JP3628151B2 - Color conversion method and apparatus, and recording medium - Google Patents

Description

【００４８】
即ち、式（６）は上記カラーモニタにおけるｒ’，ｇ’，ｂ’からＸ，Ｙ，Ｚへの色変換関係を表している。
【００４９】
また、式（６）に示す行列の逆行列を求めることによって、式（６）とは逆のＸ，Ｙ，Ｚからｒ’，ｇ’，ｂ’への色変換関係を求めると、式（７）のように表される。
【００５０】
【数２】

【００５１】
即ち、式（７）は、上記カラーモニタ用としてのＸ，Ｙ，Ｚからｒ’，ｇ’，ｂ’への色変換関係を表している。
【００５２】
従って、上記したステップＳ２０では、式（７）に示す行列演算によって、ＸＹＺからｒ’ｇ’ｂ’への色変換を行なう。
【００５３】
ところで、測色値であるＸＹＺとして、上記カラーモニタにおける赤，緑，青の各蛍光体によって再現し得る色域（上記カラーモニタ固有のガメット［ｇａｍｕｔ］）の中にある色を変換した場合には、変換によって得られるｒ’，ｇ’，ｂ’の値は、何れも０から１の範囲に納まることになる。しかし、ＸＹＺとして、上記した色域の外にある色を変換した場合には、変換によって得られるｒ’，ｇ’，ｂ’の値は、何れかが０から１の範囲外になってしまう。
【００５４】
次に、本実施例では、ステップＳ２０で得られたｒ’ｇ’ｂ’から上記カラーモニタに入力すべき赤，緑，青の表示用信号であるＲＧＢに変換する処理を行なう（ステップＳ２２）。
【００５５】
本実施例において、Ｒ，Ｇ，Ｂは、それぞれ、上記カラーモニタに入力すべき赤，緑，青の表示用信号の各信号強度（即ち、印加電圧）を、最小値が０，最大値が１となるように規格化（正規化）したものである。
【００５６】
ここで、本実施例におけるｒ’ｇ’ｂ’→ＲＧＢ変換処理について説明する前に、本実施例と従来技術とを比較するために、従来におけるｒ’ｇ’ｂ’→ＲＧＢ変換処理について説明する。
【００５７】
従来においては、上記カラーモニタにおけるγ特性に基づいて、式（８）に示す変換関数によりｒ’ｇ’ｂ’からＲＧＢへの変換を行なっていた。
【００５８】
【数３】

【００５９】
式（８）において、Ｌは、赤，緑，青の各蛍光体が発光した際の各輝度を表すものであり、上記したｒ’，ｇ’またはｂ’に対応する。また、Ｖは、上記カラーモニタに入力すべき赤，緑，青の表示用信号の各信号強度（即ち、印加電圧）を表すものであり、上記したＲ，ＧまたはＢに対応する。
【００６０】
従って、ｒ’ｇ’ｂ’→ＲＧＢ変換処理では、赤，緑，青毎に、それぞれ、ｒ’をＲに、ｇ’をＧに、ｂ’をＢに、それぞれ変換することになる。
【００６１】
従来におけるｒ’ｇ’ｂ’→ＲＧＢ変換処理では、前段のＸＹＺ→ｒ’ｇ’ｂ’変換処理においてｒ’，ｇ’，ｂ’の何れかの値として０より小さい値（即ち、Ｌ＜０）が得られた場合、式（８）から明らかなように、その値はすべてＲ，ＧまたはＢの値として０（即ち、Ｖ＝０）に変換されてしまう。
【００６２】
従って、前述したように、前段のＸＹＺ→ｒ’ｇ’ｂ’変換処理において、例えば、ＸＹＺして、上記カラーモニタにおける赤，緑，青の各蛍光体によって再現し得る色域の外にある色を変換し、その変換により得られるｒ’，ｇ’，ｂ’の値の何れかが０より小さい値になった場合（即ち、０から１の範囲外になった場合）は、その値はすべてＲ，Ｇ，Ｂの値として０に変換されて、一律に０で打ち切られることになる。このため、上記した色域の外にある色の部分については、ｒ’ｇ’ｂ’→ＲＧＢ変換処理を行なった際に、その色の部分の情報が失われてしまい、前述したように、その色の部分の色の調子がなくなったり、色相が変化したりするなどの不具合が生じてしまう。
【００６３】
図３は従来の色変換方法によっては救済できない色の領域を示す説明図である。図３では、３次元の色空間を赤−緑の平面に射影して示したものである。図３において、赤方向の軸と緑方向の軸と（即ち、２つの矢印）で挟まれた領域は実際に存在する色の領域を示しており、また、格子状に仕切られた矩形は、上記カラーモニタにおける赤，緑，青の各蛍光体によって再現し得る色域（即ち、ガメット）ｇａを示している。
【００６４】
従って、実際に存在する色であっても、図３において斜線で示した領域ＯＡ内の色のように、色域ｇａの外にある色については、前述したように、ｒ’ｇ’ｂ’→ＲＧＢ変換処理を行なった際に、その色の部分の情報が失われてしまうため、従来の色変換方法によっては、領域ＯＡ内の色を救済することができなかった。
【００６５】
一方、このような従来の色変換方法に対し、本実施例の色変換方法においては、ｒ’ｇ’ｂ’→ＲＧＢ変換処理として、図１に示すような処理を行なう。
【００６６】
即ち、本実施例においては、まず、ステップＳ２０で得られたｒ’，ｇ’またはｂ’の値をＬとおき（ステップＳ２４）、そのＬの値がｉ以上であるか否かを判定する（ステップＳ２６）。ここで、ｉは後述するようにｊのγ乗であり、０より大きい値である。
【００６７】
そして、判定の結果、Ｌの値がｉより小さい場合には、式（９）に従って、ｃの値を求める処理を行なう（ステップＳ２７）。
【００６８】
【数４】

【００６９】
但し、式（９）において、Ｍ，Ｎは、ステップＳ２０で得られたｒ’，ｇ’，ｂ’の値のうち、ステップＳ２４でＬとおかれた値以外の値である。即ち、例えば、ステップＳ２４において、ｒ’の値をＬとおいた場合は、Ｍ，Ｎはそれぞれｇ’，ｂ’の各値となる。また、式（９）において、ｆは式（１０）に示すようなＭまたはＮの関数である。
【００７０】
【数５】

【００７１】
但し、式（１０）において、ｄはＭまたはＮである。
【００７２】
従って、ｒ’の値をＬとおいた場合を例に採ると、ｇ’及びｂ’の値が０以上１以下の範囲に入っている場合は、式（１０）よりｆ（ｇ’）＝ｇ’，ｆ（ｂ’）＝ｂ’となるので、式（９）から明らかなように、ｃの値はｇ’とｂ’の平均値（ｇ’＋ｂ’）／２となる。また、ｇ’及びｂ’の値が何れも０より小さい場合は式（１０）よりｆ（ｇ’）＝０，ｆ（ｂ’）＝０となり、ｇ’及びｂ’の値が何れも１より大きい場合は式（１０）よりｆ（ｇ’）＝１，ｆ（ｂ’）＝１となるので、ｃの値は必ず０以上１以下の範囲に入る。
【００７３】
さて、以上のようにして、ステップＳ２７においてｃの値を求めたら、その後、そのｃの値に基づいて、式（１１）に示す変換関数によりＬからＶへの変換を行なう（ステップＳ２８）。
【００７４】
【数６】

【００７５】
但し、式（１１）において、ｉはｊのγ乗であり、ｉは０より大きい。また、ｊ，ｋは、それぞれ、予め設定された０より大きな所定の値である。
【００７６】
一方、ステップＳ２６における判定の結果、Ｌの値がｉ以上の場合には、式（１２）に示す変換関数によりＬからＶへの変換を行なう（ステップＳ３０）。
【００７７】
【数７】

【００７８】
図４及び図５はｒ’ｇ’ｂ’→ＲＧＢ変換処理において用いられるＬからＶへの変換特性を、従来例と本実施例とで比較して示したグラフである。これら図において、図５は図４の（Ｌ，Ｖ）＝（０，０．０３）近傍の部分を拡大して示した拡大図である。
【００７９】
図４及び図５において、実線で示された曲線Ｃ０は従来例における変換特性を表し、具体的には、式（８）に示す変換関数によって得られる変換特性を表している。一方、破線で示された曲線Ｃ１は本実施例における変換特性を表し、具体的には、式（１１），（１２）に示す変換関数によって得られる変換特性を表している。なお、図４及び図５においては、式（８），（１２）におけるγの値を「３」，式（１１）におけるｋの値を「０．１」，式（１１）におけるｊの値を「０．０３」としている。
【００８０】
本実施例における変換特性（曲線Ｃ１）は、図５において破線で示すように、Ｖの値が０以上ｊ未満の範囲内においては直線となっている。しかも、その直線の傾き（ｄＶ／ｄＬ）は、式（９）より明らかなように、ｊ／（ｉ＋ｃｋ）となり、また、Ｌ切片は−ｃｋとなるので、それぞれ、ステップＳ２７で得られたｃの値に基づいて変化する。また、このとき、前述したように、ｃの値は必ず０以上１以下の範囲に入り、ｋの値は０より大きいので、Ｌ切片である−ｃｋの値は−ｋ以上０以下の範囲に入っている。
【００８１】
さて、従来においては、図４において、実線で示された曲線Ｃ０が表すように、Ｌの値が０より小さい場合（Ｌ＜０）は、すべて、Ｖの値として０に変換されてしまうため、上記色域ｇａの外にある色については、ｒ’ｇ’ｂ’→ＲＧＢ変換処理を行なった際に、その色の部分の情報が失われていた。
【００８２】
これに対し、本実施例においては、図４及び図５において、破線で示された曲線Ｃ１が表すように、Ｌの値が−ｃｋ以上でｉ未満の場合（−ｃｋ≦Ｌ＜ｉ）は、Ｌの一次関数によって、Ｖの値として０以上ｊ未満の値に変換される。従って、−ｃｋ以上０未満の範囲（−ｃｋ≦Ｌ＜０）は上記範囲（−ｋｃ≦Ｌ＜ｉ）内であるので、上記色域ｇａの外にある色でも、Ｌの値として−ｃｋ以上０未満の範囲（−ｃｋ≦Ｌ＜０）に入る色の部分については情報が失われることがない。但し、ｃの値は、前述したように０以上１以下の範囲で変化するので、ｃの値が０に近いほど−ｃｋ以上０未満の範囲は狭くなり、ｃの値が１に近いほど−ｃｋ以上０未満の範囲は広くなる。
【００８３】
なお、Ｌの値が−ｃｋより小さい場合（Ｌ＜−ｃｋ）には、式（１１）からも明らかなように、そのＬの値は、従来の場合と同様に、Ｖの値として０に変換されることになる。
【００８４】
一方、Ｌの値がｉ以上で１未満の場合（ｉ≦Ｌ＜１）は、Ｖの値としてｊ以上１未満の値に変換される。このとき、Ｌの値は、式（１２）に示すように、従来における式（８）と同じＬの１／γ乗の関数、即ち、上記カラーモニタのγ特性に応じた関数によって、Ｖの値に変換されるため、上記範囲では曲線Ｃ１は従来の曲線Ｃ０と重なることになる。
【００８５】
なお、Ｌの値が１以上の場合（１≦Ｌ）には、式（１２）からも明らかなように、そのＬの値は、従来の場合と同様に、Ｖの値として１に変換されることになる。
【００８６】
さて、以上のようにして、ステップＳ２８またはＳ３０において、Ｖの値が得られたら、次に、本実施例においては、Ｖの値を、ステップＳ２４のｒ’，ｇ’，ｂ’に対応させて、Ｒ，ＧまたはＢとおく（ステップＳ３２）。
【００８７】
従って、以上のｒ’ｇ’ｂ’→ＲＧＢ変換処理（ステップＳ２２）によって、赤，緑，青毎に、それぞれ、ｒ’はＲに、ｇ’はＧに、ｂ’はＢに、それぞれ変換される。
【００８８】
図６は本実施例の色変換方法におけるｒ’ｇ’ｂ’→ＲＧＢ変換処理によって救済される色の領域と、変換前と変換後の各色の位置を示す説明図である。図６は、図３と同様に、３次元の色空間を赤−緑の平面に射影して示したものである。図６において、赤方向の軸と緑方向の軸と（即ち、２つの矢印）で挟まれた領域は実際に存在する色の領域を示しており、また、格子状に仕切られた実線の矩形は、上記カラーモニタにおける赤，緑，青の各蛍光体によって再現し得る色域（即ち、ガメット）ｇａを示している。
【００８９】
この色域ｇａの中にある色のうち、図６において、格子状に仕切られた一点鎖線の矩形で表した変換領域Ａ１の中にある色については、上記したｒ’ｇ’ｂ’→ＲＧＢ変換処理のうち、ステップＳ３０の処理によって、それぞれ、同じ変換領域Ａ１に変換される。
【００９０】
一方、上記色域ｇａの外にある色であっても、図６において、散点にて示した領域ＰＡ内の色については、上記したｒ’ｇ’ｂ’→ＲＧＢ変換処理のうち、ステップＳ２８の処理によって、それぞれ、上記色域ｇａ内の斜線で示した変換領域Ａ２に変換される。即ち、領域ＰＡは本実施例のｒ’ｇ’ｂ’→ＲＧＢ変換処理によって救済される色の領域であり、変換領域Ａ２がそれら救済領域ＰＡ内にある色の変換先の領域である。
【００９１】
なお、上記色域ｇａの中にある色のうち、変換領域Ａ２の中にある色については、救済領域ＰＡの中の色と一緒に、ステップＳ２８の処理によって、それぞれ、変換領域Ａ２に変換される。
【００９２】
ここで、救済領域ＰＡは、Ｌの値として見た場合に前述した−ｃｋ以上０未満の範囲に対応し、従って、その幅ｗ０はｃｋとなる。ｃの値は、式（９），（１０）に従って変化するため、救済領域ＰＡの幅ｗ０は次のように変化する。即ち、例えば、ｒ’の値をＬとおいた場合を例に採ると、ｆ（ｇ’）＝０，ｆ（ｂ’）＝０である場合、ｃ＝０となるので、幅ｗ０は０となり、最小となる。ｆ（ｇ’）＝１，ｆ（ｂ’）＝１である場合は、ｃ＝１となるので、幅ｗ０はｋとなり、最大となる。ｆ（ｇ’）＝ｇ’，ｆ（ｂ’）＝ｂ’である場合は、ｃ＝（ｇ’＋ｂ’）／２となるので、幅ｗ０は（ｇ’＋ｂ’）ｋ／２となる。このとき、幅ｗ０はｇ’，ｂ’の各値に比例するため、図６に示すように、赤についての救済領域の幅ｗ０は、緑及び青（ｇ’，ｂ’）の各値（図６では青の値は紙面に垂直な方向）が大きくなるのに従って、リニアに広くなっていく。
【００９３】
また、救済領域ＰＡの幅ｗ０の最大幅をｗ１とすると、その値はＬの値としてみた場合に、前述したようにｋに対応する。これに対し、変換領域Ａ２の幅ｗ２はＶの値として見た場合に前述したｊに対応する。また、色域ｇａの一辺の幅はＬまたはＶの値として見た場合に「１」に対応する。本実施例においては、前述したように、ｋの値を「０．１」、ｊの値を「０．０３」に設定しているので、変換領域Ａ２の幅ｗ２は救済領域ＰＡの最大幅ｗ１より狭くなっている。
【００９４】
従って、救済領域ＰＡのうち、幅ｗ０が変換領域Ａ２の幅ｗ２よりも広い領域については、その領域内にある色は、ｒ’ｇ’ｂ’→ＲＧＢ変換処理によって、より幅の狭い変換領域Ａ２に、圧縮して押し込まれることになる。これに対し、救済領域ＰＡのうち、幅ｗ０が変換領域Ａ２の幅ｗ２よりも狭い領域については、その領域内にある色は、より幅の広い変換領域Ａ２に、拡張して押し込まれることになる。よって、前者の色については、変換先の変換領域Ａ２において、色度点の分散状態が密になるのに対し、後者の色については粗にすることができる。
【００９５】
また、元々色域ｇａの中にあった色のうち、変換領域Ａ１の中にあった色は、ｒ’ｇ’ｂ’→ＲＧＢ変換処理によって、同じ変換領域Ａ１に変換されることになるので、それらの色については、変換前後における色の位置（色度点）のずれがほとんどない。
【００９６】
また、図６において、赤方向の軸と緑方向の軸と（即ち、２つの矢印）で挟まれた領域（即ち、狭角の領域）以外の領域（即ち、広角の領域）は、実際には存在しない色の領域であるが、本実施例においては、この領域内に上記救済領域ＰＡが少しも含まれていないので、上記したｒ’ｇ’ｂ’→ＲＧＢ変換処理による変換先の領域（即ち、変換領域Ａ２）には、確実に色が存在することになる。
【００９７】
さて、以上説明したように、本実施例によれば、測色値であるＸＹＺとして、上記カラーモニタにおける赤，緑，青の各蛍光体によって再現し得る色域（即ち、ガメット）ｇａの外にある色を変換した場合に、変換によって得られるｒ’，ｂ’，ｇ’の何れかの値が、０より小さい値になったとしても（Ｌ＜０）、その値が−ｃｋ以上であれば（Ｌ≧−ｃｋ）、Ｒ，Ｇ，Ｂの値として０以上ｊ未満の値に変換される（０≦Ｖ＜ｊ）。従って、上記色域ｇａの外にある色でも、ｒ’，ｂ’，ｇ’値として−ｃｋ以上０未満の範囲（−ｃｋ≦Ｌ＜０）に入る色の部分については情報が失われることがないため、Ｒ，Ｇ，Ｂの値として保存される。従って、上記した色の部分において、色の調子が無くなったり、色相が変化したりすることがない。
【００９８】
ところで、上記した実施例においては、カラー印刷用にスキャンされたＲＧＢ色信号をカラーモニタ用のＲＧＢ色信号に変換してカラーモニタに与える場合を想定していたため、測色値であるＸＹＺから表示用信号であるＲＧＢへの色変換を対象としていた。
【００９９】
しかし、カラーモニタ用のＲＧＢ色信号をカラー印刷用のＣＭＹＫ色信号に変換して印刷機等に与えたり、あるいは、カラーモニタ用のＲＧＢ色信号をＸＹＺ等の測色値に変換して記録媒体等に記録したりする場合には、上記とは逆の色変換、即ち、表示用信号であるＲＧＢから測色値であるＸＹＺへの色変換を対象としても良い。
【０１００】
即ち、この場合は、図１のステップＳ２２で行なったｒ’ｇ’ｂ’→ＲＧＢ変換処理とは逆の変換処理であるＲＧＢ→ｒ’ｇ’ｂ’変換処理を行なった後、図１のステップＳ２０で行なったＸＹＺ→ｒ’ｇ’ｂ’変換処理とは逆の変換処理であるｒ’ｇ’ｂ’→ＸＹＺ変換処理を行なうようにすれば良い。
【０１０１】
図７は本発明の他の実施例としての色変換方法の処理手順を示すフローチャートである。
【０１０２】
即ち、本実施例の色変換方法においては、図７に示すように、まず、赤，緑，青の表示用信号であるＲＧＢを、赤，緑，青について輝度リニアな値となるｒ’ｇ’ｂ’に色変換する処理を行なう（ステップＳ４０）。なお、前述した実施例におけるｒ’ｇ’ｂ’→ＲＧＢ変換処理では、ＬからＶへの変換を行なっていたが、本実施例におけるステップＳ４０のＲＧＢ→ｒ’ｇ’ｂ’変換処理においては、上記とは逆のＶからＬへの変換を行なうことになる。
【０１０３】
具体的には、まず、Ｒ，ＧまたはＢの値をＶとおき（ステップＳ４４）、そのＶの値がｊ以上であるか否かを判定する（ステップＳ４６）。但し、Ｒ，Ｇ，Ｂの各値は、それぞれ、０以上１以下の範囲には入っている。
【０１０４】
そして、判定の結果、Ｖの値がｊより小さい場合には、式（１３）に従って、Ｍ，Ｎの値を求める処理を行なう（ステップＳ５０）。
【０１０５】
【数８】

【０１０６】
但し、式（１３）において、Ｔ，Ｕは、Ｒ，Ｇ，Ｂの値のうち、ステップＳ４４でＶとおかれた値以外の値である。即ち、例えば、ステップＳ４４において、Ｒの値をＶとおいた場合は、Ｔ，ＵはそれぞれＧ，Ｂの各値となる。
【０１０７】
式（１３）に示す変換関数は、従来の変換関数である式（８）の変換関数とは逆の変換関数となっており、前述したように、例えば、Ｔ，ＵをそれぞれＧ，Ｂの各値とした場合には、式（１３）によって、Ｍ，Ｎとして暫定的なｇ’，ｂ’の値を得ることができる。
【０１０８】
次に、ステップＳ５０で得られたＭ，Ｎの各値に基づいて、前述の式（９）により、ｃの値を求める処理を行ない（ステップＳ５２）、その後、そのｃの値に基づいて、式（１４）に示す変換関数によりＶからＬへの変換を行なう（ステップＳ５４）。
【０１０９】
【数９】

【０１１０】
一方、ステップＳ４６における判定の結果、Ｖの値がｊ以上の場合には、式（１５）に示す変換関数によりＶからＬへの変換を行なう（ステップＳ４８）。
【０１１１】
【数１０】

【０１１２】
即ち、前述の実施例におけるｒ’ｇ’ｂ’→ＲＧＢ変換処理では、ＬからＶへの変換を、図４に示すような変換特性に基づいて行なっていたが、この変換特性では、図４から明らかなように、ｃの値さえ決まれば、曲線Ｃ１が一つ確定するため、Ｌの値に対するＶの値が一意的に決定される。従って、逆の変換においても、ｃの値さえ決まれば、Ｖの値に対するＬの値が一意的に決定されるので、ＲＧＢ→ｒ’ｇ’ｂ’変換処理におけるＶからＬへの変換も、図４に示す変換関係を用いて容易に行なうことができる。
【０１１３】
さて、以上のようにして、ステップＳ４８またはＳ５４において、Ｌの値が得られたら、次に、本実施例においては、Ｌの値を、ステップＳ４４のＲ，Ｇ，Ｂに対応させてｒ’，ｇ’またはｂ’とおく（ステップＳ５６）。
【０１１４】
従って、以上のＲＧＢ→ｒ’ｇ’ｂ’変換処理（ステップＳ４０）によって、赤，緑，青毎に、それぞれ、Ｒはｒ’に、Ｇはｇ’に、Ｂはｂ’に、それぞれ変換される。
【０１１５】
次に、ステップＳ４０で得られたｒ’ｇ’ｂ’を、測色値であるＸＹＺに色変換する処理を行なう（ステップＳ４２）。
【０１１６】
即ち、前述の実施例におけるＸＹＺ→ｒ’ｇ’ｂ’変換処理は、式（７）に示す行列演算によって行なっていたが、本実施例におけるステップＳ２２のｒ’ｇ’ｂ’→ＸＹＺ変換処理は、式（７）とは逆の式（６）に示す行列演算によって行なうことができる。
【０１１７】
以上のようにして、本実施例においては、表示用信号であるＲＧＢから測色値であるＸＹＺへの色変換を行なうことができる。
【０１１８】
ところで、前述の図１に示した実施例においては色変換方法について説明したが、その色変換方法は図８に示すような色変換装置によって実現することができる。
【０１１９】
図８は図１に示す色変換方法を実現することが可能な色変換装置を示すブロック図である。図８に示す色変換装置は、主として、ＣＰＵ２０、メモリ３０、Ｉ／Ｏインタフェース４０、ディスク・インタフェース５０を備えており、これらは相互にバス６０によって接続されている。
【０１２０】
また、Ｉ／Ｏインタフェース４０には、マウス７０，キーボード８０，カラーモニタ９０，ネットワークカード１３０が接続されており、マウス７０，キーボード８０から入力された指示や命令をＣＰＵ２０に伝えたり、表示用信号をカラーモニタ９０に与えたり、ネットワークカード１３０との間で通信データのやり取りを行なったりする。なお、ネットワークカード１３０はネットワーク回線１４０に接続されており、このネットワーク回線１４０を介して、例えば、サーバ１５０などと通信を行なう。
【０１２１】
また、ディスク・インタフェース５０には、ハードディスクドライブ１００，ＣＤ−ＲＯＭドライブ１１０，フロッピディスクドライブ１２０が接続されており、ハードディスクドライブ１００内のハードディスクに対しデータ等の読み書きを行なったり、ＣＤ−ＲＯＭドライブ１１０に挿入されたＣＤ−ＲＯＭよりプログラムやデータ等を読み出したり、フロッピディスクドライブ１２０に挿入されたフロッピディスク１２５に対しデータ等の読み書きを行なったりする。
【０１２２】
また、ＣＰＵ２０は、メモリ３０に蓄えられたコンピュータプログラムに従って動作し、ＸＹＺ→ｒ’ｇ’ｂ’変換処理部２２やｒ’ｇ’ｂ’→ＲＧＢ変換処理部２４として機能する。ここで、ＸＹＺ→ｒ’ｇ’ｂ’変換処理部２２は、主に、図１に示したＸＹＺ→ｒ’ｇ’ｂ’変換処理（ステップＳ２０）を実行し、また、ｒ’ｇ’ｂ’→ＲＧＢ変換処理部２４は、主に、図１に示したｒ’ｇ’ｂ’→ＲＧＢ変換処理（ステップＳ２２）を実行する。なお、色変換すべき測色値であるＸＹＺのデータは予めハードディスクドライブ１００内のハードディスクに記憶されており、このハードディスクから供給されるか、或いは、サーバ１５０に格納されており、このサーバ１５０からネットワーク回線１４０，ネットワークカード１３０を介して供給される。また、処理中に得られたデータはメモリ３０や上記ハードディスクに一時的に格納される。色変換によって得られた表示用信号であるＲＧＢのデータは、上記ハードディスクに記憶されるか或いは上記サーバに格納される。また、カラーモニタ９０が対象としているカラーモニタと同種のカラーモニタである場合には、得られた表示用信号であるＲＧＢをカラーモニタ９０に与えて、モニタリングしても良い。
【０１２３】
また、ＣＰＵ２０にＸＹＺ→ｒ’ｇ’ｂ’変換処理部２２やｒ’ｇ’ｂ’→ＲＧＢ変換処理部２４として機能させるコンピュータプログラムは、最終的には、前述したようにメモリ３０に蓄えられるが、元は、ＣＤ−ＲＯＭ１１５やフロッピディスク１２５に記録されている。即ち、上記したコンピュータプログラムはＣＤ−ＲＯＭ１１５やフロッピディスク１２５からＣＤ−ＲＯＭドライブ１１０やフロッピディスクドライブ１２０によって読み取られ、例えば、一旦ハードディスクドライブ１００内のハードディスクに書き込まれた後、メモリ３０に転送される。
【０１２４】
なお、上記したコンピュータプログラムを記録するための記録媒体としては、ＣＤ−ＲＯＭ１１５やフロッピディスク１２５やハードディスクの他、光磁気ディスク，磁気テープ，ＩＣカード，ＲＯＭカートリッジ，パンチカード、或いは、バーコードなどの符号が印刷された印刷物など、コンピュータによって読み取り可能な種々の記録媒体を用いることができる。
【０１２５】
また、上記したコンピュータプログラムとして、前述したようにＣＤ−ＲＯＭ１１５やフロッピディスク１２５に記録されていたものをメモリ３０に転送することもできるが、サーバ１５０に格納されていたものをネットワーク回線１４０，ネットワークカード１３０を介してメモり３０に転送するようにしても良い。この場合、サーバ１５０がコンピュータプログラム提供装置として機能する。なお、本実施例では、サーバへの接続手段として、ネットワーク回線とネットワークカードとの組合せを用いたが、その他、公衆回線とモデムやターミナルアダプタとの組合せを用いるようにしても良い。
【０１２６】
さて、以上の説明においては、図１に示す色変換方法によって測色値であるＸＹＺから表示用信号であるＲＧＢへの色変換を行なう場合と、図１の色変換とは逆の色変換方法によって表示用信号であるＲＧＢから測色値であるＸＹＺへの色変換を行なう場合と、についてそれぞれ説明した。そこで次に、図１の色変換方法によって得られた表示用信号であるＲＧＢ（即ち、ＲＧＢ色信号）から印刷用信号であるＣＭＹＫ（即ち、ＣＭＹＫ色信号）への色変換を行なって印刷機、製版用画像記録装置等に与える場合を考えてみる。なお、前述したように、図１の色変換方法は図８の色変換装置によって実現することができるので、図１の色変換方法によって得られた表示用信号であるＲＧＢは、図８の色変換装置によって得られた表示用信号であるＲＧＢと言い換えることができる。
【０１２７】
図８の色変換装置によって得られた表示用信号であるＲＧＢから印刷用信号であるＣＭＹＫへの色変換は、図９に示すよう色変換装置によって行なわれる。
【０１２８】
図９は本発明の別の実施例として色変換装置を示すブロック図である。図９に示すように、本実施例の色変換装置１６０は、色変換用ルックアップテーブル１７０と色変換用補間回路１８０とを備えている。
【０１２９】
図９に示す色変換装置１６０に、図８の色変換装置によって得られた表示用信号であるＲＧＢが入力されると、色変換用補間回路１８０は、入力されたＲＧＢの値の組合せが色変換用ルックアップテーブル１７０におけるアドレスの値として存在する場合には、そのＲＧＢの値の組合せを色変換用ルックアップテーブル１７０に入力する。色変換用ルックアップテーブル１７０では、色変換用補間回路１８０から入力されたＲＧＢの値の組合せの示すアドレス内に格納されているＣＭＹＫの値を読み出して、色変換用補間回路１８０に出力する。色変換用補間回路１８０は読み出されたＣＭＹＫをそのまま出力する。
【０１３０】
一方、色変換装置１６０に入力されたＲＧＢの値の組合せが色変換用ルックアップテーブル１７０におけるアドレスの値として存在しない場合には、色変換用ルックアップテーブル１７０におけるアドレスの値として存在するＲＧＢの値の組合せのうち、入力された上記ＲＧＢの値の組合せに近いものを複数選択して、色変換用ルックアップテーブル１７０に入力する。色変換用ルックアップテーブル１７０では、色変換用補間回路１８０から入力されたＲＧＢの値の各組合せの示す複数のアドレスから、各々の中に格納されているＣＭＹＫの値をそれぞれ読み出して、色変換用補間回路１８０に出力する。色変換用補間回路１８０は、読み出された複数のＣＭＹＫの値を用いて補間演算を行なって、色変換装置１６０に入力されたＲＧＢに対応するＣＭＹＫを出力する。
【０１３１】
図１０は図９の色変換用ルックアップテーブルにおけるアドレスの値として存在するＲＧＢの値の組合せとそれらアドレス内に格納されているＣＭＹＫの値の一例を示す説明図である。従って、例えば、色変換用ルックアップテーブル１７０に、色変換用補間回路１８０からＲＧＢの値の組合せとして（Ｒ，Ｇ，Ｂ）＝（０，０，８）が入力されたとすると、その組合せの示すアドレス内に格納されているＣＭＹＫの値は、図１０に示すように、（Ｃ，Ｍ，Ｙ，Ｋ）＝（Ｃ_２，Ｍ_２，Ｙ_２，Ｋ_２）であるので、この値が色変換用補間回路１８０に出力される。なお、図１０に示すように、色変換用ルックアップテーブル１７０の全てのアドレス内には必ずＣＭＹＫの値が格納されている。
【０１３２】
さて、本実施例においては、図８に示す色変換装置のｒ’ｇ’ｂ’→ＲＧＢ変換処理部２４において行なわれているｒ’ｇ’ｂ’→ＲＧＢ変換処理が、例えば、図４及び図５に示す変換特性に基づくものである場合、Ｒ，Ｇ，Ｂの何れかの値としてｊを含むＲＧＢの値の組合せが、少なくとも、色変換用ルックアップテーブル１７０におけるアドレスの値として存在するように、色変換用ルックアップテーブル１７０を構成するようにする。
【０１３３】
例えば、今、ｊ＝８と設定すると、Ｒ，Ｇ，Ｂの何れかの値として８を含むＲＧＢの値の組合せが、色変換用ルックアップテーブル１７０におけるアドレスの値として存在するように、色変換用ルックアップテーブル１７０を構成する。図１０に示す例では、アドレスの値として、（Ｒ，Ｇ，Ｂ）＝（０，０，８）、（Ｒ，Ｇ，Ｂ）＝（０，８，０）、（Ｒ，Ｇ，Ｂ）＝（８，０，０）、（Ｒ，Ｇ，Ｂ）＝（０，８，８）、（Ｒ，Ｇ，Ｂ）＝（８，０，８）、（Ｒ，Ｇ，Ｂ）＝（８，８，０）、（Ｒ，Ｇ，Ｂ）＝（８，８，８）、…など、Ｒ，Ｇ，Ｂの何れかの値として８を含むＲＧＢの値の組合せが存在しているので、まさに、図１０に示す例は上記したルックアップテーブルに該当する。
【０１３４】
従って、前述したように、色変換用ルックアップテーブル１７０の全てのアドレス内には必ずＣＭＹＫの値が格納されているため、色変換用ルックアップテーブル１７０を上記したように構成することによって、Ｒ，Ｇ，Ｂの何れかの値としてｊを含むＲＧＢの値の組合せのうち、アドレスの値として存在するものについては、必ず、それに対応するＣＭＹＫの値が色変換用ルックアップテーブル１７０内に実存することになる。
【０１３５】
ｒ’ｇ’ｂ’からＲＧＢへの変換では、図４及び図５に示す変換特性から明らかなように、Ｒ，ＧまたはＢ＝ｊ（即ち、Ｖ＝ｊ）の前後において、変換の傾向が全く異なっている。従って、ＲＧＢからＣＭＹＫへ変換する場合も、Ｒ，ＧまたはＢ＝ｊの前後において、変換の傾向が異なることになる。このため、ｊを含むＲＧＢの値の組合せに近い組合せについて、それぞれ、対応するＣＭＹＫの値（即ち、変換によって得られるＣＭＹＫの値）を求めると、それら値同士はかけ離れたものとなっており、連続性がない。
【０１３６】
一方、ｊを含むＲＧＢの値の組合せが、全く、色変換用ルックアップテーブル１７０のアドレスの値として存在せず、このＲＧＢの値の組合せに対応するＣＭＹＫの値が色変換用ルックアップテーブル１７０に実存しないものと仮定すると、ｊを含むＲＧＢの値の組合せが色変換装置１６０に入力された場合は、このＲＧＢの値の組合せに近い組合せについて、それぞれ、対応するＣＭＹＫの値が複数、色変換用ルックアップテーブル１７０から読み出されて、それらＣＭＹＫの値を用いて補間演算が行なわれることになる。しかし、前述したように、ｊを含むＲＧＢの値の組合せに近い組合せについては、それぞれ、対応するＣＭＹＫの値同士はかけ離れたものとなっており、連続性がないため、それらＣＭＹＫの値を用いて補間演算が行なわれたとしても、その補間演算によっては、ｊを含むＲＧＢの値の組合せについての正しいＣＭＹＫの値を得ることができない。
【０１３７】
これに対し、本実施例のように、ｊを含むＲＧＢの値の組合せが少なくとも色変換用ルックアップテーブル１７０のアドレスの値として存在し、このＲＧＢの値の組合せに対応するＣＭＹＫの値が色変換用ルックアップテーブル１７０に実存するように構成した場合には、ｊを含むＲＧＢの値の組合せが色変換装置１６０に入力されると、補間演算が行なわれることなく、そのＲＧＢの値の組合せに対応するＣＭＹＫの値が色変換用ルックアップテーブル１７０から読み出されて出力されるため、ｊを含むＲＧＢの値の組合せについての正しいＣＭＹＫの値を得ることができる。
【０１３８】
なお、本発明は上記した実施例や実施形態に限られるものではなく、その要旨を逸脱しない範囲において種々の態様にて実施することが可能である。
【０１３９】
即ち、上記した実施例においては、測色値としてＸＹＺを用いるようにしたが、ＸＹＺの代わりに、測色値としてＬ^＊ａ^＊ｂ^＊を用いるようにしても良い。
【０１４０】
また、上記した実施例において、ｒ’ｇ’ｂ’→ＲＧＢ変換処理におけるＬからＶへの変換特性は、図５において破線で示したように、Ｖの値が０以上ｊ未満の範囲内において、直線（即ち、Ｌの一次関数）となっている。しかし、本発明はこれに限定されるものではなく、上記範囲においては、Ｌの一次関数以外の関数であっても、単調に増加する関数であれば、ＬからＶへの変換に用いることができる。
【０１４１】
また、上記した実施例においては、表示用信号であるＲＧＢを与える対象としてカラーモニタを用いていたが、本発明は、カラーモニタに限定されるものではなく、カラー液晶ディスプレイやカラープラズマディスプレイなどの他のカラー表示デバイスを用いるようにしても良い。
【図面の簡単な説明】
【図１】本発明の一実施例としての色変換方法の処理手順を示すフローチャートである。
【図２】ＩＴＵ−Ｒ ７０９の赤，緑，青の各蛍光体の表す色の色度座標及びＤ５０の白色の色度座標を示す説明図である。
【図３】従来の色変換方法によっては救済できない色の領域を示す説明図である。
【図４】ｒ’ｇ’ｂ’→ＲＧＢ変換処理において用いられるＬからＶへの変換特性を、従来例と本発明の一実施例とで比較して示したグラフである。
【図５】図４の変換特性の要部を拡大して示したグラフである。
【図６】本発明の一実施例の色変換方法におけるｒ’ｇ’ｂ’→ＲＧＢ変換処理によって救済される色の領域と、変換前と変換後の各色の位置を示す説明図である。
【図７】本発明の他の実施例としての色変換方法の処理手順を示すフローチャートである。
【図８】図１に示す色変換方法を実現することが可能な色変換装置を示すブロック図である。
【図９】本発明の別の実施例として色変換装置を示すブロック図である。
【図１０】図９の色変換用ルックアップテーブルにおけるアドレスの値として存在するＲＧＢの値の組合せとそれらアドレス内に格納されているＣＭＹＫの値の一例を示す説明図である。
【符号の説明】
２０…ＣＰＵ
２２…ＸＹＺ→ｒ’ｇ’ｂ’変換処理部
２４…ｒ’ｇ’ｂ’→ＲＧＢ変換処理部
３０…メモリ
４０…Ｉ／Ｏインタフェース
５０…ディスク・インタフェース
６０…バス
７０…マウス
８０…キーボード
９０…カラーモニタ
１００…ハードディスクドライブ
１１０…ＣＤ−ＲＯＭドライブ
１１５…ＣＤ−ＲＯＭ
１２０…フロッピディスクドライブ
１２５…フロッピディスク
１３０…ネットワークカード
１４０…ネットワーク回線
１５０…サーバ
１６０…色変換装置
１７０…色変換用ルックアップテーブル
１８０…色変換用補間回路[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for converting a colorimetric value into a display signal input to a color display device and a technique for inversely converting the display signal into the colorimetric value.
[0002]
[Prior art]
Conventionally, an original color image (for example, a color film, color photographic paper, or a color print) is read using a color scanner to obtain three color signals of RGB, or is necessary for confirming the read color image. In order to reliably perform such processing, it is widely performed to display a color image as an object of dialogue by giving the obtained RGB color signal to a color monitor.
[0003]
However, since the RGB color signal obtained by reading with a color scanner is a signal that depends only on the characteristics of the color separation filter of the color scanner, even if it is an RGB color signal, it is not applied to the color monitor as it is. However, an image close to the original color image cannot be obtained.
[0004]
On the other hand, in the image processing for printing and newspapers, since color printing is performed using four colors of CMYK ink, each pixel of the image for color printing is finally subjected to color conversion to be CMYK. It is composed of four color signals. Therefore, in this case, in order to foresee the processing process or the screen for predicting the final print finish in an intermediate stage of image processing, the CMYK color signal for color printing is converted into an RGB color signal, which is then applied to the color monitor to be displayed in Displaying images is also widely performed.
[0005]
However, it is difficult to obtain a color image close to an actual color-printed image simply by converting the CMYK color signal for color printing into an RGB color signal and applying it to the color monitor.
[0006]
In other words, RGB color signals that depend only on the characteristics of the color separation filter as described above and CMYK signals for color printing are first converted into RGB color signals for a color monitor to be displayed by color conversion. The expected color image can be displayed on the color monitor.
[0007]
However, the color monitor can only display colors within the color gamut (gamut specific to the color monitor) surrounded by the colors of the three RGB phosphors, whereas the original color image (gamut) or color In many cases, the printed image (the gamut) includes a color that is outside the color gamut surrounded by the colors of the three color phosphors of the color monitor.
[0008]
Therefore, when the RGB color signal value for the color monitor obtained by color conversion is normalized from 0 to 1, it falls outside the color gamut surrounded by the colors of the RGB phosphors of the color monitor. Regarding the color, at least one of the RGB color signals for color monitor has a value smaller than 0 (negative value) or a value larger than 1 (value exceeding 100%).
[0009]
Here, normalization of the RGB color signal value for the color monitor from 0 to 1 means that the RGB color signal value necessary for expressing white on the color monitor is 1 (ie, MAX), respectively, and it is completely black. This means that the value of the RGB color signal necessary for expressing the value is 0 (that is, MIN), so that the phosphor cannot be illuminated with a value larger than 1, and even smaller than 0. That is, it is a consultation that cannot be made at all, such as making the phosphor shine at a value smaller than 0.
[0010]
Therefore, in such a case, conventionally, even if a value of 1 or more or a value of 0 or less is required as the value of the RGB color signal for the color monitor, all must be terminated at MAX1 or MIN0.
[0011]
[Problems to be solved by the invention]
For this reason, if a part of the color to be expressed included in the original color image or the color printed image is outside the color gamut that can be reproduced by the phosphor in the color monitor, the color part (for example, As for the color portion with high saturation, the value of the RGB color signal for the color monitor is uniformly cut off at MAX1 or MIN0, so that the color tone of that color portion is lost. There was a bug.
[0012]
In addition, when the color to be expressed contained in the original color image or the color print image moves from the inside of the color gamut that can be reproduced by the phosphor in the color monitor to the outside, at the boundary of the color gamut, RGB The color signal values are not interrupted by MAX1 or MIN0 at the same time, but only the color signal values that are about to exceed MAX1 or MIN0 are interrupted by MAX1 or MIN0. Since the color continues to change, there is a fear that the hue starts to change suddenly near the boundary of the color gamut, leading to a change in hue.
[0013]
In general, high-saturation color parts are often very important parts in an image, and it is an unacceptable defect that the color tone of the important part is lost or the hue changes. Is likely to be.
[0014]
Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art, and in a specific color portion outside the reproducible color gamut, there is no loss of color tone or hue change. The object is to provide a color conversion method and a color conversion device.
[0015]
[Means for solving the problems and their functions and effects]
In order to achieve at least a part of the above object, the color conversion method of the present invention provides an additive color mixture primary color each having specific chromaticity coordinates, which is input corresponding to the additive color mixture primary color. A color is displayed by mixing with a mixing amount corresponding to the signal strength of the signal, and a value within a predetermined specified range can be taken as the signal strength of the display signal, and the mixing amount corresponding to the signal strength For a color display device that can take a value within a predetermined reproducible range, the colorimetric value is converted into the display signal in order to obtain the display signal to be input to the color display device from the colorimetric value. A color conversion method for
(A) converting the colorimetric values into respective mixed amounts of the primary colors of the additive color mixture;
(B) A step of converting each additive amount of the additive-mixed primary colors obtained by the conversion into a signal intensity of the display signal corresponding to the additive-mixed primary colors for each of the additive-mixed primary colors. When,
With
The step (b)
Of the additive primary colors, at least one specific primary color is out of the reproducible range based on the mixture amount of the remaining primary colors other than the specific primary color obtained by the conversion in the step (a). And within a maximum relief range set in advance, a relief range is set,
The mixing amount of the specific primary color obtained by the conversion in the step (a) is the reproducible range. First area preset in Is converted into a value within the first conversion range preset within the specified range, as the signal strength of the display signal,
The mixing amount of the specific primary color is a value outside the reproducible range and a value within the relief range Or a value in the remaining second region other than the first region within the reproducible range. In this case, the gist is to convert the signal intensity of the display signal into a value in the remaining second conversion range other than the first conversion range in the specified range.
[0016]
Further, the color conversion device of the present invention mixes the additive primary colors having specific chromaticity coordinates with a mixing amount corresponding to the signal intensity of the display signal input corresponding to each of the additive primary colors. By displaying the color, the signal intensity of the display signal can take a value within a predetermined specified range, and the mixing amount according to the signal intensity can be a value within a predetermined reproducible range. A color conversion device that converts the colorimetric value into the display signal in order to obtain the display signal to be input to the color display device from the colorimetric value for a color display device that can be taken,
First conversion means for converting the colorimetric values into respective mixed amounts of the additive mixed primary colors;
A second conversion for converting each additive amount of the additive-mixed primary colors obtained by the conversion into a signal intensity of the display signal corresponding to each of the additive-mixed primary colors for each of the additive-mixed primary colors Means,
With
The second conversion means includes
Among the additive primary colors, at least one specific primary color is out of the reproducible range on the basis of the mixture amounts of the remaining primary colors other than the specific primary color obtained by conversion by the first conversion means. And within the maximum relief range set in advance, the relief range is set,
The mixing amount of the specific primary color obtained by conversion by the first conversion means is the reproducible range. First area preset in Is converted into a value within the first conversion range preset within the specified range, as the signal strength of the display signal,
When the mixing amount of the specific primary color is outside the reproducible range and is within the relief range Or a value in the remaining second region other than the first region within the reproducible range. The gist is to convert the signal intensity of the display signal into a value in the remaining second conversion range other than the first conversion range in the specified range.
[0017]
As described above, in the color conversion method and apparatus of the present invention, first, the colorimetric value is converted into each mixing amount of the primary color of the additive color mixture, and then each mixture of the primary colors of the additive color mixture obtained by the conversion is obtained. The quantity is converted into the signal strength of the display signal. At this time, at least one of the additive primary colors is outside the reproducible range based on the mixing amount of the remaining primary colors other than the specific primary color obtained by conversion, and A relief range is set within a preset maximum relief range. And the mixing amount of the specific primary color obtained by conversion is in a reproducible range First area preset in The signal intensity of the display signal is converted to a value within the first conversion range set in advance within the specified range, and the mixing amount of the specific primary color is a value outside the reproducible range. If the value is within the relief range Or a value in the remaining second region other than the first region within the reproducible range. Is converted into a value in the remaining second conversion range other than the first conversion range in the specified range as the signal strength of the display signal.
[0018]
Therefore, according to the color conversion method and the color conversion apparatus of the present invention, for example, even a color portion outside the color gamut that can be reproduced by the color display device is converted from the colorimetric value of the color portion. Of the obtained additive primary color mixture amounts, when the specific primary color mixture amount is a value within the relief range, the value is converted into a value within the second conversion range as the signal intensity of the display signal. Therefore, the specific primary color is stored as the value of the signal intensity of the display signal without losing the information on the color portion described above. Therefore, the color tone does not disappear or the hue does not change in the color portion described above.
[0019]
In the color conversion method of the present invention,
In the step (b), when the relief range is set based on the mixture amounts of the remaining primary colors, the relief range is set to be larger as the mixture amount of the remaining primary colors is larger, and the remaining primary colors are set. It is preferable that the relief range is set to be smaller as the mixing amount is smaller.
[0020]
By setting the relief range in this way, colors that are outside the reproducible range of the color display device and are to be rescued can be included in the relief range accurately and without excess or deficiency. Moreover, since the second conversion range does not include unnecessary conversion values, the range can be used effectively.
[0021]
In the color conversion method of the present invention,
In the setting of the relief range in the step (b), the mixing amount of the specific primary color is First region If the value is within I don't know It is preferable.
[0022]
With this configuration, it is not necessary to perform the relief range setting process when it is not necessary, so that the overall processing speed can be improved.
[0023]
In the color conversion method of the present invention,
The ratio of the second conversion range to the specified range is preferably smaller than the ratio of the maximum relief range to the reproducible range.
[0024]
Conversion to colors in the reproducible color gamut is as much as possible when the second conversion range that is the conversion destination of the relief range is made as narrow as possible and the first conversion range that is the conversion destination of the reproducible range is as wide as possible. The amount of deviation between the front and rear chromaticity points can be reduced.
[0025]
In the color conversion method of the present invention,
The step (b)
The conversion from the mixing amount of the specific primary color to the signal intensity of the display signal corresponding to the specific primary color is performed using a conversion function representing a conversion relationship between the mixing amount and the signal intensity,
The mixing amount of the specific primary color is First region A conversion function including a first function is used, and the mixing amount of the specific primary color is a value within the maximum relief range. Or a value in the second region In this case, a conversion function that is different from the first function and includes a second function that changes based on the mixture amount of the remaining primary colors other than the specific primary color may be used.
[0026]
With this configuration, when the mixing amount of the specific primary color is a value within the maximum relief range, conversion corresponding to each mixing amount of the remaining primary colors other than the specific primary color can be performed.
[0027]
Further, according to another color conversion method of the present invention, an additive color mixture primary color each having a specific chromaticity coordinate is mixed in accordance with the signal intensity of a display signal input corresponding to the additive color mixture primary color. By mixing the colors, the color is displayed, the signal intensity of the display signal can take a value within a predetermined specified range, and the mixing amount according to the signal intensity is within a predetermined reproducible range. A color conversion method for converting the display signal to be input to the color display device into a colorimetric value for a color display device capable of taking a value,
(A) converting the signal intensity of the display signal corresponding to each of the primary colors of the additive color mixture into a mixing amount of the primary colors of the additive color mixture for each of the primary colors of the additive color mixture;
(B) converting each mixing amount of the primary color of the additive color mixture obtained by conversion into the colorimetric value;
With
The step (a)
Among the additive primary colors, at least one specific primary color is out of the reproducible range and set in advance based on the mixture amount of the remaining primary colors other than the specific primary color obtained in advance. Within the maximum relief range, set a relief range,
When the signal intensity of the display signal corresponding to the specific primary color is a value within a first conversion range set in advance within the specified range, the reproducible range is obtained as the mixing amount of the specific primary color. First area preset in To the value in
When the signal intensity of the display signal corresponding to the specific primary color is a value within the remaining second conversion range other than the first conversion range within the specified range, the mixing amount of the specific primary color , A value outside the reproducible range and a value within the relief range Or a value in the remaining second region other than the first region within the reproducible range. The gist is to convert to.
[0028]
By using such a color conversion method, a display signal to be input to the color display device can be converted into a colorimetric value.
[0029]
In addition, another color conversion apparatus of the present invention includes a look-up table including representative points for color conversion, and corresponds to each of the additive color mixture primary colors obtained by conversion by the color conversion apparatus of the present invention. A color conversion device that converts the signal intensity of the display signal into a value of a predetermined color system using a representative point of the color conversion lookup table and an interpolation operation based on the representative point;
The color conversion lookup table includes the first conversion range and the second conversion among combinations of signal intensity values of the display signal respectively corresponding to the additive primary color obtained by conversion. At least a specific combination including a boundary value located at the boundary with the range as a signal intensity value of the display signal corresponding to any one of the specific primary colors is used as an address value of the color conversion lookup table With
The gist of the invention is that an address represented by the specific combination stores a value of the color system obtained by converting the specific combination.
[0030]
By adopting such a configuration, it is possible to avoid interpolating a region that spans the first conversion range and the second conversion range with the same coefficient, so that a correct color system value can be obtained.
[0031]
The recording medium of the present invention mixes the additive primary colors having specific chromaticity coordinates with a mixing amount corresponding to the signal intensity of the display signal input corresponding to each of the additive primary colors. In addition to displaying colors, the signal intensity of the display signal can take a value within a predetermined specified range, and the mixing amount according to the signal intensity can take a value within a predetermined reproducible range For a display device, a computer-readable recording medium recording a computer program for converting the colorimetric value into the display signal in order to obtain the display signal to be input to the color display device from the colorimetric value Because
A first function for converting the colorimetric value into each additive amount of the additive primary color;
Each additive amount of the additive mixed primary colors obtained by conversion is converted into signal intensity of the display signal corresponding to each of the additive mixed primary colors for each of the additive mixed primary colors,
Of the additive primary colors, at least one specific primary color is out of the reproducible range based on the mixture amount of the remaining primary colors other than the specific primary color obtained by conversion by the first function. And within a maximum relief range set in advance, a relief range is set,
The mixing amount of the specific primary color obtained by conversion by the first function is the reproducible range. First area preset in Is converted into a value within the first conversion range preset within the specified range, as the signal strength of the display signal,
The mixing amount of the specific primary color is a value outside the reproducible range and a value within the relief range Or a value in the remaining second region other than the first region within the reproducible range. The second function of converting the signal strength of the display signal into a value in the remaining second conversion range other than the first conversion range within the specified range,
The gist is that a computer program to be realized by a computer is recorded.
[0032]
The recording medium includes a flexible disk, CD-ROM, magneto-optical disk, IC card, ROM cartridge, punch card, printed matter on which a code such as a barcode is printed, an internal storage device of a computer (memory such as RAM or ROM). ) And external storage devices can be used.
[0033]
When a computer program recorded on such a recording medium is executed by a computer, processing similar to that of the color conversion method and color conversion apparatus of the present invention described above is performed, and the color conversion method and color conversion apparatus of the present invention Similar effects can be achieved.
[0034]
Other aspects of the invention
The present invention also includes other aspects as described below. In other words, as one aspect, there is an aspect as a program supply apparatus that supplies a computer program that causes a computer to realize the functions of the respective steps or means of the present invention via a communication path. In such an aspect, the above-described color conversion method and color conversion device can be realized by placing the program on a server or the like on a network, downloading the necessary program to a computer via a communication path, and executing the program. it can.
[0035]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described based on examples. FIG. 1 is a flowchart showing a processing procedure of a color conversion method as an embodiment of the present invention.
[0036]
For example, in the intermediate stage of image processing for printing (for example, in the middle of the plate making process), in order to predict the final printing result, the RGB color signal scanned for color printing is used as the RGB color signal for color monitor. Let's think about converting to, giving to a color monitor, and displaying a color image.
[0037]
In general, when converting RGB color signals of a scanner reading transparent film into RGB color signals for display (display signals RGB), color conversion is performed in the order of RGB → XYZ → RGB. Here, XYZ is representatively shown via a device-independent color measurement system or color system, and L ^* a ^* b ^* Or L ^* u ^* v ^* Etc., or some of them. Among these, the color conversion method of the present embodiment is applied to the case where color conversion is performed from XYZ, which is a colorimetric value, to RGB, which is a display signal (that is, XYZ → RGB).
[0038]
That is, in the color conversion method of the present embodiment, as shown in FIG. 1, first, XYZ that is a colorimetric value is a luminance linear value for red, green, and blue (that is, proportional to the luminance) r. Processing for color conversion to “g” b ”is performed (step S20). These r′g′b ′ will be described later.
[0039]
FIG. 2 is an explanatory diagram showing chromaticity coordinates of colors represented by red, green, and blue phosphors of ITU-R 709 and white chromaticity coordinates of D50.
[0040]
In this embodiment, it is assumed that ITU-R709 (formerly CCIR 709) phosphor is used as red, green, and blue phosphors in a color monitor. Here, ITU-R is an international organization related to telecommunications (International Telecommunication Union). In its Recommendation 709 (HDTV standard), red (red), green (blue), and blue (blue) The chromaticity coordinates (light emission chromaticity coordinates) of the color represented by the phosphor are defined as shown in FIG.
Further, by determining the chromaticity coordinates of white as well as the chromaticity coordinates of the colors represented by the respective phosphors of red, green and blue, the maximum luminance of red, green and blue (that is, red, The ratio of the maximum luminance when the green and blue phosphors emit light can be determined.
[0041]
For example, when white of D50 (color temperature 5000 degrees) is used as white, the chromaticity coordinates are given as shown in FIG. Therefore, the ratio of the maximum luminance of red, green, and blue in the color monitor is such that Ew, Er, Eg, and Eb have chromaticity coordinates of white, red, green, and blue, respectively, and a Y component (that is, a luminance component). Can be expressed as shown in Equation (1).
[0042]
Ew = 0.24989Er + 0.6980999Eg + 0.0520963Eb (1)
[0043]
Here, the above-mentioned r ′, g ′, and b ′ are the red, green, and blue luminances in the color monitor (that is, the respective luminances when the red, green, and blue phosphors emit light), Each is a value obtained by normalization (normalization) so that the minimum luminance is approximately 0 and the maximum luminance is approximately 1. In general, in a color monitor, red, green, and blue phosphors emit light with appropriate luminances, and these lights are mixed to display a desired color. Therefore, the above-mentioned r ′, g ′, and b ′ represent the respective mixing amounts of red, green, and blue when a desired color is displayed by mixing each color of red, green, and blue on the color monitor. It will be.
[0044]
On the other hand, the relationship between x, y, z, X, Y, and Z is expressed as in equations (2) to (5).
[0045]
x = X / (X + Y + Z) (2)
y = Y / (X + Y + Z) (3)
z = Z / (X + Y + Z) (4)
z = 1-xy (5)
[0046]
Accordingly, when the color conversion relationship from r ′, g ′, b ′ to X, Y, Z is obtained using the above equation and values, it is expressed as equation (6).
[0047]
[Expression 1]

[0048]
That is, Expression (6) represents the color conversion relationship from r ′, g ′, b ′ to X, Y, Z in the color monitor.
[0049]
Further, by obtaining an inverse matrix of the matrix shown in Expression (6), a color conversion relationship from X, Y, Z to r ′, g ′, b ′ opposite to Expression (6) is obtained. 7).
[0050]
[Expression 2]

[0051]
That is, Expression (7) represents the color conversion relationship from X, Y, Z to r ′, g ′, b ′ for the color monitor.
[0052]
Accordingly, in step S20 described above, color conversion from XYZ to r′g′b ′ is performed by the matrix operation shown in Expression (7).
[0053]
By the way, when a color in the color gamut (gamut specific to the color monitor) that can be reproduced by the red, green, and blue phosphors in the color monitor is converted as XYZ that is a colorimetric value. The values of r ′, g ′, and b ′ obtained by conversion all fall within the range of 0 to 1. However, when a color outside the above-described color gamut is converted as XYZ, one of the values of r ′, g ′, and b ′ obtained by the conversion is outside the range of 0 to 1. .
[0054]
Next, in this embodiment, a process of converting r′g′b ′ obtained in step S20 into RGB, which are red, green, and blue display signals to be input to the color monitor (step S22). .
[0055]
In this embodiment, R, G, and B represent the signal intensities (that is, applied voltages) of the red, green, and blue display signals to be input to the color monitor, respectively, with a minimum value of 0 and a maximum value. It is normalized (normalized) to be 1.
[0056]
Here, before describing the r′g′b ′ → RGB conversion process in the present embodiment, the conventional r′g′b ′ → RGB conversion process will be described in order to compare the present embodiment with the prior art. To do.
[0057]
Conventionally, conversion from r′g′b ′ to RGB is performed by a conversion function shown in Expression (8) based on the γ characteristic in the color monitor.
[0058]
[Equation 3]

[0059]
In Expression (8), L represents each luminance when each of the red, green, and blue phosphors emits light, and corresponds to the above-described r ′, g ′, or b ′. V represents the signal intensity (that is, applied voltage) of the red, green, and blue display signals to be input to the color monitor, and corresponds to R, G, or B described above.
[0060]
Therefore, in the r′g′b ′ → RGB conversion process, r ′ is converted to R, g ′ is converted to G, and b ′ is converted to B for each of red, green, and blue.
[0061]
In the conventional r′g′b ′ → RGB conversion processing, any value of r ′, g ′, b ′ in the preceding XYZ → r′g′b ′ conversion processing is smaller than 0 (ie, L < When 0) is obtained, as is apparent from the equation (8), all of the values are converted to 0 (that is, V = 0) as R, G, or B values.
[0062]
Therefore, as described above, in the previous XYZ → r′g′b ′ conversion processing, for example, XYZ is out of the color gamut that can be reproduced by the red, green, and blue phosphors in the color monitor. When a color is converted and any of the values of r ′, g ′, b ′ obtained by the conversion becomes a value smaller than 0 (that is, when it falls outside the range of 0 to 1), the value Are all converted to 0 as the values of R, G, B, and are uniformly censored at 0. For this reason, when the color portion outside the above-described color gamut is subjected to r′g′b ′ → RGB conversion processing, information on the color portion is lost. Problems such as the color tone of the color portion being lost or the hue changing occur.
[0063]
FIG. 3 is an explanatory diagram showing a color region that cannot be relieved by a conventional color conversion method. FIG. 3 shows a three-dimensional color space projected onto a red-green plane. In FIG. 3, an area sandwiched between the red axis and the green axis (that is, two arrows) indicates an actually existing color area, and the rectangle partitioned in a lattice shape is A color gamut (that is, a gamut) ga that can be reproduced by the red, green, and blue phosphors in the color monitor is shown.
[0064]
Therefore, even if the color actually exists, as described above, the color r′g′b ′ for the color outside the color gamut ga, such as the color in the area OA indicated by the hatching in FIG. → When the RGB conversion process is performed, information on the color portion is lost, so that the color in the area OA cannot be relieved by the conventional color conversion method.
[0065]
On the other hand, in contrast to such a conventional color conversion method, in the color conversion method of the present embodiment, processing as shown in FIG. 1 is performed as r′g′b ′ → RGB conversion processing.
[0066]
That is, in this embodiment, first, the value of r ′, g ′ or b ′ obtained in step S20 is set to L (step S24), and it is determined whether or not the value of L is i or more. (Step S26). Here, i is j to the γ-th power, as will be described later, and is a value greater than zero.
[0067]
If the value of L is smaller than i as a result of the determination, a process for obtaining the value of c is performed according to equation (9) (step S27).
[0068]
[Expression 4]

[0069]
However, in Equation (9), M and N are values other than the values set to L in Step S24 among the values of r ′, g ′, and b ′ obtained in Step S20. That is, for example, when the value of r ′ is set to L in step S24, M and N are values of g ′ and b ′, respectively. Moreover, in Formula (9), f is a function of M or N as shown in Formula (10).
[0070]
[Equation 5]

[0071]
However, in Formula (10), d is M or N.
[0072]
Therefore, taking the case where the value of r ′ is L as an example, when the values of g ′ and b ′ are in the range of 0 to 1, f (g ′) = g from equation (10). Since “, f (b ′) = b ′, the value of c becomes an average value (g ′ + b ′) / 2 of g ′ and b ′, as is clear from the equation (9). Further, when both the values of g ′ and b ′ are smaller than 0, f (g ′) = 0 and f (b ′) = 0 from the equation (10), and the values of g ′ and b ′ are both 1. If larger, f (g ′) = 1 and f (b ′) = 1 from equation (10), so the value of c always falls within the range of 0 to 1.
[0073]
Now, when the value of c is obtained in step S27 as described above, after that, based on the value of c, conversion from L to V is performed by the conversion function shown in equation (11) (step S28).
[0074]
[Formula 6]

[0075]
However, in Formula (11), i is j to the γth power, and i is larger than 0. J and k are predetermined values larger than 0 set in advance.
[0076]
On the other hand, if the result of determination in step S26 is that the value of L is equal to or greater than i, conversion from L to V is performed by the conversion function shown in equation (12) (step S30).
[0077]
[Expression 7]

[0078]
4 and 5 are graphs showing the conversion characteristics from L to V used in the r′g′b ′ → RGB conversion processing in comparison with the conventional example and the present example. In these drawings, FIG. 5 is an enlarged view showing an enlarged portion in the vicinity of (L, V) = (0, 0.03) in FIG.
[0079]
4 and 5, the curve C0 indicated by the solid line represents the conversion characteristic in the conventional example, and specifically represents the conversion characteristic obtained by the conversion function shown in Expression (8). On the other hand, a curve C1 indicated by a broken line represents the conversion characteristic in the present embodiment, and specifically represents the conversion characteristic obtained by the conversion functions shown in the equations (11) and (12). 4 and 5, the value of γ in the equations (8) and (12) is “3”, the value of k in the equation (11) is “0.1”, and the value of j in the equation (11). Is "0.03".
[0080]
The conversion characteristic (curve C1) in the present embodiment is a straight line within a range where the value of V is 0 or more and less than j, as indicated by a broken line in FIG. Moreover, the slope (dV / dL) of the straight line is j / (i + ck) and the L intercept is −ck, as is clear from the equation (9). Varies based on the value of. At this time, as described above, the value of c is always in the range of 0 to 1, and the value of k is greater than 0. Therefore, the value of −ck, which is the L intercept, is in the range of −k to 0. In.
[0081]
Conventionally, in FIG. 4, when the value of L is smaller than 0 (L <0), as indicated by the curve C0 indicated by the solid line, all values are converted to 0 as the value of V. With respect to colors outside the color gamut ga, when the r′g′b ′ → RGB conversion process is performed, information on the color portion is lost.
[0082]
On the other hand, in this embodiment, when the value of L is greater than or equal to −ck and less than i (−ck ≦ L <i), as shown by the curve C1 indicated by the broken line in FIGS. , L is converted to a value of V that is 0 or more and less than j by a linear function. Accordingly, since the range from −ck to less than 0 (−ck ≦ L <0) is within the range (−kc ≦ L <i), even if the color is outside the color gamut ga, the value of L is −ck. Information is not lost for color portions that fall within the range of less than 0 (−ck ≦ L <0). However, since the value of c changes in the range of 0 or more and 1 or less as described above, the closer the value of c is to 0, the narrower the range of −ck or more and less than 0, and the closer the value of c is to 1— The range from ck to less than 0 becomes wide.
[0083]
When the value of L is smaller than −ck (L <−ck), as is clear from the equation (11), the value of L is set to 0 as the value of V as in the conventional case. Will be converted.
[0084]
On the other hand, when the value of L is i or more and less than 1 (i ≦ L <1), the value of V is converted to a value of j or more and less than 1. At this time, as shown in Expression (12), the value of L is determined by the same function of L to the 1 / γ power as in Expression (8) in the past, that is, a function corresponding to the γ characteristic of the color monitor. Since it is converted into a value, the curve C1 overlaps the conventional curve C0 in the above range.
[0085]
When the value of L is 1 or more (1 ≦ L), the L value is converted to 1 as the value of V, as is clear from the equation (12), as in the conventional case. Will be.
[0086]
If the value of V is obtained in step S28 or S30 as described above, then in this embodiment, the value of V is made to correspond to r ′, g ′, b ′ in step S24. Then, R, G, or B is set (step S32).
[0087]
Therefore, by the above r′g′b ′ → RGB conversion process (step S22), r ′ is converted into R, g ′ is converted into G, and b ′ is converted into B for each of red, green, and blue. Is done.
[0088]
FIG. 6 is an explanatory diagram showing the color area relieved by the r′g′b ′ → RGB conversion process in the color conversion method of the present embodiment, and the position of each color before and after conversion. FIG. 6 shows a three-dimensional color space projected onto a red-green plane, as in FIG. In FIG. 6, the area sandwiched between the red axis and the green axis (that is, two arrows) indicates an actually existing color area, and is a solid rectangle partitioned in a grid pattern. Indicates a color gamut (namely, gamut) ga that can be reproduced by the red, green, and blue phosphors in the color monitor.
[0089]
Among the colors in the color gamut ga, for the colors in the conversion area A1 represented by the dashed-dotted rectangles partitioned in a grid pattern in FIG. 6, the above-described r′g′b ′ → RGB Of the conversion processes, each is converted into the same conversion area A1 by the process of step S30.
[0090]
On the other hand, even if the color is out of the color gamut ga, the steps in the r′g′b ′ → RGB conversion process described above for the color in the area PA indicated by the dotted points in FIG. By the processing of S28, each is converted into the conversion area A2 indicated by the oblique lines in the color gamut ga. That is, the area PA is a color area that is relieved by the r′g′b ′ → RGB conversion process of the present embodiment, and the conversion area A2 is a color conversion destination area in the relief area PA.
[0091]
Of the colors in the gamut ga, the colors in the conversion area A2 are converted to the conversion area A2 by the processing in step S28 together with the colors in the relief area PA. The
[0092]
Here, the relief area PA corresponds to the above-described range of −ck or more and less than 0 when viewed as the value of L, and therefore the width w0 thereof is ck. Since the value of c changes according to equations (9) and (10), the width w0 of the relief area PA changes as follows. In other words, for example, when the value of r ′ is set to L, when f (g ′) = 0 and f (b ′) = 0, c = 0, so the width w0 is 0. The minimum. When f (g ′) = 1 and f (b ′) = 1, since c = 1, the width w0 is k, which is the maximum. When f (g ′) = g ′ and f (b ′) = b ′, c = (g ′ + b ′) / 2, and therefore the width w0 is (g ′ + b ′) k / 2. . At this time, since the width w0 is proportional to each value of g ′ and b ′, as shown in FIG. 6, the width w0 of the relief region for red is the values of green and blue (g ′, b ′) ( In FIG. 6, the blue value increases linearly as the value (in the direction perpendicular to the paper surface) increases.
[0093]
Further, when the maximum width of the width w0 of the relief area PA is w1, the value corresponds to k as described above when viewed as an L value. On the other hand, the width w2 of the conversion area A2 corresponds to j described above when viewed as a value of V. The width of one side of the color gamut ga corresponds to “1” when viewed as an L or V value. In the present embodiment, as described above, since the value of k is set to “0.1” and the value of j is set to “0.03”, the width w2 of the conversion area A2 is the maximum width of the relief area PA. It is narrower than w1.
[0094]
Therefore, in the relief area PA, for the area where the width w0 is wider than the width w2 of the conversion area A2, the color in the area is converted into a conversion area with a narrower width by r′g′b ′ → RGB conversion processing. It is compressed and pushed into A2. On the other hand, in the relief area PA, in the area where the width w0 is narrower than the width w2 of the conversion area A2, the colors in the area are expanded and pushed into the wider conversion area A2. Become. Therefore, for the former color, the dispersion state of the chromaticity points becomes dense in the conversion area A2 of the conversion destination, whereas the latter color can be made coarse.
[0095]
Further, among the colors originally in the color gamut ga, the color in the conversion area A1 is converted into the same conversion area A1 by r′g′b ′ → RGB conversion processing. For these colors, there is almost no shift in the color position (chromaticity point) before and after conversion.
[0096]
In FIG. 6, the region (that is, the wide angle region) other than the region (that is, the narrow angle region) sandwiched between the red direction axis and the green direction axis (that is, two arrows) is actually Is a non-existing color area, but in this embodiment, since the relief area PA is not included at all in this area, the area to be converted by the above r′g′b ′ → RGB conversion process In other words, the color is surely present in the conversion area A2.
[0097]
As described above, according to the present embodiment, the colorimetric values XYZ are outside the color gamut (that is, the gamut) ga that can be reproduced by the red, green, and blue phosphors in the color monitor. Even if the value of r ′, b ′, or g ′ obtained by the conversion becomes a value smaller than 0 (L <0), the value is −ck or more. If there is (L ≧ −ck), the values of R, G, B are converted to values of 0 or more and less than j (0 ≦ V <j). Therefore, even for colors outside the color gamut ga, information is lost for the color portion that falls within the range of −ck to less than 0 (−ck ≦ L <0) as r ′, b ′, and g ′ values. Since there is no value, it is stored as R, G, and B values. Therefore, the color tone does not disappear or the hue does not change in the color portion described above.
[0098]
By the way, in the above-described embodiment, it is assumed that the RGB color signal scanned for color printing is converted to the RGB color signal for color monitor and applied to the color monitor. It was intended for color conversion to RGB, which is an industrial signal.
[0099]
However, the RGB color signal for the color monitor is converted into a CMYK color signal for color printing and given to a printing machine, or the RGB color signal for the color monitor is converted into a colorimetric value such as XYZ. For example, the color conversion opposite to the above, that is, color conversion from RGB as a display signal to XYZ as a colorimetric value may be targeted.
[0100]
That is, in this case, the RGB → r′g′b ′ conversion process, which is the reverse of the r′g′b ′ → RGB conversion process performed in step S22 of FIG. The r′g′b ′ → XYZ conversion process, which is the reverse of the XYZ → r′g′b ′ conversion process performed in step S20, may be performed.
[0101]
FIG. 7 is a flowchart showing a processing procedure of a color conversion method as another embodiment of the present invention.
[0102]
That is, in the color conversion method of this embodiment, as shown in FIG. 7, first, RGB, which is a display signal for red, green, and blue, is converted to a luminance linear value for red, green, and blue. A process of color conversion to “b” is performed (step S40). In the r′g′b ′ → RGB conversion process in the above-described embodiment, the conversion from L to V is performed. However, in the RGB → r′g′b ′ conversion process in step S40 in the present embodiment, the conversion is performed. The reverse conversion from V to L is performed.
[0103]
Specifically, first, the value of R, G, or B is set to V (step S44), and it is determined whether the value of V is j or more (step S46). However, each value of R, G, B is in the range of 0 or more and 1 or less.
[0104]
If the result of determination is that the value of V is smaller than j, processing for obtaining the values of M and N is performed according to equation (13) (step S50).
[0105]
[Equation 8]

[0106]
However, in Equation (13), T and U are values other than the values set as V in step S44 among the values of R, G, and B. That is, for example, when the value of R is set to V in step S44, T and U are values of G and B, respectively.
[0107]
The conversion function shown in Expression (13) is a conversion function opposite to the conversion function of Expression (8), which is a conventional conversion function, and as described above, for example, T and U are changed to G and B, respectively. In the case of each value, provisional values of g ′ and b ′ can be obtained as M and N by Expression (13).
[0108]
Next, based on the values of M and N obtained in step S50, a process for obtaining the value of c is performed by the above-described equation (9) (step S52), and thereafter, based on the value of c. Conversion from V to L is performed by the conversion function shown in Expression (14) (step S54).
[0109]
[Equation 9]

[0110]
On the other hand, if the result of determination in step S46 is that the value of V is j or more, conversion from V to L is performed by the conversion function shown in equation (15) (step S48).
[0111]
[Expression 10]

[0112]
That is, in the r′g′b ′ → RGB conversion processing in the above-described embodiment, the conversion from L to V is performed based on the conversion characteristics as shown in FIG. As can be seen from the above, if only the value of c is determined, one curve C1 is established, so that the value of V with respect to the value of L is uniquely determined. Accordingly, even in the reverse conversion, if only the value of c is determined, the value of L with respect to the value of V is uniquely determined. Therefore, the conversion from V to L in the RGB → r′g′b ′ conversion process is also performed. This can be easily performed using the conversion relationship shown in FIG.
[0113]
As described above, when the value of L is obtained in step S48 or S54, in the present embodiment, the value of L is then made to correspond to R, G, B of step S44 and r ′. , G ′ or b ′ (step S56).
[0114]
Therefore, by the above RGB → r′g′b ′ conversion process (step S40), R is converted into r ′, G is converted into g ′, and B is converted into b ′ for each of red, green, and blue. Is done.
[0115]
Next, r′g′b ′ obtained in step S40 is color-converted to XYZ as colorimetric values (step S42).
[0116]
That is, the XYZ → r′g′b ′ conversion process in the above-described embodiment is performed by the matrix operation shown in Expression (7), but the r′g′b ′ → XYZ conversion process in step S22 in the present embodiment. Can be performed by the matrix operation shown in the equation (6) opposite to the equation (7).
[0117]
As described above, in this embodiment, color conversion from RGB as a display signal to XYZ as a colorimetric value can be performed.
[0118]
Incidentally, although the color conversion method has been described in the embodiment shown in FIG. 1, the color conversion method can be realized by a color conversion apparatus as shown in FIG.
[0119]
FIG. 8 is a block diagram showing a color conversion apparatus capable of realizing the color conversion method shown in FIG. The color conversion apparatus shown in FIG. 8 mainly includes a CPU 20, a memory 30, an I / O interface 40, and a disk interface 50, which are connected to each other by a bus 60.
[0120]
The I / O interface 40 is connected with a mouse 70, a keyboard 80, a color monitor 90, and a network card 130. The I / O interface 40 transmits instructions and commands input from the mouse 70 and the keyboard 80 to the CPU 20, and displays signals. Is sent to the color monitor 90, or communication data is exchanged with the network card 130. The network card 130 is connected to a network line 140, and communicates with the server 150, for example, via the network line 140.
[0121]
The hard disk drive 100, the CD-ROM drive 110, and the floppy disk drive 120 are connected to the disk interface 50, and data is read from and written to the hard disk in the hard disk drive 100, and the CD-ROM drive 110. The program and data are read from the CD-ROM inserted in the CD-ROM, and the data and the like are read from and written to the floppy disk 125 inserted in the floppy disk drive 120.
[0122]
The CPU 20 operates in accordance with a computer program stored in the memory 30 and functions as an XYZ → r′g′b ′ conversion processing unit 22 and an r′g′b ′ → RGB conversion processing unit 24. Here, the XYZ → r′g′b ′ conversion processing unit 22 mainly executes the XYZ → r′g′b ′ conversion process (step S20) shown in FIG. 1, and r′g′b. The '→ RGB conversion processing unit 24 mainly executes r′g′b ′ → RGB conversion processing (step S22) shown in FIG. Note that XYZ data, which is a colorimetric value to be color-converted, is stored in advance in the hard disk in the hard disk drive 100 and is supplied from this hard disk or stored in the server 150. It is supplied via the network line 140 and the network card 130. Data obtained during processing is temporarily stored in the memory 30 or the hard disk. RGB data, which is a display signal obtained by color conversion, is stored in the hard disk or stored in the server. If the color monitor 90 is the same type of color monitor as the target color monitor, the obtained display signal RGB may be supplied to the color monitor 90 for monitoring.
[0123]
Further, the computer program that causes the CPU 20 to function as the XYZ → r′g′b ′ conversion processing unit 22 and the r′g′b ′ → RGB conversion processing unit 24 is finally stored in the memory 30 as described above. However, it is originally recorded on the CD-ROM 115 and the floppy disk 125. That is, the above-described computer program is read from the CD-ROM 115 or the floppy disk 125 by the CD-ROM drive 110 or the floppy disk drive 120, for example, once written on the hard disk in the hard disk drive 100 and then transferred to the memory 30. .
[0124]
In addition to the CD-ROM 115, the floppy disk 125, and the hard disk, the recording medium for recording the above-described computer program includes a magneto-optical disk, a magnetic tape, an IC card, a ROM cartridge, a punch card, or a barcode. Various recording media readable by a computer, such as a printed matter on which a code is printed, can be used.
[0125]
Further, as described above, the computer program recorded on the CD-ROM 115 or the floppy disk 125 as described above can be transferred to the memory 30. However, the computer program stored in the server 150 can be transferred to the network line 140, the network. You may make it transfer to the memory 30 via the card | curd 130. FIG. In this case, the server 150 functions as a computer program providing device. In the present embodiment, a combination of a network line and a network card is used as a connection means to the server. However, a combination of a public line and a modem or a terminal adapter may be used.
[0126]
In the above description, when color conversion from XYZ as a colorimetric value to RGB as a display signal is performed by the color conversion method shown in FIG. 1, the color conversion method opposite to the color conversion of FIG. The case where color conversion from RGB as a display signal to XYZ as a colorimetric value is performed has been described. Therefore, next, color conversion from RGB (that is, RGB color signal), which is a display signal obtained by the color conversion method of FIG. 1, to CMYK (that is, CMYK color signal), which is a printing signal, is performed. Consider the case of giving to an image recording apparatus for plate making. As described above, since the color conversion method of FIG. 1 can be realized by the color conversion device of FIG. 8, RGB which is a display signal obtained by the color conversion method of FIG. In other words, it can be paraphrased as RGB which is a display signal obtained by the converter.
[0127]
Color conversion from RGB, which is a display signal obtained by the color conversion device of FIG. 8, to CMYK, which is a printing signal, is performed by the color conversion device as shown in FIG.
[0128]
FIG. 9 is a block diagram showing a color conversion apparatus as another embodiment of the present invention. As shown in FIG. 9, the color conversion device 160 of this embodiment includes a color conversion lookup table 170 and a color conversion interpolation circuit 180.
[0129]
When RGB, which is a display signal obtained by the color conversion device shown in FIG. 8, is input to the color conversion device 160 shown in FIG. 9, the color conversion interpolation circuit 180 determines that the combination of input RGB values is a color. If there is an address value in the conversion lookup table 170, the RGB value combination is input to the color conversion lookup table 170. In the color conversion lookup table 170, the CMYK values stored in the address indicated by the combination of RGB values input from the color conversion interpolation circuit 180 are read and output to the color conversion interpolation circuit 180. The color conversion interpolation circuit 180 outputs the read CMYK as it is.
[0130]
On the other hand, when the combination of RGB values input to the color conversion device 160 does not exist as the address value in the color conversion lookup table 170, the RGB values that exist as the address value in the color conversion lookup table 170 are displayed. Among the value combinations, a plurality of combinations close to the input RGB value combinations are selected and input to the color conversion lookup table 170. The color conversion lookup table 170 reads out CMYK values stored in each of a plurality of addresses indicated by combinations of RGB values input from the color conversion interpolation circuit 180, and performs color conversion. Output to the interpolation circuit 180. The color conversion interpolation circuit 180 performs an interpolation operation using the read CMYK values, and outputs CMYK corresponding to RGB input to the color conversion device 160.
[0131]
FIG. 10 is an explanatory diagram showing an example of combinations of RGB values existing as address values in the color conversion lookup table of FIG. 9 and CMYK values stored in these addresses. Therefore, for example, if (R, G, B) = (0, 0, 8) is input as a combination of RGB values from the color conversion interpolation circuit 180 to the color conversion lookup table 170, As shown in FIG. 10, the value of CMYK stored in the indicated address is (C, M, Y, K) = (C ₂ , M ₂ , Y ₂ , K ₂ Therefore, this value is output to the color conversion interpolation circuit 180. As shown in FIG. 10, CMYK values are always stored in all addresses of the color conversion lookup table 170.
[0132]
In this embodiment, the r′g′b ′ → RGB conversion processing performed in the r′g′b ′ → RGB conversion processing unit 24 of the color conversion apparatus shown in FIG. In the case of being based on the conversion characteristics shown in FIG. 5, a combination of RGB values including j as one of R, G, and B exists as at least an address value in the color conversion lookup table 170. Thus, the color conversion lookup table 170 is configured.
[0133]
For example, if j = 8 is set now, the color value is such that a combination of RGB values including 8 as any of R, G, B values exists as an address value in the color conversion lookup table 170. A conversion lookup table 170 is configured. In the example shown in FIG. 10, the address values are (R, G, B) = (0, 0, 8), (R, G, B) = (0, 8, 0), (R, G, B). ) = (8,0,0), (R, G, B) = (0,8,8), (R, G, B) = (8,0,8), (R, G, B) = (8,8,0), (R, G, B) = (8,8,8), etc., there are combinations of RGB values including 8 as any value of R, G, B. Therefore, the example shown in FIG. 10 corresponds to the lookup table described above.
[0134]
Accordingly, as described above, CMYK values are always stored in all the addresses of the color conversion lookup table 170. Therefore, by configuring the color conversion lookup table 170 as described above, R For any combination of RGB values including j as one of the values G, B and B, the CMYK values corresponding to the combinations exist in the color conversion lookup table 170. Will do.
[0135]
In the conversion from r'g'b 'to RGB, as is apparent from the conversion characteristics shown in FIGS. 4 and 5, there is a tendency for conversion before and after R, G or B = j (ie, V = j). It is completely different. Accordingly, even when converting from RGB to CMYK, the conversion tendency differs before and after R, G, or B = j. For this reason, when the corresponding CMYK values (that is, CMYK values obtained by the conversion) are obtained for combinations close to the RGB value combinations including j, these values are far from each other. There is no continuity.
[0136]
On the other hand, there is no combination of RGB values including j as the address value of the color conversion lookup table 170, and the CMYK values corresponding to the RGB value combinations are the color conversion lookup table 170. If a combination of RGB values including j is input to the color conversion device 160, there are a plurality of corresponding CMYK values for the combinations close to the RGB value combinations. It is read from the conversion look-up table 170, and interpolation calculation is performed using these CMYK values. However, as described above, for the combinations close to the combination of RGB values including j, the corresponding CMYK values are different from each other, and there is no continuity, so these CMYK values are used. Even if the interpolation calculation is performed, a correct CMYK value for a combination of RGB values including j cannot be obtained by the interpolation calculation.
[0137]
On the other hand, as in this embodiment, a combination of RGB values including j exists as at least an address value of the color conversion lookup table 170, and a CMYK value corresponding to this RGB value combination is a color. When configured to exist in the conversion lookup table 170, when a combination of RGB values including j is input to the color conversion device 160, the combination of RGB values is not performed without performing an interpolation operation. Since the CMYK values corresponding to are read out from the color conversion lookup table 170 and output, the correct CMYK values can be obtained for combinations of RGB values including j.
[0138]
The present invention is not limited to the above-described examples and embodiments, and can be implemented in various modes without departing from the scope of the invention.
[0139]
That is, in the above-described embodiment, XYZ is used as the colorimetric value, but L as the colorimetric value instead of XYZ. ^* a ^* b ^* May be used.
[0140]
Further, in the above-described embodiment, the conversion characteristic from L to V in the r′g′b ′ → RGB conversion processing is within the range where the value of V is 0 or more and less than j, as indicated by the broken line in FIG. , A straight line (that is, a linear function of L). However, the present invention is not limited to this, and in the above range, even a function other than a linear function of L can be used for conversion from L to V as long as it is a monotonically increasing function. it can.
[0141]
In the above-described embodiments, a color monitor is used as a target for providing RGB as a display signal. However, the present invention is not limited to a color monitor, and may be a color liquid crystal display or a color plasma display. Other color display devices may be used.
[Brief description of the drawings]
FIG. 1 is a flowchart showing a processing procedure of a color conversion method according to an embodiment of the present invention.
FIG. 2 is an explanatory diagram showing chromaticity coordinates of colors represented by red, green, and blue phosphors of ITU-R 709 and white chromaticity coordinates of D50.
FIG. 3 is an explanatory diagram showing a region of a color that cannot be relieved by a conventional color conversion method.
FIG. 4 is a graph showing the conversion characteristics from L to V used in r′g′b ′ → RGB conversion processing in a conventional example and an embodiment of the present invention.
FIG. 5 is a graph showing an enlarged main part of the conversion characteristics of FIG. 4;
FIG. 6 is an explanatory diagram showing a color area relieved by r′g′b ′ → RGB conversion processing and a position of each color before and after conversion in the color conversion method according to the embodiment of the present invention;
FIG. 7 is a flowchart showing a processing procedure of a color conversion method as another embodiment of the present invention.
8 is a block diagram showing a color conversion apparatus capable of realizing the color conversion method shown in FIG. 1. FIG.
FIG. 9 is a block diagram showing a color conversion apparatus as another embodiment of the present invention.
10 is an explanatory diagram showing an example of combinations of RGB values existing as address values in the color conversion lookup table of FIG. 9 and CMYK values stored in those addresses; FIG.
[Explanation of symbols]
20 ... CPU
22 ... XYZ → r'g'b 'conversion processing unit
24... R′g′b ′ → RGB conversion processing unit
30 ... Memory
40 ... I / O interface
50 ... Disk interface
60 ... Bus
70 ... Mouse
80 ... Keyboard
90 ... Color monitor
100: Hard disk drive
110 ... CD-ROM drive
115 ... CD-ROM
120 ... floppy disk drive
125 ... floppy disk
130: Network card
140: Network line
150 ... Server
160 ... Color conversion device
170 ... Color conversion lookup table
180 ... Interpolation circuit for color conversion

Claims (9)

The additive color mixture primary colors each having a specific chromaticity coordinate are mixed in a mixing amount corresponding to the signal intensity of the display signal input corresponding to each of the additive color mixture primary colors, thereby displaying the color, For a color display device that can take a value within a predetermined specified range as the signal intensity of the display signal, and a color display device that can take a value within a predetermined reproducible range as the mixing amount according to the signal intensity, A color conversion method for converting the colorimetric value into the display signal in order to obtain the display signal to be input to the color display device from:
(A) converting the colorimetric values into respective mixed amounts of the primary colors of the additive color mixture;
(B) A step of converting each additive amount of the additive-mixed primary colors obtained by the conversion into a signal intensity of the display signal corresponding to the additive-mixed primary colors for each of the additive-mixed primary colors. When,
With
The step (b)
Of the additive primary colors, at least one specific primary color is out of the reproducible range based on the mixture amount of the remaining primary colors other than the specific primary color obtained by the conversion in the step (a). And within a maximum relief range set in advance, a relief range is set,
When the mixing amount of the specific primary color obtained by the conversion in the step (a) is a value within the first region preset within the reproducible range , the signal intensity of the display signal is , Converted into a value within a first conversion range set in advance within the specified range,
The mixing amount of the specific primary color is a value outside the reproducible range, and a value within the relief range , or a remaining second region other than the first region within the reproducible range. If the value is a value , the signal intensity of the display signal is converted into a value in the remaining second conversion range other than the first conversion range in the specified range. . The color conversion method according to claim 1,
In the step (b), when the relief range is set based on the mixture amounts of the remaining primary colors, the relief range is set to be larger as the mixture amount of the remaining primary colors is larger, and the remaining primary colors are set. A color conversion method characterized in that the relief range is set to be smaller as the mixing amount of each of these is smaller. The color conversion method according to claim 1 or 2,
The setting of the relief range in step (b), a color conversion method characterized by not adversely rows when the mixing amount of the specific primary color is the value of the first region. The color conversion method according to any one of claims 1 to 3, wherein
A color conversion method, wherein a ratio of the second conversion range to the specified range is smaller than a ratio of the maximum relief range to the reproducible range. The color conversion method according to any one of claims 1 to 4, wherein:
The step (b)
The conversion from the mixing amount of the specific primary color to the signal intensity of the display signal corresponding to the specific primary color is performed using a conversion function representing a conversion relationship between the mixing amount and the signal intensity,
When the mixing amount of the specific primary color is a value in the first region , a conversion function including a first function is used, and the mixing amount of the specific primary color is a value within the maximum relief range or the second If the value is within a region, a conversion function including a second function that is different from the first function and that changes based on the mixture amount of the remaining primary colors other than the specific primary color is used. Characteristic color conversion method. The additive color mixture primary colors each having a specific chromaticity coordinate are mixed in a mixing amount corresponding to the signal intensity of the display signal input corresponding to each of the additive color mixture primary colors, thereby displaying the color, A color display device that can take a value within a predetermined specified range as the signal intensity of the display signal, and a color display device that can take a value within a predetermined reproducible range as the mixing amount according to the signal intensity. A color conversion method for converting the display signal to be input to a device into a colorimetric value,
(A) converting the signal intensity of the display signal corresponding to each of the primary colors of the additive color mixture into a mixing amount of the primary colors of the additive color mixture for each of the primary colors of the additive color mixture;
(B) converting each mixing amount of the primary color of the additive color mixture obtained by conversion into the colorimetric value;
With
The step (a)
Among the additive primary colors, at least one specific primary color is out of the reproducible range and set in advance based on the mixture amount of the remaining primary colors other than the specific primary color obtained in advance. Within the maximum relief range, set a relief range,
When the signal intensity of the display signal corresponding to the specific primary color is a value within a first conversion range set in advance within the specified range, the mixture amount of the specific primary color is within the reproducible range . Is converted into a value in the first area set in advance ,
When the signal intensity of the display signal corresponding to the specific primary color is a value within the remaining second conversion range other than the first conversion range within the specified range, the mixing amount of the specific primary color Converting to a value outside the reproducible range and within the relief range or a value within the remaining second region other than the first region within the reproducible range. Characteristic color conversion method. The additive color mixture primary colors each having a specific chromaticity coordinate are mixed in a mixing amount corresponding to the signal intensity of the display signal input corresponding to each of the additive color mixture primary colors, thereby displaying the color, For a color display device that can take a value within a predetermined specified range as the signal intensity of the display signal, and a color display device that can take a value within a predetermined reproducible range as the mixing amount according to the signal intensity, A color conversion device that converts the colorimetric value into the display signal in order to obtain the display signal to be input to the color display device from:
First conversion means for converting the colorimetric values into respective mixed amounts of the additive mixed primary colors;
A second conversion for converting each additive amount of the additive-mixed primary colors obtained by the conversion into a signal intensity of the display signal corresponding to each of the additive-mixed primary colors for each of the additive-mixed primary colors Means,
With
The second conversion means includes
Among the additive primary colors, at least one specific primary color is out of the reproducible range on the basis of the mixture amounts of the remaining primary colors other than the specific primary color obtained by conversion by the first conversion means. And within the maximum relief range set in advance, the relief range is set,
When the mixing amount of the specific primary color obtained by the conversion by the first conversion means is a value in the first region preset within the reproducible range , the signal intensity of the display signal As a value within the first conversion range set in advance within the specified range,
When the mixing amount of the specific primary color is a value outside the reproducible range and a value within the relief range , or the remaining second region other than the first region within the reproducible range the value in the area, said as a signal strength of the display signal, the specified range color conversion apparatus characterized by converting the value of the remaining second conversion range other than the first conversion range in the . A lookup table comprising representative points for color conversion is provided, and the signal intensity of the display signal respectively corresponding to the primary color of the additive color mixture obtained by conversion by the color conversion device according to claim 7 A color conversion device that converts a value into a predetermined color system using a representative point of a conversion lookup table and an interpolation operation based on the point,
The color conversion lookup table includes the first conversion range and the second conversion among combinations of signal intensity values of the display signal respectively corresponding to the additive primary color obtained by conversion. At least a specific combination including a boundary value located at the boundary with the range as a signal intensity value of the display signal corresponding to any one of the specific primary colors is used as an address value of the color conversion lookup table With
The color conversion apparatus, wherein an address represented by the specific combination stores a value of the color system obtained by converting the specific combination. The additive color mixture primary colors each having a specific chromaticity coordinate are mixed in a mixing amount corresponding to the signal intensity of the display signal input corresponding to each of the additive color mixture primary colors, thereby displaying the color, For a color display device that can take a value within a predetermined specified range as the signal intensity of the display signal, and a color display device that can take a value within a predetermined reproducible range as the mixing amount according to the signal intensity, A computer-readable recording medium recording a computer program for converting the colorimetric values into the display signals in order to obtain the display signals to be input to the color display device from:
A first function for converting the colorimetric value into each additive amount of the additive primary color;
Each additive amount of the additive mixed primary colors obtained by conversion is converted into signal intensity of the display signal corresponding to each of the additive mixed primary colors for each of the additive mixed primary colors,
Of the additive primary colors, at least one specific primary color is out of the reproducible range based on the mixture amount of the remaining primary colors other than the specific primary color obtained by conversion by the first function. And within a maximum relief range set in advance, a relief range is set,
When the mixing amount of the specific primary color obtained by the conversion by the first function is a value within the first region preset within the reproducible range , the signal intensity of the display signal is , Converted into a value within a first conversion range set in advance within the specified range,
The mixing amount of the specific primary color is a value outside the reproducible range, and a value within the relief range , or a remaining second region other than the first region within the reproducible range. If it is a value, the second function of converting the signal strength of the display signal into a value in the remaining second conversion range other than the first conversion range within the specified range,
A recording medium on which a computer program to be realized by a computer is recorded.

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