
上記方法によれば、例えば、入力平均信号レベルに係わらず最大出力輝度が一定であるような表示装置（液晶表示装置等）を用いても、全体的に明るい画像を再生するときの画面のまぶしさを軽減するとともに直視の際の網膜漂白化現象による一時的な盲目化を防止することが可能となる。したがって、上記方法は、表示装置の種類によらず高い表示品位で画像を再生することが可能な画像再生方法を提供することができるという効果を奏する。
【０１６５】
また、本発明の画像表示装置は、上記の課題を解決するために、全ての画素信号のレベルの平均を平均信号レベルとして演算した後、上記平均信号レベルに応じて表示装置の画素の最大出力輝度が変化するように画像を再生する方法である。
【０１６６】
上記方法によれば、例えば、入力平均信号レベルに係わらず最大出力輝度が一定であるような表示装置（液晶表示装置等）を用いても、全体的に明るい画像を再生するときの画面のまぶしさを軽減するとともに直視の際の網膜漂白化現象による一時的な盲目化を防止することが可能となる。したがって、上記方法は、表示装置の平均信号レベル−最大出力輝度特性によらず高い表示品位で画像を再生することが可能な画像再生方法を提供することができるという効果を奏する。
【０１６７】
上記各画像再生方法では、入力信号−出力輝度特性を指数関数で近似表現した場合の指数値が、平均信号レベルの上昇に伴ってより大きい値となるように画像を再生することが望ましい。これにより、全体的に明るい画像（平均輝度が高い画像）を再生するときの画面のまぶしさを軽減することができるとともに、直視の際の網膜漂白化現象による一時的な盲目化を防止することができる。したがって、上記方法は、全体的に明るい画像を視認性に優れた画像として再生することができるという効果を奏する。
【０１６８】
また、上記各画像再生方法では、最大出力輝度が、上記平均信号レベルが上昇するにしたがってより小さくなるように画像を再生することが望ましい。これにより、全体的に暗い画像（平均輝度が低い画像）における暗部の視認性を向上させるとともに、全体的に明るい画像（平均輝度が高い画像）における白つぶれや眩しさを防止して明部の視認性を向上させることができる。したがって、上記方法は、全体的に暗い画像も全体的に明るいい画像も視認性に優れた画像として再生することができるという効果を奏する。
【図面の簡単な説明】
【図１】本発明の実施の一形態の画像再生方法に用いられる画像表示装置の概略構成を示すブロック図である。
【図２】上記画像再生方法のより好ましい形態の画像再生方法に用いられる画像表示装置であって、独立に制御される表示素子および発光素子を表示部に備える画像表示装置の概略構成を示すブロック図である。
【図３】図２に示す画像表示装置における画像再生方法の流れを模式的に示すブロック図である。
【図４】本発明の他の実施の形態の画像再生方法に用いられる画像表示装置であって、発光型表示素子を表示部に備える画像表示装置の概略構成を示すブロック図である。
【図５】図４に示す画像表示装置における画像再生方法の流れを模式的に示すブロック図である。
【図６】逆ガンマ補正された画像信号が種々の表示装置を通して出力されたときの入力信号−出力輝度特性の変化を表示装置の入出力特性とともに模式的に表す説明図である。
【図７】線形処理された画像信号が種々の表示装置を通して出力されたときの入力信号−出力輝度特性の変化を表示装置の入出力特性とともに模式的に表す説明図である。
【図８】ＣＲＴ表示装置の背景部の入力輝度信号レベルが一定である場合における、ボックスの入力輝度信号レベルに対するボックスの出力輝度の変化を示すグラフである。
【図９】ＣＲＴ表示装置の平均入力輝度信号レベルに対するボックスの出力輝度の変化を示すグラフである。
【図１０】ＣＲＴ表示装置における、平均入力輝度信号レベルに対する、規格化された最大出力輝度およびガンマ値の変化を示すグラフである。
【図１１】液晶表示装置の背景部の入力輝度信号レベルが一定である場合における、ボックスの入力輝度信号レベルに対するボックスの出力輝度の変化を示すグラフである。
【図１２】平均入力輝度信号レベルが０％であるときの液晶表示装置の入力信号−出力輝度特性を示すグラフである。
【図１３】平均入力輝度信号レベルが７５％である時の液晶表示装置の入力信号−出力輝度特性を示すグラフである。
【図１４】本発明を適用しなかった場合の液晶表示装置の入力信号−出力輝度特性を示すグラフである。
【符号の説明】
１ 平均信号レベル演算回路
２ 入力信号−出力輝度特性設定回路
３ 最大出力輝度調整回路
４ 信号補正部
５ γ（Ｇ）補正回路
６ 逆特性補正回路
７ 画像信号補正装置
７’ 画像信号補正装置
８ 表示装置
９ スイッチ
１０ 逆ガンマ補正回路
１１ 遅延回路
１２ 最大出力輝度設定回路
１３ 輝度出力制御回路
１５ 表示素子駆動形態変換回路
１６ 表示素子
１７ 発光素子駆動形態変換回路
１８ 発光素子
２１ 信号変換回路
２２ 発光型表示素子駆動形態変換回路
２３ 発光型表示素子
２７ 画像信号補正装置
２８ 表示装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image reproduction method for reproducing an image using a display device.
[0002]
[Prior art]
Conventionally, gamma correction and luminance correction corresponding to an input image signal have been performed in an image display device. The gamma correction is to correct an input signal-output luminance characteristic (change in output luminance with respect to a change in input signal; called gamma characteristic) of the image display apparatus. By such correction, it is possible to display an image that is substantially equal to the input original image by adjusting the luminance, color (chromaticity), and contrast ratio of the output image. It is also possible to freely adjust the display quality by displaying an image with the contrast ratio adjusted with respect to the input original image.
[0003]
As a gamma correction technique relating to a liquid crystal display device, Japanese Patent Application Laid-Open No. 10-126648 discloses an input analog in a gamma correction circuit that converts an input analog image signal into a digital signal by an AD (analog-digital) converter and then performs gamma correction. The image signal is amplified with an amplification degree selected corresponding to the front end of the AD converter (analog input voltage range), then converted into a digital signal by the AD converter, and then determined in association with the selection information of the amplification degree. A gamma correction circuit that performs gamma correction using a gamma correction characteristic is disclosed. With this configuration, it is possible to perform gamma correction with a wide range and high accuracy with a conversion process with a small number of bits, and it is possible to reduce the cost of a circuit necessary for performing gamma correction with a wide range and high accuracy. .
[0004]
JP-A-5-64037 discloses a gamma correction curve adapted to a liquid crystal display device so as to linearize a function representing an input voltage-output luminance characteristic based on the transmittance characteristic measured by a luminance meter, There is disclosed a gamma correction circuit that performs an appropriate gamma correction process on signals of R (red), G (green), and B (blue) colors using the obtained gamma correction curve.
[0005]
Furthermore, as a gamma correction technique related to a CRT (cathode ray tube) display device, Japanese Patent Laid-Open No. 5-145942 measures the gamma characteristics of each color of RGB of the CRT display device, and uses the measured gamma characteristics to It is disclosed that color correction and gamma correction are performed so that the luminance ratio of each level is constant.
[0006]
Japanese Patent Laid-Open No. 1-158416 (Japanese Patent Publication No. 7-109456) discloses a luminance correction technique for a color light source for a liquid crystal display, and the color balance is lost due to a decrease in the relative visibility of each color of RGB at low luminance. In order to prevent the light emission from being seen, the first luminance adjustment unit that adjusts the luminance levels of the light emitting element groups for RGB colors in common, and the green emission according to the luminance level adjusted by the first luminance adjustment unit It is disclosed to provide a second luminance adjusting means for adjusting a balance of luminance levels among the element group, the red light emitting element group, and the blue light emitting element group.
[0007]
Conventionally, an image signal subjected to inverse gamma correction is usually input to a CRT display device. Inverse gamma correction uses an inverse function of a non-linear function representing an input signal-output luminance characteristic (gamma characteristic) of the image display device in order to make the light intensity of the original image proportional to the luminance of each pixel of the image display device. Thus, the image signal is corrected. A nonlinear function representing the gamma characteristic of a CRT display device can be approximated by an exponential function, and an exponent value called a gamma value is generally about 2.2. Therefore, an image signal subjected to inverse gamma correction is normally input to the CRT display device assuming that the gamma value of the CRT display device is 2.2.
[0008]
Gamma value γ (G) and maximum output luminance with respect to average input luminance signal level (average luminance signal level over the entire screen of the input image signal) G in two types of CRT display devices D and E of different manufacturers that are generally marketed i _max The result of measuring the change in (G) is shown in FIG. Of the curves shown in FIG. 10, the curve indicated by γD represents the gamma value γ (G) of one CRT display device D, and the curve indicated by γE represents the gamma value γ (G) of the other CRT display device E. A curve representing i, _max The curve indicated by D is the maximum output luminance i of the CRT display device D. _max A curve representing (G), i _max The curve indicated by E is the maximum output luminance i of the CRT display device E. _max It is a curve showing (G). The average input luminance signal level G is a relative value expressed with the maximum value being 100%, and the maximum output luminance is a value normalized so that the maximum value is 1.
[0009]
From the measurement data shown in FIG. 10, the maximum output luminance i for most input images in which the average input luminance signal level G is within a certain range. _max (G) and gamma value γ (G) are substantially constant, but in an input image having an average input luminance signal level G that is out of the range, the maximum output luminance i _max The feature that (G) is reduced can be read.
[0010]
According to FIG. 10, when the average input luminance signal level G is about 60%, the actually measured gamma value γ (G) is also about 2.2. Therefore, at this time, the CRT display device faithfully reproduces an image obtained by linearizing the input image signal, that is, an original image (photographed image) before being subjected to inverse gamma correction.
[0011]
However, when the average input luminance signal level G is lower than about 60% or higher than about 60%, the gamma value γ (G) of the image output of the CRT display device does not show 2.2. The processing is not complete and a display faithful to the original image (captured image) cannot be obtained.
[0012]
However, according to the study by the inventors of the present application, when the average input luminance signal level is low, an input in which the output luminance of the dark portion is relatively increased as shown in FIG. Since the signal-output luminance characteristic is obtained, it has been found that the display advantage that the visibility of the dark portion is improved can be obtained.
[0013]
Further, according to the study by the inventors of the present application, with such display characteristics, when the average input luminance signal level is high, the output luminance of the bright part is relatively lowered as shown in FIG. Since the input signal-output luminance characteristic is obtained and the output luminance of the entire screen is relatively lowered, visibility is improved without any white-out in a bright part and without feeling glare. I understood.
[0014]
It is generally not recognized that the visibility of an image on display is improved by such display characteristics of a CRT display device. Note that not all CRT display devices exhibit such display characteristics. However, in general CRT display devices, display brightness is used to prevent CRT burnout due to an increase in drive current unique to the CRT display device. Since a circuit (automatic luminance limiting circuit) for limiting an increase in drive current accompanying an increase in the number of pixels is provided, such display characteristics are usually exhibited.
[0015]
[Problems to be solved by the invention]
On the other hand, in a display device configured by a light emitting element such as a backlight and an optical switching element such as a liquid crystal panel, for example, a liquid crystal display device, when reproducing an image signal on the display device, the maximum luminance of the display image is light emission. The input signal-output luminance characteristic is generally determined by the characteristics of the optical switching element. The maximum luminance of the display image and the input signal-output luminance characteristic are characteristics independent of each other. In such a display device, as is apparent from FIG. 11 showing the measurement result in the case of the liquid crystal display device, the maximum output luminance i in the display image is displayed. _max (G) and an exponent value (gamma value) γ (G) obtained by approximating the nonlinear function representing the input signal-output luminance characteristic with an exponential function are the average input luminance signal level G (background portion) of the input image. It is constant regardless of the input luminance signal level H.
[0016]
Here, when the inventor of the present application subjectively compares the display characteristics (luminance characteristics) of the consumer CRT display device and the display characteristics (luminance characteristics) of such a liquid crystal display device, In terms of points, it has been found that it is somewhat desirable to have display characteristics (luminance characteristics) like those of a CRT display device.
[0017]
In FIG. 11, in the liquid crystal display device, the inverse gamma correction of the input image signal and the correction of the voltage-optical conversion characteristic of the liquid crystal (correction of deviation from the linear characteristic) were performed by the signal processing circuit in the liquid crystal display device. The obtained input signal-output luminance characteristics are shown.
[0018]
When an image signal subjected to inverse gamma correction such as an image signal transmitted from a TV broadcasting station is input to the liquid crystal display device having the input signal-output luminance characteristic shown in FIG. 11, reproduction on the display surface of the liquid crystal display device is performed. The output luminance characteristics of the image are as shown in FIG.
[0019]
In this output luminance characteristic, the normalized luminance is considerably higher than the linear characteristic in a portion where the luminance where the normalized luminance signal level is 0.4 or higher is high. With this output luminance characteristic, an image that gives a strong impression that the entire image is white when the image is viewed is obtained, and the input image cannot be reproduced correctly.
[0020]
In addition, when the output luminance characteristics of the reproduced image as shown in FIG. 11 are displayed, when the image having a high average luminance is displayed, the luminance is increased overall. For this reason, the viewer feels the entire screen dazzling, and the minute brightness difference in the bright part cannot be sufficiently recognized, so that the viewer feels like whitening. Conversely, when displaying a dark image with a low average luminance, the dark portion is reproduced with substantially linear characteristics, but gives an impression that the dark portion is not sufficiently visible because the dark portion is entirely dark.
[0021]
On the other hand, in the CRT display device, since the maximum output luminance when the average luminance is low is relatively high, the visibility of the dark part is felt relatively well. Further, when displaying an overall bright image with a high average luminance, the maximum output luminance is relatively low, so that the glare is suppressed and the overall visibility is slightly improved.
[0022]
Such defects in color tone such as visibility and whitening due to display characteristics are prominently seen in CRT display devices, flat display devices such as liquid crystal displays and plasma displays, and projection display devices.
[0023]
By the way, the conventional gamma correction technique is based on the premise that the gamma characteristic of the display device does not change depending on the type of image, and correction is performed with the same set value (gamma value) for any image. ing. Therefore, the lack of visibility in the liquid crystal display device as described above cannot be improved.
[0024]
The conventional luminance correction technique relates to the output adjustment of the color light source, and does not take into consideration the level of the image signal input to the liquid crystal display. Therefore, even with this technique, the lack of visibility in the liquid crystal display device as described above cannot be improved.
[0025]
Essentially, in order for a display image to be reproduced faithfully from an input signal, it is necessary to exhibit an input signal-output luminance characteristic that is almost linear on at least the image display surface of the display device. In order to realize a natural image for the viewer, it may be possible to arbitrarily adjust input / output characteristics such as luminance characteristics and tone characteristics of image reproduction. This causes problems such as a complicated configuration of the signal processing circuit of the image display device and an increase in cost.
[0026]
The present invention has been made in view of the above-described conventional problems, and an object thereof is to provide an image reproduction method capable of reproducing an image with high display quality.
[0027]
[Means for Solving the Problems]
In order to solve the above problems, an image reproduction method of the present invention is an image reproduction method for reproducing an image on a display device having a plurality of pixels based on an image signal including a pixel signal representing information for each pixel. After calculating an average of all pixel signal levels as an average signal level, an input signal-output luminance characteristic representing a change in luminance of the pixel with respect to the level of the pixel signal is set according to the average signal level, and then set. An image is reproduced so as to satisfy the input signal-output luminance characteristic.
[0028]
According to the above method, for example, even if a display device (liquid crystal display device or the like) whose input signal-output luminance characteristic is constant regardless of the average signal level, the input signal- The exponential value (gamma value) when the output luminance characteristics are approximated by an exponential function can be reduced, so the overall darkness of images (images with low average luminance) is excellent, and overall Therefore, it is possible to reproduce an image excellent in visibility of a bright part by preventing whitening and glare in a bright image (image having a high average luminance). Therefore, according to the above method, it is possible to reproduce an image with high display quality regardless of whether or not the input signal-output luminance characteristic of the display device changes according to the average signal level.
[0029]
Further, a preferred form of the image reproduction method is a method of reproducing an image such that the maximum output luminance of the pixel of the display device changes according to the average signal level.
[0030]
According to the above method, for example, even when a display device (liquid crystal display device or the like) whose maximum output luminance is constant regardless of the input average signal level is displayed according to the increase in the average signal level of the image display device. Since the maximum output brightness of the device can be lowered, the glare of the screen when reproducing an overall bright image can be reduced and temporary blinding due to retinal bleaching phenomenon during direct viewing can be prevented. It becomes possible. Therefore, according to the above method, it is possible to reproduce an image with high display quality regardless of the type of display device.
[0031]
In addition, in order to solve the above-described problem, the image display device of the present invention reproduces an image on a display device having a plurality of pixels based on an image signal including a pixel signal representing information for each pixel. In the above, after calculating the average of the levels of all the pixel signals as the average signal level, the image is reproduced so that the maximum output luminance of the pixels of the display device changes according to the average signal level.
[0032]
According to the above method, for example, even when a display device (liquid crystal display device or the like) whose maximum output luminance is constant regardless of the input average signal level is displayed according to the increase in the average signal level of the image display device. Since the maximum output brightness of the device can be lowered, the glare of the screen when reproducing an overall bright image can be reduced and temporary blinding due to retinal bleaching phenomenon during direct viewing can be prevented. It becomes possible. Therefore, according to the above method, an image can be reproduced with high display quality regardless of the average signal level-maximum output luminance characteristic of the display device.
[0033]
The setting of the maximum output luminance value and / or the input signal-output luminance characteristic according to the average signal level in each of the image reproduction methods can be arbitrarily set.
[0034]
The calculation of the average signal level is preferably performed by calculating the average of the levels of all the pixel signals within the unit period. The unit period may be the entire period constituting one image such as one frame, or may be a part of a period constituting one image such as a period over one field or several fields. Good. The calculation of the average signal level may be performed by sampling the instantaneous voltage level of all the pixel signals at an appropriate sampling period and calculating the average of each sampled voltage level.
[0035]
In each of the above image reproduction methods, it is desirable to reproduce an image so that an exponent value when an input signal-output luminance characteristic is approximated by an exponential function becomes a larger value as the average signal level increases. This reduces the glare of the screen when playing an overall bright image (an image with a high average brightness) and prevents temporary blinding due to retinal bleaching during direct viewing. Can do. Therefore, according to the above method, a bright image as a whole can be reproduced as an image with excellent visibility.
[0036]
In each of the above image reproduction methods, it is desirable to reproduce an image such that the maximum output luminance becomes smaller as the average signal level increases. This improves the visibility of dark areas in images that are generally dark (images with low average brightness), and prevents white spots and glare in images that are bright overall (images with high average brightness). Visibility can be improved. Therefore, it is possible to reproduce both an overall dark image and an overall bright image as images with excellent visibility.
[0037]
In each of the above image reproduction methods, the image signal used for image reproduction may be a black and white video signal composed of a luminance signal representing luminance information for each pixel, and a luminance signal representing luminance information for each pixel and each It may be a color video signal composed of chromaticity signals representing chromaticity information for each pixel, or a color video signal including each color component signal of three primary colors or more primary colors.
[0038]
When the input image signal includes a luminance signal representing luminance information for each pixel, it is desirable that the average signal level is calculated by calculating the average of all the luminance signal levels. Thereby, an image can be reproduced by an image display device having a simple configuration.
[0039]
That is, in a standard image reproduction method, a color video signal composed of a luminance signal and a color difference signal is used as an image signal. In this case, of the signals constituting the color video signal, it is the luminance signal that affects the input signal-output luminance characteristics and the maximum output luminance of the display device. Therefore, the average signal level is calculated by using only the luminance signal rather than the average of both the luminance signal and the color difference signal. Therefore, an image can be reproduced by an image display device having a simple configuration.
[0040]
Further, when the input image signal is a color video signal including each color component signal of three primary colors (for example, RGB) or more primary colors, the calculation of the average signal level is a value corresponding to a luminance value from the color component signal. It is desirable to do this by calculating the average of the levels. As a method for calculating the average of the level of the value corresponding to the luminance value, even if the value corresponding to the luminance is calculated from the color component signals of all colors by the conversion formula, the calculated value is averaged. Alternatively, after averaging the color component signals of all colors, the obtained average value may be converted into an average value corresponding to the luminance by a conversion formula. Further, as a method of calculating the average of the luminance signal levels, a method of calculating the average signal level using only the color component signals of some colors without using the color component signals of all colors may be adopted. Good. Also in this case, as in the case of using the color component signal components of all the colors described above, the order of the conversion to the value corresponding to the luminance and the calculation of the average value is arbitrary.
[0041]
Further, when a color video signal including such color component signals is input, the calculation of the average signal level does not have to calculate the average of the levels corresponding to the luminance values. An average of at least one level may be calculated as an average signal level. That is, for example, when RGB three primary color signals are input as image signals, only the G signal may be extracted and the average value of the G signal level may be calculated as the average signal level, or the average level of each color component signal The value may be calculated as an average signal level.
[0042]
When the input image signal includes a luminance signal representing luminance information for each pixel, an input luminance signal-output luminance characteristic representing a change in luminance of the pixel with respect to the level of the luminance signal is set according to the average signal level, It is preferable to correct the luminance signal so as to satisfy the set input luminance signal-output luminance characteristic.
[0043]
Further, when the input image signal includes each color component signal of three primary colors (for example, RGB) or more primary colors, an input signal-output luminance representing a change in pixel luminance with respect to at least one level of each color component signal. Preferably, the characteristic is set according to the average signal level, and at least one of the color component signals is corrected so as to satisfy the set input signal-output luminance characteristic.
[0044]
The input signal-output luminance characteristic may be set by calculating a parameter representing the input signal-output luminance characteristic from the average signal level using a calculation formula. In setting the input signal-output luminance characteristic, a lookup table that associates the average signal level with the input signal-output luminance characteristic is stored in a storage device such as a memory, and the input signal is referenced with reference to the lookup table. -The output luminance characteristic may be set. The above look-up table may be created based on the measurement results obtained by measuring various input signal-output luminance characteristics in advance.
[0045]
When the image signal is corrected by an arithmetic process using the input signal-output luminance characteristic parameter, the pixel signal is input with an input / output characteristic corresponding to the input signal-output luminance characteristic by the arithmetic process using the input signal-output luminance characteristic parameter. After the conversion, it is desirable to correct the deviation from the linear characteristic in the input signal-output luminance characteristic of the display unit (display device).
[0046]
According to the above configuration, the combination of the input / output characteristic of the second correction and the input signal-output luminance characteristic of the display device is a linear characteristic. Therefore, the first correction only needs to perform a simple calculation process using only the input signal-output luminance characteristic parameter, and the calculation is simplified.
[0047]
On the other hand, when the second correction is not performed, it is necessary to correct the image signal using both the input signal-output luminance characteristic parameter and the input signal-output luminance characteristic parameter of the display unit, and the calculation is complicated. become.
[0048]
In the second correction, the pixel signal may be converted using an inverse function of the function representing the input signal-output luminance characteristic of the display device. However, when displaying on a plurality of different types of display devices, since the type of the display device is not specified, the input signal-output luminance characteristics of the display device may change to various characteristics. Therefore, in this case, input signal-output luminance characteristics of various display devices are stored in advance in a storage device such as a RAM or a storage medium such as a hard disk in association with the type of the display device, and the stored contents are referred to. Thus, it is preferable to correct the image signal with the input / output characteristics opposite to the input signal-output luminance characteristics of the display device.
[0049]
The display device may include a light emitting element and an optical switching element for controlling light from the light emitting element for each pixel. Therefore, the display device is configured to include a light-emitting optical switching element (light-emitting display element) having a light-emitting element function, for example, a CRT, a light-emitting diode, a plasma display panel (PDP), an FED (Field Emission Display), and the like. The light-emitting element may include a light-emitting element and a non-light-emitting optical switching element (non-light-emitting display element) that controls or modulates light from the light-emitting element without itself emitting light, such as a liquid crystal display element. May be.
[0050]
When the display device includes a light-emitting element and a non-light-emitting optical switching element, such as a transmissive liquid crystal display device, and these can be controlled independently of each other, the maximum output luminance is calculated, It is preferable to output the calculation result to the light emitting element. The calculation of the maximum output luminance is preferably performed by calculating the standardized maximum output luminance and then calculating the maximum output luminance based on the calculation result and the luminance reference value given from the outside. .
[0051]
On the other hand, when the display device is configured to include a light-emitting element that also functions as an optical switching element, that is, a light-emitting optical switching element, the maximum output luminance is calculated and corrected according to the set input signal-output luminance characteristic. It is desirable that the converted image signal is converted based on the calculation result of the maximum output luminance, and the converted image signal is output to the light-emitting optical switching element.
[0052]
Each arithmetic processing is preferably realized by hardware by a circuit, but may be realized by software. In other words, each arithmetic processing described above may be realized by a storage unit such as a RAM that stores a computer program describing the arithmetic processing of each arithmetic unit and a CPU (Central Processing Unit) that executes the computer program.
[0053]
Next, the process in which the present inventors have made the present invention will be described in detail with reference to FIGS.
[0054]
6 and 7 are diagrams schematically showing display characteristics of various display devices with respect to an input image signal. 6 and 7 show a state in which image signals S1 and S5 are input to three display devices 30A to 30C having different display characteristics, respectively.
[0055]
In FIG. 6, an image signal S1 subjected to inverse gamma correction as in general TV broadcasting is input. Accordingly, the slope of the curve C1 representing the change in the output level (voltage level of the image signal) with respect to the input level (light intensity of the original image) of the image signal S1 decreases as the input level increases. The reason why the image signal S1 is input is that the gamma characteristic as indicated by the curve C2, that is, the slope of the curve C2 representing the change in the output level (luminance) with respect to the input level (voltage level of the image signal). This is because it is assumed that an image is reproduced on the CRT display device 30A having display characteristics that increase as the input level increases, and reverse gamma correction is performed so as to cancel this change in tilt. Therefore, if the display device has a gamma characteristic like the CRT display device 30A of FIG. 6, the output level (luminance) of the image S2 output (displayed) from the CRT display device 30A is as shown by a curve C5. It becomes linear with respect to the input level (light intensity of the original image).
[0056]
Generally, a display device has various input / output characteristics (display characteristics) with respect to an input (light intensity) from an image signal source (original image). For this reason, the input image reproduced almost faithfully on the CRT display device 30A may be represented by another display device, for example, the display device 30B having input / output characteristics represented by the curve C3 or the curve C4. In the display device 30C having an input / output characteristic, as shown by curves C6 and C7, images S3 and S4 having a non-linear output level with respect to the input level from the original image are output, and the original image is reproduced faithfully. It is happening not to be done.
[0057]
Further, when the image signal S5 having a linear change in the output level (voltage level of the image signal) with respect to the input level (light intensity of the original image) as indicated by the curve C8 in FIG. 7 is input, for example, a computer or the like In the case where the image data created in (1) is directly input, if the input image signal S5 is not corrected, the image signal S5 is represented by the curves C9 to C11 in the display devices 30A to 30C. It is converted into an image having characteristics corresponding to the input / output characteristics of the display devices 30A to 30C and output. Therefore, in the display devices 30A to 30C, as indicated by the curves C12 to C14, S6 to S8 having a non-linear output level with respect to the input level from the original image are output. Therefore, it can be seen that the original image (input image) is not correctly reproduced on the display screens of the display devices 30A to 30C.
[0058]
The input / output characteristics of such a display device (display device) will be described in more detail.
[0059]
In a display device such as a CRT display device, an input signal voltage-output luminance conversion characteristic is generally called a gamma (γ) characteristic, and is expressed as the following equation (1).
[0060]
[Expression 1]

[0061]
Here, I is an output luminance, V is an input signal voltage, and an exponent value γ is a non-linear parameter called a gamma value. A and b are offset constants.
[0062]
The input signal is stipulated in the SMPTE standard 170M or 240M, which is a television standard of the American Film and Television Engineers Association (SMTPE), or the conventional radio wave law / CCIR (International Radiocommunication Advisory Committee) recommendation 624 / RS170A Consider the case of a normal TV image signal that has been subjected to inverse gamma correction by the luminance-signal voltage conversion formula. Since this TV image signal has been subjected to inverse gamma correction on the assumption that it is input to a CRT display device with γ = 2.2, if it is displayed on the CRT display device, special processing is performed under certain conditions. The original image is almost linearized and output. Since the input signal voltage is substantially proportional to the input luminance signal level, a = b = 0 in equation (1) for the sake of simplification with respect to the input luminance signal level g subjected to inverse gamma processing, and g is the maximum value. Maximum brightness (maximum output brightness) i _max Can be expressed as the following equation (2).
[0063]
[Expression 2]

[0064]
In this way, the relationship between the input and the output can be considered in the signal processing part by removing the restriction of the display device. Here, in order to explain the concept, the constant term of the formula (1) is omitted, but the formula (1) may be used as it is, and the characteristics of the CRT display device may be described in more detail. This further improves the accuracy of the equations described below.
[0065]
Taking a CRT display as an example, the maximum output luminance i in the system according to the average input luminance signal level G _max Is determined structurally. Therefore, the maximum luminance i in equation (2) _max Is a function i of the average input luminance signal level G _max When rewritten as (G), I becomes a function of g and G, and can be expressed as the following equation (3).
[0066]
[Equation 3]

[0067]
Here, the average input luminance signal level G is the level g of the luminance signal input corresponding to each pixel P (x, y) of the display device. _xy It can be expressed as the following equation.
[0068]
[Expression 4]

[0069]
The pixel P (x, y) is an x-th row (1 ≦ x ≦ i) y-column among a plurality of pixels arranged in a matrix of i rows (i ≧ 2) × j columns (j ≧ 2). Let us denote the pixel of the eye (1 ≦ y ≦ j).
[0070]
However, in a general CRT display device, the maximum output luminance i with respect to the average input luminance signal level G as shown in FIGS. _max When the change of the gamma value γ is examined, as shown in FIG. _max In addition, the gamma value γ also changes with the change in the average input luminance signal level G. Therefore, the maximum output brightness i _max Similarly, the gamma value γ needs to be rewritten as a function of the average input luminance signal level G.
[0071]
That is, based on the characteristics shown in FIG. 8, the expression (3) can be rewritten as the following expression (5).
[0072]
[Equation 5]

[0073]
According to Equation (5), the maximum output luminance i corresponding to the average input luminance signal level G _max If (G) and a gamma value γ (G) are set, the input luminance signal can be displayed with input / output characteristics similar to those of the CRT display device using a display device other than the CRT display device, and more reproducible. It can be seen that image reproduction can be performed with good input / output characteristics.
[0074]
Therefore, a function D representing a standardized input signal-output luminance characteristic of an arbitrary display device is expressed as a standardized input luminance signal level g. _input And normalized output luminance level g _output Is defined using
g _output = D (g _input (6)
It becomes the relationship.
[0075]
Here, the reason for standardizing the signal level and characteristics is that the accuracy of the input and output signals differs depending on the type of display device, and the scale of output luminance varies depending on the type of display device. This is because it is assumed. That is, by adjusting the display device, for example, when the input signal is 8 bits and the output signal is 10 bits, or when the input signal is 8 bits and the output signal is 6 bits, the output luminance scale is This is because there may occur a case where a setting value of 10 becomes 20 in another setting value.
[0076]
In actual signal processing, g _input , G _output And i _max If the value of is standardized so as to be a value in the range of 0 to 1, the conversion to the final signal form is performed by converting the operation result into the maximum number of representations of the signal (if it is an n-bit digital signal, Maximum number of expressions 2 ⁿ Since it is sufficient to multiply -1), the calculation can be easily performed.
[0077]
From the above, it has been found that in order to obtain an optimal display output, various image signals need to be corrected with respect to the input image signal.
[0078]
DETAILED DESCRIPTION OF THE INVENTION
[Embodiment 1]
An embodiment of the image reproduction method of the present invention will be described below with reference to FIG.
[0079]
As shown in FIG. 1, the image display device used in the image reproduction method of this embodiment includes a display device 8 having a plurality of pixels (not shown) for displaying an image and an image signal correction device 7. Image signal g containing pixel signal representing information for each pixel ₀ (Luminance signal level g ₀ ) Is input to the display device 8 via the image signal correction device 7.
[0080]
The image signal correction device 7 includes an average signal level calculation circuit 1 that calculates an average of all pixel signal levels as an average input luminance signal level G, and an input signal-output luminance that represents a change in luminance of the pixel with respect to the level of the pixel signal. Input signal-output luminance characteristic setting circuit 2 for setting the characteristic according to the average input luminance signal level G, and maximum output luminance adjustment for adjusting the maximum output luminance of the pixels of the display device 8 according to the average input luminance signal level G The circuit 3 and the image signal g so as to satisfy the set input signal-output luminance characteristics ₀ And a signal correction unit 4 for correcting.
[0081]
The signal correction unit 4 has an input / output characteristic equal to the set input signal-output luminance characteristic, and the image signal g ₀ Γ (G) correction circuit 5 for correcting the image signal, and an input / output characteristic opposite to the input / output characteristic (input signal-output luminance characteristic) of the display device 8 and the image signal g ₁ And a reverse characteristic correction circuit 6 for correcting the above.
[0082]
Next, an image reproduction method of the present invention using the image display device will be described.
[0083]
First, an input image signal g that has been subjected to inverse gamma correction in the average signal level calculation circuit 1. ₀ Luminance signal level g ₀ From this, the average input luminance signal level G is calculated.
[0084]
Next, in the input signal-output luminance characteristic setting circuit 2, an exponent value (gamma value) γ (G) when the input signal-output luminance characteristic of the image display device is approximated by an exponential function is expressed as an average input luminance signal level G. Calculate from. Specifically, an exponent value (gamma value) γ (G) when the input signal-output luminance characteristic of the image display device is approximated by an exponential function is averaged as shown by the curve γD in FIG. The luminance signal level G is set to increase as the level increases.
[0085]
Further, the input image signal so that the exponent value (gamma value) when the signal correction unit 4 approximates the input signal-output luminance characteristic of the image display device with an exponential function matches the set value γ (G). g ₀ And the corrected image signal g ₀ Is output to the display device 8. Specifically, first, the γ (G) correction circuit 5 uses an exponential function with the set value γ (G) as an exponential value, and the image signal g ₀ Luminance signal level g ₀ To image signal g ₁ Luminance signal level g ₁ To calculate the luminance signal level g ₁ Image signal g ₁ Is generated. Next, the inverse characteristic correction circuit 6 uses the inverse function of the function representing the input signal-output luminance characteristic of the display device 8 to obtain the image signal g. ₁ Luminance signal level g ₁ To image signal g _out Luminance signal level g _out To calculate the luminance signal level g _out Image signal g _out Is output to the display device 8.
[0086]
Further, the maximum output luminance adjustment circuit 3 adjusts the maximum output luminance of the pixels of the display device 8 according to the average input luminance signal level G. Specifically, the maximum output luminance i of the display device 8 is calculated from the average input luminance signal level G. _max (G) is calculated, and the calculation result is calculated as the maximum output luminance i. _out To the display device 8. Maximum output brightness i _max The calculation of (G) is performed by, for example, the curve i in FIG. _max As in D, the maximum output luminance i increases as the average input luminance signal level G increases. _max The calculation is such that (G) becomes small.
[0087]
In this case, the image signal g input to the display element of the display device 8. _out Luminance signal level g _out And the maximum output luminance i of the display device 8 _out Can be expressed by the following equations (7) and (8).
[0088]
[Formula 6]

[0089]
Further, the final output luminance I of the display device 8 can be expressed as the following equation (9).
[0090]
[Expression 7]

[0091]
G _out Is g in the above formula (6) _input Represents a standardized input luminance signal level corresponding to ^-1 () Is a standardized output luminance level g of the display device 8. _d (G in equation (6) above _output Function D (g) corresponding to _out ) Is the inverse function.
[0092]
As described above, in the image reproduction method of the present invention, the image signal g after the inverse gamma correction is performed. ₀ Gamma value γ (G) and maximum output luminance i with reference to the average input luminance signal level G at _max By setting (G), a high-quality image with good reproducibility can be displayed regardless of the type of the display device 8.
[0093]
In the above formulas (2) to (9), the processing for the luminance signal of the input image signal is shown. However, in an actual image display device, the luminance signal is used as the driving voltage v for driving the display element. ₁ And the maximum output luminance i _out The driving voltage v of the light emitting element of the display device 8 ₂ The process of converting to is generally included.
[0094]
Therefore, Expression (9) can be expressed as the following Expression (10).
[0095]
[Equation 8]

[0096]
Here, V in the equation indicates an average input voltage value of the luminance signal voltage.
[0097]
Also, the luminance signal level g _out To the drive voltage v of the display element of the display device 8 ₁ Conversion to maximum output brightness i _out To the driving voltage v of the light emitting element of the display device 8 ₂ Is converted to the linear function V ₁ And V ₂ Can be expressed as the following equations (11A) and (11B).
[0098]
v ₁ = V ₁ (G _out (11A)
v ₂ = V ₂ (I _out (11B)
Expressions (10), (11A), and (11B) are for the case where the operation mode of the display device 8 is driven by a change in voltage value. Even in the case of formation driven by a signal such as heat, light, pressure, vibration, sound wave, etc., the same expression as Expression (10), Expression (11A), and (11B) holds.
[0099]
FIG. 1 shows the configuration of the image display device when the display device 8 includes a display element and a light emitting element that can be controlled independently of each other. In this case, the image signal g obtained by the equation (7) _out (Luminance signal level g _out ) And the maximum output luminance i obtained by equation (8) _out May be input to the display element and the light emitting element, respectively.
[0100]
In the case where the display device 8 includes a switching element having a light emitting element function, that is, a so-called light emitting display element (self light emitting switching element), the maximum output luminance i _out This adjustment needs to be performed by signal processing of the image signal.
[0101]
Therefore, the image signal g before the reverse characteristic correction circuit 6 corrects the reverse characteristic of the input signal-output luminance characteristic of the display device 8. ₁ Maximum output brightness i _out Multiply by. That is, in this case, the image signal g output from the image signal correction device 7 _out Luminance signal level g _out Is represented by the following equation (12).
[0102]
[Equation 9]

[0103]
The final output luminance I can be expressed by the following equation (13). Note that the signal representation is the same as described above.
[0104]
[Expression 10]

[0105]
Here, an image signal g subjected to inverse gamma correction such as an image for television broadcasting is used. ₀ The outline of the processing of the image signal when is input has been described. Therefore, in this case, it is not necessary to perform pre-processing. However, for example, when a linearized image signal is input from an apparatus capable of generating an image signal such as a computer, inverse gamma correction is performed. The above process may be performed after the application.
[0106]
In addition, the above-described image signal processing can be similarly performed in an analog signal system and a digital signal system. However, in the digital signal system, it can be easily performed by calculating numerical values, and parameters can be easily changed. It is. When an analog signal is input in a digital signal system, the above-described processing may be performed after the analog signal is converted into a digital signal. In this process, the gradation expression bit number of the digital data can be processed with high precision if it is at least 8 bits or more. However, if the precision is not so necessary or the configuration is simplified, the expression bit number is set. Processing may be performed with a size smaller than 8 bits.
[0107]
In the above processing, the display device 8 may be such that a switching element and a light emitting element such as a transmissive liquid crystal display device are independent, or a light emitting element such as an FED (Field Emission Display) or PDP. Can be applied in the same manner even if they are integrated with the switching element, and the display quality can be easily set.
[0108]
In the above processing, the gamma value γ (G), which is the luminance parameter of the image display device, and the maximum output luminance i _max (G) can be set freely. Therefore, the gamma value γ (G) and the maximum output luminance i _max If various setting patterns of (G) are stored in the storage device and the stored contents are read as necessary, it is easy to unify the video display quality even if different display devices 8 are used. Become.
[0109]
In the signal processing by the image signal correction device 7 described above, the input image signal is converted into the luminance signal g. ₀ And a color difference signal, the output luminance signal g _out Signal level g _out Is the input luminance signal g ₀ Signal level g ₀ If it is α (0 ≦ α ≦ 1) times, the color difference signal may be multiplied by the same coefficient α and the obtained color difference signal may be output.
[0110]
Further, in the signal processing by the image signal correction device 7 described above, the output luminance obtained by the above-described calculation also when the input image signal is composed of each color component signal and the luminance is expressed by a combination of each color component signal. Signal g _out Signal level g _out Is the input luminance signal g ₀ Signal level g ₀ If it becomes β (0 ≦ β ≦ 1) times, the result obtained by multiplying each color component signal by β may be output as an output signal.
[0111]
In the above processing, the processing based on the linearized luminance signal of the input image signal has been described. However, the processing is independently performed for each of the R, G, and B primary color components signals or the multi-primary color component signals. It can also be set to perform processing. In that case, it is only necessary to change the luminance signal level in the equation to each color component signal level and perform the respective processing. When this method is used, it is possible to perform correction with higher accuracy than when correction is performed only with a luminance signal. On the other hand, the number of independent parameters increases, and a device such as a storage device or an arithmetic processing circuit that stores parameters. Cost increases. Therefore, this method may be applied when priority is given to improvement in accuracy over apparatus cost.
[0112]
[Embodiment 2]
Next, a preferred embodiment of the embodiment described in Embodiment 1 will be described with reference to FIGS. For convenience of explanation, members having the same functions as those shown in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.
[0113]
As shown in FIG. 2, the image display device used in the image reproduction method of the present embodiment includes an image signal correction device 7 ′ and a display device 8 in which a light emitting element and a display element (switching element) can be controlled independently. I have.
[0114]
The display device 8 includes a non-light emitting display element 16 such as a liquid crystal panel having a plurality of pixels (not shown) and an image signal g output from the image signal correction device 7. _out Display drive signal (drive signal) S _out A display element drive form conversion circuit 15 such as a liquid crystal drive circuit for conversion into a light source, a light emitting element 18 such as a backlight, and a maximum output luminance i output from the image signal correction device 7. _out Is a signal I in a form corresponding to the input of the light emitting element 18 _out Light-emitting element driving form conversion circuit 17 for converting to, for example, maximum output luminance i _out And a variable voltage source for generating a voltage corresponding to.
[0115]
The image signal correction device 7 ′ includes a switch 9 in addition to the average signal level calculation circuit 1, the input signal-output luminance characteristic setting circuit 2, the maximum output luminance adjustment circuit 3, and the signal correction unit 4 similar to those in the first embodiment. , An inverse gamma correction circuit 10 and a delay circuit 11 are provided.
[0116]
The switch 9 is an image signal g from the inverse gamma correction circuit 10. ₀ And an externally input gamma corrected image signal g ₀ Are selectively output to both the delay circuit 11 and the average signal level calculation circuit 1. Further, the inverse gamma correction circuit 10 receives the linearly corrected image signal g input from the reverse side. ₀ 'Is inverse gamma corrected and the corrected image signal g ₀ Is output to the switch 9.
[0117]
Further, the delay circuit 11 receives the image signal g ₀ Is output to the input signal-output luminance characteristic setting circuit 2 and the timing at which the gamma value γ (G), which is a setting parameter for the input signal-output luminance characteristic of the image display device, is output to the signal correction unit 4. In order to synchronize, the image signal g is equal to the time required for the calculation of the average input luminance signal level G in the average signal level calculation circuit 1 and the calculation of the gamma value γ (G) in the input signal-output luminance characteristic setting circuit 2. ₀ Is to delay.
[0118]
The maximum output brightness adjustment circuit 3 of the present embodiment is based on the average input brightness signal level G, and the standardized maximum output brightness setting value I _max And a maximum output luminance setting circuit 12 for calculating the standardized maximum output luminance I _max And the maximum output luminance i based on the luminance reference value α given from the outside _max And a luminance output control circuit 13 that outputs the calculation result to the light emitting element driving form conversion circuit 17 of the display device 8.
[0119]
In FIG. 2, the inverse gamma corrected image signal g ₀ And the linearly corrected input image signal g ₀ 'And both are shown, but these are not necessarily entered at the same time. Also, here, for convenience of explanation, the image signal subjected to inverse gamma correction and the input image signal g subjected to linear correction are used. ₀ The image signal obtained by further reverse-gamma-correcting ' ₀ However, it does not indicate that these are the same signal.
[0120]
In the present embodiment, the inverse gamma corrected image signal g ₀ And linearly corrected image signal (original image signal) g ₀ However, it is also possible to adopt a configuration corresponding to only one of the input image signals. For example, the switch 9 and the inverse gamma correction circuit 10 are omitted, and the inverse gamma corrected image signal g ₀ It is good also as a structure corresponding only to this input.
[0121]
Next, an image reproduction method using the image display device will be described with reference to FIGS. Here, the luminance signal level g ₀ , G ₁ , G _out , Average input luminance signal level G, maximum output luminance (illumination luminance adjustment level) I _max (G), i _out , And the luminance reference value (external luminance adjustment level) α are standardized so as to be all values from 0 to 1.
[0122]
Image signal g input to the input terminal (signal input port) of the display device 8 ₀ Is a linearly processed image signal g ₀ 'And an image signal g that has been subjected to inverse gamma correction for a CRT display device. ₀ It may be the case. Therefore, first, the linearly processed image signal g ₀ When 'is input, the inverse gamma correction circuit 10 performs inverse gamma correction. On the other hand, the image signal g that has been subjected to inverse gamma correction in advance. ₀ When is input, reverse gamma correction is not performed.
[0123]
Next, the average signal level calculation circuit 1 performs inverse gamma correction on the image signal g. ₀ The average AVE of pixel signal levels in an image signal for a certain signal amount (g _{0 xy} ) (= F ₁ (G ₀ )) Is calculated as the average input luminance signal level G. Image signal g for a certain amount of signal to be averaged ₀ Is an image signal g for one field image. ₀ Alternatively, it may be a voltage value of a pixel signal sampled at an appropriate interval. Furthermore, the input image signal g ₀ Is a luminance-color difference signal (YPbPr or YCbCr), the average signal level of the luminance signal Y may be calculated. The input image signal g ₀ Is the three primary color signals (RGB), the average signal level of the three primary color signals (RGB) may be calculated, and after the three primary color signals (RGB) are converted into the luminance signal Y, the average signal level of the luminance signal Y is calculated. You may calculate.
[0124]
Next, in the input signal-output luminance characteristic setting circuit 2, according to the average input luminance signal level G, the gamma value γ (G) (= f ₂ (G)) is set. Further, the maximum output brightness i corresponding to the average input brightness signal level G in the maximum output brightness adjustment circuit 3. _max (G) (= i _out ) Is set. Specifically, first, the maximum output luminance setting circuit 12 standardizes the set value I of the maximum output luminance based on the average input luminance signal level G. _max (G) is calculated. Next, the luminance output control circuit 13 normalizes the set value I of the maximum output luminance. _max Is multiplied by the luminance reference value α given from the outside, and the obtained value α · I _max The maximum output brightness i _max (G) is output to the light emitting element drive form conversion circuit 17.
[0125]
Thereafter, the γ (G) correction circuit 5 uses the exponential function represented by the following formula (14) with the set gamma value γ (G) as an exponential value, and the image signal g ₀ Luminance signal level g ₀ To image signal g ₁ Luminance signal level g ₁ Is calculated.
[0126]
[Expression 11]

[0127]
At this time, the input image signal g ₀ Before being supplied to the γ (G) correction circuit 5, the average input luminance signal level G in the average signal level calculation circuit 1 and the gamma value γ (G) in the input signal-output luminance characteristic setting circuit 2 are calculated. Is delayed by the time required for the average input luminance signal level G to be equal to the input image signal g at the same time. ₀ It is planned to reflect on.
[0128]
Next, an inverse function g of a function representing the input signal-output luminance characteristic of the display device 8 in the inverse characteristic correction circuit 6. _out = D ^-1 (G ₁ ), The image signal g ₁ Luminance signal level g ₁ To image signal g _out Luminance signal level g _out To calculate the luminance signal level g _out Image signal g _out Is output to the display element drive form conversion circuit 15.
[0129]
After that, the image signal g _out Is a signal S of the input form of the display element 16 in the display element drive form conversion circuit 15. _out Converted to. Specifically, for example, as shown in FIG. ₁ = V ₁ (G _out ) To drive voltage level v corresponding to display element 16 ₁ (= S _out ). At this time, the maximum luminance value g _out Is input to the display element drive form conversion circuit 15 as data, the display element drive form conversion circuit 15 outputs the maximum luminance value g. _out N bits of digital signals (0 to 2) ⁿ -1), the n-bit digital signal is converted to the drive voltage level v ₁ (= S _out ).
[0130]
On the other hand, the maximum luminance value i _out Is a signal I corresponding to the driving mode of the light emitting element 18 in the light emitting element driving mode conversion circuit 17. _out Is converted to Specifically, for example, as shown in FIG. ₂ = V ₂ (I _out ) To drive voltage level v corresponding to display element 16 ₂ (= I _out ). At this time, the maximum luminance value i _out Is input to the light emitting element driving form conversion circuit 17 as data, the light emitting element driving form conversion circuit 17 outputs the maximum luminance value i. _out N bits of digital signals (0 to 2) ⁿ -1), the n-bit digital signal is converted to the drive voltage level v ₂ (= I _out ).
[0131]
In the present embodiment, by performing such image signal processing, any luminance characteristic (average input luminance signal level−gamma) can be used as long as the display device 8 can independently control the display element 16 and the light emitting element 18. Even when the display device 8 having value characteristics and average input luminance signal level-maximum output luminance characteristics is used, an image display device having optimum luminance characteristics can be realized, and high-quality display can be performed. .
[0132]
[Embodiment 3]
The following will describe another embodiment of the present invention with reference to FIGS. For convenience of explanation, members having the same functions as those shown in the first or second embodiment are denoted by the same reference numerals, and description thereof is omitted.
[0133]
As shown in FIG. 4, the image display device used in the image reproduction method of the present embodiment includes an image signal correction device 27 and a display device 28 in which the display element (switching element) itself functions as a light emitting element.
[0134]
The display device 28 includes a light emitting display element 23 such as a CRT having a plurality of pixels (not shown), and an image signal g output from the image signal correction device 27. _out 'Display drive signal (drive signal) S _out And a light emitting display element driving form conversion circuit 22 for converting to '.
[0135]
In the image signal correction device 27, the signal conversion circuit 21 is inserted between the γ (G) correction circuit 5 and the inverse characteristic correction circuit 6 in the image signal correction device 7 of the second embodiment, and the luminance output control circuit 13 Except that the output is supplied to the signal conversion circuit 21, it is the same as the image signal correction apparatus 7 of the second embodiment.
[0136]
The signal conversion circuit 21 receives the image signal g output from the γ (G) correction circuit 5. ₁ Signal level g ₁ And maximum output brightness i ₁ (= I _max (G)) and the resulting image signal g ₂ Is output to the reverse characteristic correction circuit 6.
[0137]
Next, an image reproduction method using the image display device will be described with reference to FIGS. Here, the luminance signal level g ₀ , G ₁ , G ₂ , G _out ', Average input luminance signal level G, maximum output luminance (illumination luminance adjustment level) I _max (G), i ₁ , And the luminance reference value (external luminance adjustment level) α are standardized so as to be all values from 0 to 1.
[0138]
First, the input image signal g ₀ Appropriate maximum output brightness i ₁ (= I _max (G)) and the gamma value γ (G) are calculated, and the γ (G) correction circuit 5 calculates the input image signal g ₀ Image signal g ₁ Each process until the correction is performed is the same as in the second embodiment.
[0139]
However, in the maximum output luminance adjustment circuit 3, the maximum output luminance i set according to the average input luminance signal level G is set to 100% as the maximum output luminance level that is preset in the image display device and can be output by the display device 28. _max Change (G).
[0140]
Here, the maximum output luminance level that can be output by the display device 28 is defined as Y. ₁₀₀ And the maximum output luminance i for an input image with an average input luminance signal level G _max (G) and Y ₁₀₀ If the ratio to is α, the following equation holds.
[0141]
α = I _max (G) / Y ₁₀₀ ... (15)
Here, the following formula should be set.
[0142]
Y ₁₀₀ = I _max (G ₁₀₀ ) ... (16)
In the image reproduction method of the present embodiment, the image signal g output from the γ (G) correction circuit 5 by the signal conversion circuit 21 after each of these steps. ₁ Signal level g ₁ And maximum output brightness i ₁ (= I _max (G)). That is, the image signal g ₁ Maximum output brightness i ₁ And the image signal g ₂ Ask for.
[0143]
g ₂ = I ₁ ・ G ₁ ... (17)
Thus, the image signal (luminance signal) g ₁ Maximum output brightness i ₁ The signal conversion for multiplying is performed because the maximum output luminance of the display device 28 depends on the input signal level of the light emitting display element 23.
[0144]
Thereafter, the obtained image signal g ₂ Is output to the inverse characteristic correction circuit 6, and the inverse characteristic correction circuit 6 uses the inverse function g of the function representing the input signal-output luminance characteristic of the display device 28. _out '= D ^-1 (G ₂ ), The image signal g ₂ Luminance signal level g ₂ To image signal g _out Luminance signal level g _out 'Is calculated.
[0145]
Furthermore, the luminance signal level g _out Image signal with 'g _out 'Is output to the light emitting display element driving form conversion circuit 22, and the light emitting display element driving form conversion circuit 22 inputs the signal S of the input form corresponding to the display device 28. _out Converted to '. Specifically, for example, as shown in FIG. _Three = V _Three (G _out ') Is used to drive voltage level v corresponding to light emitting display element 23 _Three (= S _out Converted to '). At this time, the maximum luminance value g _out When 'is input as data to the light emitting display element driving form conversion circuit 22, the light emitting display element driving form conversion circuit 22 has a maximum luminance value g. _out 'Data is an n-bit digital signal (0 to 2 ⁿ -1), for example, after converting into an 8-bit digital signal (0-255), the n-bit digital signal is converted to a drive voltage level v _Three (= S _out ') To convert.
[0146]
In the present embodiment, a light emitting display element having any luminance characteristic (average input luminance signal level-gamma value characteristic and average input luminance signal level-maximum output luminance characteristic) is obtained by performing such image signal processing. Even if the display device 28 provided is used, an image display device having optimum luminance characteristics can be realized, and high-quality display can be performed.
[0147]
In each of the above embodiments, high quality is achieved by adjusting both the input signal-output luminance characteristics of the image display device and the maximum output luminance of the pixels of the display unit according to the average input luminance signal level. Although the display has been obtained, even if only one of the input signal-output luminance characteristics of the image display device and the maximum output luminance of the pixels of the display unit is adjusted according to the average input luminance signal level, a display with a certain level of quality is achieved. It is possible to obtain
[0148]
【Example】
[Example 1]
First, using a commercially available HDTV (high-definition television), which is a high-quality CRT display device, the average input luminance signal level (the average value of the input luminance signal over the entire screen), the input luminance signal data, and the maximum output luminance The relationship was measured.
[0149]
For the measurement, an image in which a box having a size of 150 × 150 pixels is provided at the center of a frame image having a size of 1920 × 1035 pixels, an input luminance signal level B of the box (relative value expressed as a maximum value of 100%), and The output luminance of the box was measured with a color luminance meter while changing the input luminance signal level H (relative value expressed with the maximum value being 100%) of the background portion (portion other than the box). Further, linearized data was used for the input luminance signal.
[0150]
As a result of the measurement, the output luminance of each box when the input luminance signal level H of the background is fixed and the input luminance signal level B of the box is changed is as shown in FIG. Further, when the input luminance signal level H in the background portion is fixed and the output luminance of the box is plotted against the average input luminance signal level G on the screen, it is as shown in FIG.
[0151]
In this case, the area of the box is 1.13% of the area of the entire screen, which is sufficiently small. For this reason, the input luminance signal level H in the background portion may be regarded as being equal to the average input luminance signal level G of the entire image.
[0152]
Therefore, in this example, in order to perform display (reproduction) similar to the display characteristics of the CRT display device D (or E) in the method of the first embodiment, the setting parameter, that is, the maximum output luminance i. _max The set values of (G) and gamma value γ (G) correspond to the average input luminance signal level G and curve i shown in FIG. _max D (or i _max E) and curve γD (or γE). Thereby, using a display device other than the CRT display device, for example, a liquid crystal display device, the same as the CRT display device D (or E), the average input luminance signal level-maximum output luminance characteristic (with respect to the average input luminance signal level G) Maximum output brightness i _max (Change in (G)) and average input luminance signal level-gamma value characteristics (change in gamma value γ (G) with respect to average input luminance signal level G) can be realized.
[0153]
In this image display device, from the results of FIGS. 8 to 10, when the average input luminance signal level G is low, the gamma value γ (G) is increased and the output luminance of the dark part is relatively increased. It is expected that the visibility of the dark part is improved. In addition, when the average input luminance signal level G is high, the gamma value γ (G) is decreased and the output luminance of the bright part is relatively reduced, so that the visibility of the bright part is expected to be improved. The Further, when the average input luminance signal level G is high, the maximum output luminance i _max By reducing (G) and suppressing the glare of the screen, it is expected that visibility is increased.
[0154]
Next, the input signal-output luminance characteristics of a general liquid crystal display device were measured by the same method as that for a CRT display device. However, since the liquid crystal display device used here is a liquid crystal display device for NTSC (National Television System Committee) display, considering the difference in the number of display pixels from the HDTV, the area of the box is the entire screen. It was set to be about 1% of the area. The obtained measurement results are normalized with reference to the maximum value and shown in FIG. Note that the input signal-output luminance characteristics shown in FIG. 11 are obtained by the inverse gamma correction of the input image signal and the correction of the voltage-optical conversion characteristics of the liquid crystal (linear characteristics) by a signal processing circuit in the liquid crystal display device. 2 shows the input signal-output luminance characteristics obtained as a result of the correction of the deviation from (1). The input signal-output luminance characteristic of this liquid crystal display device was substantially constant regardless of the average input luminance signal level G (= background input luminance signal level H).
[0155]
The characteristics shown in FIG. 11 are approximated by a sixth-order polynomial and processed when approximated by a non-linear exponential function as used for a CRT display device.
[0156]
These input signal-output luminance characteristics were applied to the liquid crystal display device through signal processing shown in FIG. 3 by a simulator. FIG. 12 shows the input signal-output luminance characteristics of the liquid crystal display device when the average input luminance signal level G is approximately 0%. FIG. 12 shows the input signal of the liquid crystal display device when the average input luminance signal level G is about 75%. The output luminance characteristics are shown in FIG.
[0157]
In this processing mode, the input signal-output luminance characteristic of the liquid crystal display device shown in FIG. 11 is processed so as to be converted into a linear characteristic through the signal processing shown by Expression (9). Therefore, the input signal-output luminance characteristic shown in FIG. 10 is substantially realized in the liquid crystal display device.
[0158]
As a result of evaluation with actual images, the above-mentioned expected effect was obtained. Further, if the data corresponding to FIG. 10 is measured by a different display device and the obtained measurement data is stored in a storage device or the like, the image shown in FIG. Can be reproduced.
[0159]
[Comparative Example 1]
FIG. 14 shows the display characteristics when the input image signal is directly input to the liquid crystal display device without applying the image reproducing method according to the present invention to the liquid crystal display device used in the first embodiment. If an image signal subjected to inverse gamma correction is input, if the display result (output luminance) on the liquid crystal display device shows a linear characteristic with respect to the input level (light intensity of the original image), the original image (the captured image) Image, etc.) is reproduced faithfully. In the display characteristics of FIG. 14, since the level is relatively increased in a portion where the luminance is 40% or higher, there is a high possibility of being recognized as a whitish image with a hazy appearance.
[0160]
Moreover, as a result of evaluation with an actual image, it was visually recognized as a whitish display as a whole as compared with an image having a linear input signal-output luminance characteristic. For this reason, the color tone of the image felt slightly faded, and the texture of the image felt fresh.
[0161]
【The invention's effect】
As described above, the image reproduction method of the present invention calculates the average of all pixel signal levels as an average signal level, and then calculates an input signal-output luminance characteristic representing a change in pixel luminance with respect to the pixel signal level. This is a method of setting an image according to the average signal level and then reproducing an image so as to satisfy the set input signal-output luminance characteristic.
[0162]
According to the above method, for example, even when a display device (liquid crystal display device or the like) in which the input signal-output luminance characteristic is constant regardless of the average signal level, the visibility of dark portions in a dark image as a whole is improved. It is possible to reproduce an image that is excellent and that is excellent in the visibility of a bright portion in an overall bright image. Therefore, the above method provides an image reproduction method capable of reproducing an image with high display quality regardless of whether or not the input signal-output luminance characteristic of the display device changes according to the average signal level. There is an effect that can be.
[0163]
Further, a preferred form of the image reproduction method is a method of reproducing an image such that the maximum output luminance of the pixel of the display device changes according to the average signal level.
[0164]
According to the above method, for example, even when a display device (such as a liquid crystal display device) whose maximum output luminance is constant regardless of the input average signal level is used, the screen is brightly displayed when an overall bright image is reproduced. It is possible to reduce the blindness and prevent temporary blinding due to the retinal bleaching phenomenon during direct viewing. Therefore, the above method has an effect that it is possible to provide an image reproduction method capable of reproducing an image with high display quality regardless of the type of the display device.
[0165]
In order to solve the above problems, the image display device of the present invention calculates the average of all pixel signal levels as an average signal level, and then outputs the maximum output of the pixels of the display device according to the average signal level. This is a method of reproducing an image so that the luminance changes.
[0166]
According to the above method, for example, even when a display device (such as a liquid crystal display device) whose maximum output luminance is constant regardless of the input average signal level is used, the screen is brightly displayed when an overall bright image is reproduced. It is possible to reduce the blindness and prevent temporary blinding due to the retinal bleaching phenomenon during direct viewing. Therefore, the above method has an effect that it is possible to provide an image reproduction method capable of reproducing an image with high display quality regardless of the average signal level-maximum output luminance characteristic of the display device.
[0167]
In each of the above image reproduction methods, it is desirable to reproduce an image so that an exponent value when an input signal-output luminance characteristic is approximated by an exponential function becomes a larger value as the average signal level increases. This reduces the glare of the screen when playing an overall bright image (an image with a high average brightness) and prevents temporary blinding due to retinal bleaching during direct viewing. Can do. Therefore, the above method has an effect that a bright image as a whole can be reproduced as an image with excellent visibility.
[0168]
In each of the above image reproduction methods, it is desirable to reproduce an image such that the maximum output luminance becomes smaller as the average signal level increases. This improves the visibility of dark areas in images that are generally dark (images with low average brightness), and prevents white spots and glare in images that are bright overall (images with high average brightness). Visibility can be improved. Therefore, the above-described method has an effect that both a dark image and a bright image can be reproduced as an image having excellent visibility.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a schematic configuration of an image display device used in an image reproduction method according to an embodiment of the present invention.
FIG. 2 is a block diagram illustrating a schematic configuration of an image display apparatus used in an image reproduction method according to a more preferable form of the image reproduction method, the display unit including a display element and a light emitting element that are independently controlled in a display unit; FIG.
3 is a block diagram schematically showing a flow of an image reproduction method in the image display apparatus shown in FIG.
FIG. 4 is a block diagram illustrating a schematic configuration of an image display device used in an image reproduction method according to another embodiment of the present invention, the display device including a light-emitting display element in a display unit;
5 is a block diagram schematically showing a flow of an image reproduction method in the image display apparatus shown in FIG.
FIG. 6 is an explanatory diagram schematically showing changes in input signal-output luminance characteristics together with input / output characteristics of a display device when an inverse gamma corrected image signal is output through various display devices.
FIG. 7 is an explanatory diagram schematically illustrating a change in input signal-output luminance characteristics together with input / output characteristics of a display device when a linearly processed image signal is output through various display devices.
FIG. 8 is a graph showing a change in the output luminance of the box with respect to the input luminance signal level of the box when the input luminance signal level of the background portion of the CRT display device is constant.
FIG. 9 is a graph showing a change in output luminance of a box with respect to an average input luminance signal level of a CRT display device.
FIG. 10 is a graph showing changes in normalized maximum output luminance and gamma value with respect to an average input luminance signal level in a CRT display device.
FIG. 11 is a graph showing a change in the output luminance of the box with respect to the input luminance signal level of the box when the input luminance signal level of the background portion of the liquid crystal display device is constant.
FIG. 12 is a graph showing input signal-output luminance characteristics of the liquid crystal display device when the average input luminance signal level is 0%.
FIG. 13 is a graph showing an input signal-output luminance characteristic of the liquid crystal display device when the average input luminance signal level is 75%.
FIG. 14 is a graph showing input signal-output luminance characteristics of a liquid crystal display device when the present invention is not applied.
[Explanation of symbols]
1 Average signal level calculation circuit
2 Input signal-output luminance characteristic setting circuit
3 Maximum output brightness adjustment circuit
4 signal correction part
5 γ (G) correction circuit
6 Reverse characteristic correction circuit
7 Image signal correction device
7 'Image signal correction device
8 display devices
9 switch
10 Inverse gamma correction circuit
11 Delay circuit
12 Maximum output brightness setting circuit
13 Luminance output control circuit
15 Display element drive form conversion circuit
16 Display element
17 Light Emitting Element Drive Form Conversion Circuit
18 Light emitting element
21 Signal conversion circuit
22 Light emitting display element drive form conversion circuit
23 Light-emitting display element
27 Image signal correction device
28 Display device

Claims (3)

In an image display method in which an image signal including a pixel signal representing information for each pixel is input to a display device having a plurality of pixels and an image is displayed on the display device.
Calculating the average of the level of the pixel signal subjected to inverse gamma correction over the entire screen as the average signal level;
The exponent value when the input signal-output luminance characteristic representing the change in luminance of the pixel with respect to the level of the pixel signal is approximated by an exponential function, and the exponent value when the average signal level rises continuously increases. Step to set so that
Correcting the image signal with input / output characteristics equal to the set input signal-output luminance characteristics by the first signal correction unit;
Correcting the image signal with an input / output characteristic opposite to the input signal-output luminance characteristic of the display device by the second signal correction unit;
Adjusting the maximum output luminance of the pixels of the display device without changing the image signal input to the display device in accordance with the average signal level. In an image display method for displaying an image on a display device having a plurality of pixels based on an image signal including a pixel signal representing information for each pixel,
Calculating the average of the level of the pixel signal subjected to inverse gamma correction over the entire screen as the average signal level;
The exponent value when the input signal-output luminance characteristic representing the change in luminance of the pixel with respect to the level of the pixel signal is approximated by an exponential function, and the exponent value when the average signal level rises continuously increases. Step to set so that
Correcting the image signal with input / output characteristics equal to the set input signal-output luminance characteristics by the first signal correction unit;
Calculating a maximum output luminance of a pixel of the display device according to the average signal level;
The step of adjusting the maximum output luminance of the pixel of the display device by converting the image signal corrected by the first signal correction unit based on the calculation result of the maximum output luminance, and the second signal correction unit by the display unit And a step of correcting the image signal with an input / output characteristic opposite to the input signal-output luminance characteristic. 3. The image display method according to claim 1 , wherein the image is displayed such that the maximum output luminance becomes smaller as the average signal level increases .

Images