JP3664836B2

JP3664836B2 - Color conversion method

Info

Publication number: JP3664836B2
Application number: JP07252497A
Authority: JP
Inventors: 央光加藤木; 敏男野村
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1997-03-26
Filing date: 1997-03-26
Publication date: 2005-06-29
Anticipated expiration: 2017-03-26
Also published as: JPH10271343A

Description

【０００１】
【発明の属する技術分野】
本発明はカラー画像処理方法に関し、例えば高速の色修正、色補正が必要なカラースキャナ、カラーカメラ、カラーハードコピー装置やカラー画像入力機器、表示機器、出力機器等で、同一の色再現特性を得るためのカラー画像処理方法に関するものである。
【０００２】
【従来の技術】
従来、カラー印刷、カラーハードコピーの分野での色変換方式としてはルックアップテーブルを用いた３次元補間が提案されており、例えば特開昭６３−１６２２４８号公報に示されている。
【０００３】
これは図１５の（ａ）のように、３次元色空間を複数の単位立方体に分割し、各立方体図形の格子点に予め計算により求めた色補正値を設定しておき、入力色が含まれる単位立方体を選択し、該単位補間立方体の複数の頂点での出力値を用いて色変換を行う方法である。この例では、図１５の（ｂ）のように補間処理に単位立方体の８個の頂点の値を用いていることが特徴であり、求めるべき修正値の点の反対の頂点と、入力点で作られる直方体の空間領域の体積を、求めるべき修正値の点における重み係数とし修正値を求めている。
【０００４】
【発明が解決しようとする課題】
しかしながら、上記の従来における色変換方式では、細かく入力色空間を分割するほど、すなわち、格子点間の距離を小さくするほど補間精度は高くなるが、反対に格子点が増加してしまう。このため、格子点に設定する色補正値を格納するメモリ容量が多くなり、その計算のための処理時間も増加し、ハード構成においても複雑になる。さらに、格納する補正値自体を求めることにも非常に時間がかかる。
【０００５】
また、一般の３次元色空間の各軸はその必要な精度が不均一であることが特徴であり、このため色変換に補間方式を使用する場合に、補間に使用するテーブルメモリの各軸のビット数は不均一であることが望ましく、総ビット数が一定の場合にはこのように不均一なビット構成をとることによりメモリ容量を増加させずに人間の視覚特性に合った高品質の色情報の補間が可能となる。しかし、従来の技術では入力信号が作る３次元色空間内の単位立方体を補間の基本単位としているため、人間の視覚特性を考慮した補間ができないという問題があった。
【０００６】
本発明は上記に鑑みてなされたものであって、３次元色空間を細かく分割して色補正を行うのではなく、特定の軸上にのみあらかじめ最適な色修正信号データを離散的に配置しておき、色変換の際に入力色の近傍の軸上に離散的に配置されている代表点を選択し重みづけで線形補間することにより色変換を行い、これによってルックアップテーブルデータが極力削減でき、簡易なハード構成で好ましい色変換処理を実現することを目的とする。
【０００７】
【課題を解決するための手段】
本願の第１の発明にかかる色変換方法は、第１の３次元色空間上の任意の座標から第２の３次元色空間上の所定の座標への変換を行う色変換方法であって、前記第１の３次元色空間において、原点を中心とし互いに直交する３本の軸Ｘａ、Ｘｂ、Ｘｃと前記３本の直交軸とは別のＮ本の直線とで計Ｎ＋３本の軸を設定し、前記Ｎ＋３本の軸上に離散的に所定数の代表点を配置し、かつ前記第１の３次元色空間上の各座標を前記Ｎ＋３本の軸との距離に従って複数の領域に分割した場合に、入力色信号値が前記複数領域のいずれに属するかを特定する領域判定工程と、前記入力色信号値に近い前記代表点を前記特定された領域の境界上に存在する前記Ｎ本の軸の少なくとも１本を含む３本の軸上でそれぞれ１つずつ選出して３つの代表点を得る代表点選出工程と、前記第１の３次元色空間から前記第２の３次元色空間への変換に際して、前記第１の３次元色空間上の入力色信号値を、該入力色信号値に対応する前記３つの代表点と、前記第１の３次元色空間における前記代表点が前記第２の３次元色空間のどの座標に変換されるかを示すルックアップテーブルとを用いて、線形もしくは所定の方法により色変換を行う補間工程とを有するものである。
【０００８】
本願の第２の発明にかかる色変換方法は、前記ルックアップテーブルを複数用意し、ユーザーが前記複数のルックアップテーブルを切り替えるものである。
本願の第３の発明にかかる色変換方法は、前記ルックアップテーブルを複数用意し、入力画像の色濃度値の統計量に応じて、前記複数のルックアップテーブルを自動的に切り替えるものである。
【０００９】
本願の第４の発明にかかる色変換方法は、第１の３次元色空間上の任意の座標から第２の３次元色空間上の所定の座標への変換を行う色変換方法であって、前記第１の３次元色空間において、原点を中心とし互いに直交する３本の軸Ｘａ、Ｘｂ、Ｘｃと前記３本の直交軸とは別のＮ本の直線とで計Ｎ＋３本の軸を設定し、前記Ｎ＋３本の軸上に離散的に所定数の代表点を配置し、かつ前記第１の３次元色空間上の各座標を前記Ｎ＋３本の軸との距離に従って複数の領域に分割した場合に、入力色信号値が前記複数領域のいずれに属するかを特定する領域判定工程と、前記入力色信号値に近い前記代表点を前記特定された領域の境界上に存在する前記Ｎ本の軸の少なくとも１本を含む３本の軸上でそれぞれ１つずつ選出して３つの代表点を得る代表点選出工程と、前記第１の３次元色空間から前記第２の３次元色空間への変換に際して、前記第１の３次元色空間上の入力色信号値を、該入力色信号値に対応する前記３つの代表点と、前記第１の３次元色空間における前記代表点が前記第２の３次元色空間のどの座標に変換されるかを定義した数式を用いて、線形もしくは所定の方法により色変換を行う補間工程とを有するものである。
【００１０】
本願の第５の発明にかかる色変換方法は、前記数式を複数用意し、ユーザーが前記複数の数式を切り替えるものである。
本願の第６の発明にかかる色変換方法は、前記数式を複数用意し、入力画像の色濃度値の統計量に応じて、前記複数の数式を自動的に切り替えるものである。
【００１１】
本願の第７の発明にかかる色変換方法は、Ｎ＝１とし、前記３本の直交軸Ｘａ、Ｘｂ、Ｘｃとは別にＸａ＝Ｘｂ＝Ｘｃを満たす１本の直線を設定するものである。
【００１２】
本願の第８の発明にかかる色変換方法は、前記３本の直交軸Ｘａ、Ｘｂ、Ｘｃとは別に設定する直線として、Ｘａ＝Ｘｂ、Ｘｂ＝Ｘｃ、Ｘｃ＝Ｘａの直線を軸として含むもしくは追加するものである。
【００１３】
本願の第９の発明にかかる色変換方法は、前記３本の直交軸Ｘａ、Ｘｂ、Ｘｃとは別に設定する直線として、前記第１の３次元色空間における３成分がそれぞれ最大となる座標を通る３本の直線を軸として含むもしくは追加するものである。
【００１４】
本願の第１０の発明にかかる色変換方法は、Ｘａ＝Ｘｂ＝Ｘｃ軸上にある離散的な代表点配置密度を他の軸より高くするものである。
【００１５】
本願の第１１の発明にかかる色変換方法は、離散的な代表点の配置密度を中間濃度部で高めるものである。
【００１６】
本願の第１２の発明にかかる色変換方法は、前記入力色信号値が対応する３本の軸に対してそれぞれ該入力色信号値から垂線をおろした足を３つの新たな代表点とし、それぞれの新たな代表点について前記第１の３次元色空間における前記新たな代表点が前記第２の３次元色空間のどの座標に変換されるかを、前記新たな代表点と同一軸上でかつ両側に配置されている２つの代表点を用いて決定するものである。
【００１７】
【発明の実施の形態】
以下、本発明に係る色変換装置の実施の形態を添付図面に基づいて説明する。
【００１８】
〔実施の形態１〕図１は実施の形態１に係る色変換処理部の概略構成を示すブロック図であり、入力値が３次元色空間内のどの領域に存在するかを判定する領域判定部１０１、それぞれ軸上に離散的に配置されている代表点を選出する代表点選出部１０２、代表点の座標と色補正データを格納したルックアップテーブル（ＬＵＴ）１０３、選択された代表点を基に重みづけによって色変換を行う線形補間部１０４によって構成される。
【００１９】
図２に代表点の配置を模式的に示す。図２において代表点はＣ軸、Ｍ軸、Ｙ軸、Ｃ＝Ｍ＝Ｙ軸の４本の軸上に離散的に配置されており、ここではＬＵＴ１０３はそれぞれの代表点の座標に対して、図３のような色補正データを持つものとするが、代表点の数および配置間隔はこれに限定されない。図３の色補正データは、本発明の色変換を適用する機器の特性に応じて決定されるものとする。
【００２０】
３次元色空間は、図２に示した４本の軸のうちから入力値と距離の近い順に軸を３本考えた場合に、どの軸が含まれるかによって３つの領域に分けられる。それぞれの領域は原点Ｏを頂点とし、Ｃ軸、Ｍ軸、Ｙ軸のうちの２本とＣ＝Ｍ＝Ｙ軸を３つの母線とする三角錐が、色空間を表す立方体で切りとられたものである。これは、四角錐ＯＡ１Ａ２Ａ３Ａ６、ＯＡ４Ａ５Ａ１Ａ６、ＯＡ３Ａ７Ａ４Ａ６となる。
【００２１】
領域判定部１０１は、入力値の３成分の大小関係により入力値が上記３つのどの領域に属するかを判定する。それぞれの領域は図２に示した４本の軸のうち３本を含むが、代表点選出部１０２によってそれぞれの軸から入力値に最も近い代表点を選ぶ。今、図４（ａ）に示すように入力値をＰ（Ｃ，Ｍ，Ｙ）とし、代表点選出部１０２が近傍３点として点Ｐ１（Ｃ１，Ｍ１，Ｙ１）、Ｐ２（Ｃ２，Ｍ２，Ｙ２）、Ｐ３（Ｃ３，Ｍ３，Ｙ３）を選んだとすると、出力値Ｐ’（Ｃ’，Ｍ’，Ｙ’）は次式で与えられる。
【００２２】
【数１】

【００２３】
ここでＰ’１（Ｃ’１，Ｍ’１，Ｙ’１）、Ｐ’２（Ｃ’２，Ｍ’２，Ｙ’２）、Ｐ’３（Ｃ’３，Ｍ’３，Ｙ’３）は図４（ｂ）に示すように、それぞれ点Ｐ１、Ｐ２、Ｐ３の色補正データを表す。
【００２４】
なお、入力値ＰがＣ軸、Ｍ軸、Ｙ軸、Ｃ＝Ｍ＝Ｙ軸のいずれかの軸上にある場合は、その軸上から入力値に最も近い２点を代表点選出部１０２によって選出し、その２点を用いた線形補間によって出力値Ｐ’を求める。
【００２５】
例えば、図５に示すように入力値Ｐと点Ｐ１、点Ｐ２が同一軸上にあり、入力値Ｐに最も近い２点として点Ｐ１、点Ｐ２を選出し、入力値Ｐが線分Ｐ１Ｐ２をＬ２：Ｌ１に内分しているとすると、出力値Ｐ’（Ｃ’，Ｍ’，Ｙ’）は次式で与えられる。
【００２６】
【数２】

【００２７】
また、入力値がＣＭ平面、ＭＹ平面、ＹＣ平面上に存在する場合は、近傍の対象となる２本の軸上より代表点をぞれぞれ選出し、重みづけによって線形補間を行う。
【００２８】
〔実施の形態２〕次に本発明の第２の実施の形態について説明する。本実施の形態の概略構成は実施の形態１と同様に図１に示されるが、ここでは代表点を配置する軸を増やしており、本実施の形態における代表点の配置を図６に示す。図６では図２と比べて代表点を配置する軸を以下のように３本追加している。すなわち、代表点はＣ軸、Ｍ軸、Ｙ軸、Ｃ＝Ｍ＝Ｙ軸、Ｃ＝Ｍ軸（Ｙ＝０）、Ｍ＝Ｙ軸（Ｃ＝０）、Ｙ＝Ｃ軸（Ｍ＝０）の７本の軸上に離散的に配置されており、ここではＬＵＴ１０３はそれぞれの代表点の座標に対して、図７のような色補正データを持つものとする。
【００２９】
代表点の数および配置間隔はこれに限定されない。図７の色補正データは、本発明の色変換を適用する機器の特性に応じて決定されるものとする。３次元色空間は図６に示した７本の軸のうちから入力値と距離の近い順に軸を３本考えた場合に、どの軸が含まれるかによって６つの領域に分けられる。ぞれぞれの領域は原点Ｏを頂点とし、Ｃ軸、Ｍ軸、Ｙ軸、Ｃ＝Ｍ軸（Ｙ＝０）、Ｍ＝Ｙ軸（Ｃ＝０）、Ｙ＝Ｃ軸（Ｍ＝０）のうち近隣の２本とＣ＝Ｍ＝Ｙ軸を母線とする三角錘である。これは、三角錘ＯＡ１Ａ２Ａ６、ＯＡ１Ａ５Ａ６、ＯＡ２Ａ３Ａ６、ＯＡ３Ａ６Ａ７、ＯＡ４Ａ５Ａ６、ＯＡ４Ａ６Ａ７となる。
【００３０】
図１の領域判定部１０１によって、入力値が上記６つのどの領域に属するかを判定し、以下実施の形態１と同じ手順で色変換処理を行う。また、入力値がＣ＝Ｍ平面、Ｍ＝Ｙ平面、Ｙ＝Ｃ平面上に存在する場合には、近傍の対象となる２本の軸より代表点をそれぞれ選出し、重みづけによって線形補間を行う。本実施の形態ではＬＵＴの代表点を配置する軸が増えているため、より高精度の色変換が可能となる。
【００３１】
〔実施の形態３〕次に本発明の第３の実施の形態について説明する。本実施の形態の概略構成は実施の形態２と同様に図１に示されるが、ここでは代表点を配置する軸を増やしており、本実施の形態における代表点の配置を図８に示す。図８では図２と比べて代表点を配置する軸を以下のように３本追加している。すなわち、代表点は新たに点Ａ６（１００，１００，１００）を中心とし、互いに直交する３本の軸（Ｃ＝Ｍ＝１００となる軸、Ｍ＝Ｙ＝１００となる軸、Ｙ＝Ｃ＝１００となる軸）を追加した計７本の軸上に離散的に配置されており、ここでＬＵＴ１０３はそれぞれの代表点の座標に対して、図９の色補正データを持つものとする。代表点の数および配置間隔はこれに限定されない。
【００３２】
図９の色補正データは、本発明の色変換を適用する機器の特性に応じて決定されるものとする。３次元色空間は図８に示した７本の軸のうちから入力値と距離の近い順に軸を３本考えた場合に、どの軸が含まれるかによって９つの領域に分けられる。
【００３３】
図１の領域判定部１０１によって、入力値が９つのどの領域に属するかを判定され、以下実施の形態２と同じ手順で色変換を行う。本実施の形態では新たにＬＵＴの代表点の配置軸を上記３本追加したことにより、高濃度部において色変換の精度向上を図ることができる。
【００３４】
〔実施の形態４〕次に、本発明の第４の実施の形態について説明する。代表点の配置によって画像の色変換精度は大きく異なるが、実施の形態１ではＣ＝Ｍ＝Ｙ軸は常に選出対象となるため、Ｃ＝Ｍ＝Ｙ軸上のデータ配置密度を高くすることにより、より高精度な色変換を行うことが可能となる。
【００３５】
〔実施の形態５〕次に、本発明の第５の実施の形態について説明する。濃度が非常に高い領域、あるいは非常に低い領域では、人間の目は色の違いにそれほど敏感でないため、これらの濃度領域の変換精度は相対的に低くてもよい。また、自然画像の場合、３次元色空間内の色データ分布は比較的中間濃度に集まりやすい。このため、３次元色空間において中間濃度部に代表点を多く配置しておくことにより、メモリ量を増加させることなく好ましい色変換が可能となる。
【００３６】
〔実施の形態６〕次に、本発明の第６の実施の形態について説明する。本実施の形態のブロック図を図１０に示すが、ＬＵＴ選択部４０１と特性の異なるＬＵＴ１（４０２）、ＬＵＴ２（４０３）、ＬＵＴ３（４０４）を備えている。ＬＵＴの数は３つに限定されない。ユーザーはＬＵＴ選択部４０１によってＬＵＴを選択することにより、自分の好みに応じて出力画像の色合いを調整できる。
【００３７】
〔実施の形態７〕次に、本発明の第７の実施の形態について説明する。本実施の形態のブロック図を図１１に示すが、フレームメモリ５０１、ヒストグラム生成部５０２を新たに設け、特性の異なるＬＵＴ１（５０４）、ＬＵＴ２（５０５）、ＬＵＴ３（５０６）を備えている。ＬＵＴの数は３つに限定されない。例えば、ＬＵＴ１は低濃度部の代表点配置密度を高くしたもの、ＬＵＴ２は中間濃度部の代表点配置密度を高くしたもの、ＬＵＴ３は高濃度部の代表点配置密度を高くしたものとする。
【００３８】
まず、フレームメモリ５０１に入力画像を読み込み、画像データのヒストグラムをヒストグラム生成部５０２で生成する。こうして得られたヒストグラムに基づき、低濃度部の分布が多い場合にはＬＵＴ１を選択し、中間濃度部の分布が多い場合にはＬＵＴ２を選択し、高濃度部の分布が多い場合にはＬＵＴ３を選択するようにする。このようにして、入力画像に合ったＬＵＴを自動的に選択することによって、より好ましい色変換が可能となる。
【００３９】
〔実施の形態８〕次に、本発明の第８の実施の形態について説明する。本実施の形態のブロック図を図１２に示す。まず、領域判定部１０１により入力点の存在領域が定まると、入力点に近い軸が３本求まる。次に、垂線の足探索部６０１によって、図１３に示すように入力点から近傍の軸へ垂線を下ろす。点Ｈ１、Ｈ２、Ｈ３は入力点Ｐから近傍の３本の軸へ垂線を下ろした時の垂線の足である。
【００４０】
次に、新代表点生成部６０５によってこの点Ｈ１、Ｈ２、Ｈ３での色補正データを、これらの点と同一軸上にあって、かつその点の両側に位置する代表点２点から内分することにより求める。こうして垂線の足における色補正データが求まれば、それらを用いて線形補間によって実施の形態１と同じ手順で色変換処理を行う。本発明は入力点により近いデータを生成して線形補間に使用することができるため、高精度な色変換が可能となる。
【００４１】
〔実施の形態９〕次に、本発明の第９の実施の形態について説明する。本実施の形態のブロック図を図１４に示す。これは、軸上に代表点を離散的に配置するのではなく、軸ごとに色補正のための数式を設けたものである。したがってＬＵＴを必要としない。領域判定部１０１により入力点の存在領域が定まると、入力点に近い軸が３本求まる。次に、垂線の足探索部６０１によって、図１３に示すように入力点から近傍の３本の軸へ垂線を下ろし、垂線の足を３点求める。
【００４２】
ここで、各軸ごとに色変換の数式が設定されているので、先に求めた垂線の足の座標値における色変換データを変換データ生成部６０２によって求める。こうしてそれぞれ近傍の３点の色変換データを求め、それらを用いて線形補間によって実施の形態１と同じ手順で色変換処理を行う。本実施の形態ではＬＵＴを必要としないことから、メモリ容量の大幅な削減が可能である。
【００４３】
以上の実施の形態は、既にカラープリンタにおいて、実施の形態の特徴である少ない演算量で従来例の色変換と同等の色変換がなし得ることを主観評価で確認しているが、これらの実施の形態は色変換対象がカラープリンタのみに限定されるのではなく、スキャナや、ＣＲＴモニタの色変換を行う場合等、種々の画像入力装置や画像出力装置に対しても同様に適用できる。
【００４４】
【発明の効果】
以上説明したように、本発明に係る色変換方法によれば、あらかじめ３次元色空間上のそれぞれの軸上にのみ離散的に代表点を配置しておき、必要最小限のＬＵＴで色変換ができるため、メモリ容量を削減することができ、３×３のマトリクス演算と小さなＬＵＴのみで高精度の色変換を行うことができ、その演算量および回路規模の削減効果は極めて顕著である。
【００４５】
また、本発明に係る色変換方法によれば、ＬＵＴの代わりに色補正のための数式を設定することにより、ＬＵＴデータ用のメモリを全く持たないことが可能である。
【００４６】
また、本発明に係る色変換方法によれば、ユーザーは複数のＬＵＴ又は数式の中から、適当なＬＵＴ又は数式を選択することにより、自分の好みに応じて出力画像の色合いを調整することができる。
【００４７】
また、本発明に係る色変換方法によれば、入力画像に応じて変換特性の異なるＬＵＴ又は数式を選択することにより、自動的に入力画像の性質に適合した色変換を行うことができる。
【００４８】
また、本発明に係る色変換方法によれば、代表点の数を増やすことにより、色変換精度の向上を図ることができる。
【００４９】
また、本発明に係る色変換方法によれば、代表点を高濃度領域にまで配置することにより、高濃度部において色変換の精度を向上させることが可能である。
【００５０】
また、本発明に係る色変換方法によれば、領域分割の際に、常にＸａ＝Ｘｂ＝Ｘｃ軸上の代表点を含んでいるため、この軸上の代表点の配置密度を高くすることによって、色変換精度の向上を図ることができる。
【００５１】
また、本発明に係る色変換方法によれば、代表点を軸上に等間隔に配置する必要がなく、代表点の配置を自由に変化させることが可能となり、３次元色空間内における代表点を人間の視覚特性を考慮して配置することができる。
【００５２】
また、本発明に係る色変換方法によれば、代表点の配置を部分的に集めることにより、メモリ容量を増大させることなく、色再現性を向上させることができる。
【００５３】
また、本発明に係る色変換方法によれば、軸上で入力点に最も近い点のデータを同一軸上で近隣の代表点２点から内分によって新たに生成することにより、高精度に色変換することができる。
【図面の簡単な説明】
【図１】実施の形態１に係る色変換部の概略構成を示すブロック図である。
【図２】実施の形態１に係るＬＵＴの代表点配置の概略図である。
【図３】実施の形態１に係るＬＵＴデータ対応図である。
【図４】近傍３点による線形補間の概略を示す説明図である。
【図５】軸上での線形補間の概略を示す説明図である。
【図６】実施の形態２に係るＬＵＴの代表点配置の概略図である。
【図７】実施の形態２に係るＬＵＴデータ対応図である。
【図８】実施の形態３に係るＬＵＴの代表点配置の概略図である。
【図９】実施の形態３に係るＬＵＴデータ対応図である。
【図１０】実施の形態６に係る色変換部の概略構成を示すブロック図である。
【図１１】実施の形態７に係る色変換部の概略構成を示すブロック図である。
【図１２】実施の形態８に係る色変換部の概略構成を示すブロック図である。
【図１３】実施の形態８において３次元色空間で入力点から近傍軸に対して求めた垂線の足の説明図である。
【図１４】実施の形態９に係る色変換部の概略構成を示すブロック図である。
【図１５】従来における代表点配置例の概略図である。
【符号の説明】
１０１領域判定部
１０２代表点選出部
１０３ＬＵＴ
１０４線形補間部
４０１、５０３ＬＵＴ選択部
４０２、５０４ＬＵＴ１
４０３、５０５ＬＵＴ２
４０４、５０６ＬＵＴ３
５０１フレームメモリ
５０２ヒストグラム生成部
６０１垂線の足探索部
６０２変換データ生成部
６０５新代表点生成部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a color image processing method, for example, a color scanner, a color camera, a color hard copy apparatus, a color image input device, a display device, and an output device that require high-speed color correction and color correction, and the same color reproduction characteristics. The present invention relates to a color image processing method for obtaining.
[0002]
[Prior art]
Conventionally, as a color conversion method in the fields of color printing and color hard copy, three-dimensional interpolation using a look-up table has been proposed, and is disclosed in, for example, Japanese Patent Laid-Open No. 63-162248.
[0003]
As shown in FIG. 15 (a), the three-dimensional color space is divided into a plurality of unit cubes, and color correction values obtained by calculation are set in advance at the lattice points of each cubic figure to include the input color. The unit cube is selected, and color conversion is performed using output values at a plurality of vertices of the unit interpolation cube. This example is characterized in that the values of eight vertices of the unit cube are used for the interpolation processing as shown in FIG. 15B, and the vertices opposite to the correction value points to be obtained and the input points are used. The correction value is obtained using the volume of the space region of the rectangular parallelepiped to be created as a weighting coefficient at the point of the correction value to be obtained.
[0004]
[Problems to be solved by the invention]
However, in the above conventional color conversion method, the finer the input color space is divided, that is, the smaller the distance between lattice points is, the higher the interpolation accuracy is. However, the number of lattice points increases. For this reason, the memory capacity for storing the color correction value set at the grid point increases, the processing time for the calculation increases, and the hardware configuration becomes complicated. Furthermore, it takes a very long time to obtain the correction value itself to be stored.
[0005]
Further, each axis of a general three-dimensional color space is characterized in that the required accuracy is non-uniform. Therefore, when an interpolation method is used for color conversion, each axis of the table memory used for interpolation is It is desirable that the number of bits is non-uniform, and when the total number of bits is constant, this non-uniform bit configuration ensures high-quality colors that match human visual characteristics without increasing memory capacity. Interpolation of information is possible. However, the conventional technique uses a unit cube in a three-dimensional color space created by an input signal as a basic unit of interpolation, and thus has a problem that interpolation considering human visual characteristics cannot be performed.
[0006]
The present invention has been made in view of the above. Instead of finely dividing a three-dimensional color space and performing color correction, optimal color correction signal data is discretely arranged in advance only on a specific axis. In advance, color conversion is performed by selecting representative points that are discretely arranged on the axis near the input color and performing linear interpolation with weighting, thereby reducing lookup table data as much as possible. It is possible to achieve a preferable color conversion process with a simple hardware configuration.
[0007]
[Means for Solving the Problems]
A color conversion method according to a first invention of the present application is a color conversion method for performing conversion from arbitrary coordinates on a first three-dimensional color space to predetermined coordinates on a second three-dimensional color space, In the first three-dimensional color space, a total of N + 3 axes are set by three axes Xa, Xb, Xc orthogonal to each other centered on the origin and N straight lines different from the three orthogonal axes. Then, a predetermined number of representative points are discretely arranged on the N + 3 axes, and each coordinate on the first three-dimensional color space is divided into a plurality of regions according to the distance from the N + 3 axes. A region determination step for specifying which of the plurality of regions the input color signal value belongs to, and the representative points that are close to the input color signal value on the boundary of the specified region . obtaining three representative points by selecting one each on the three axes, including at least one axis In the surface point selection step and the conversion from the first three-dimensional color space to the second three-dimensional color space, the input color signal value in the first three-dimensional color space is converted into the input color signal value. Using the corresponding three representative points and a look-up table indicating to which coordinate in the second three-dimensional color space the representative point in the first three-dimensional color space is converted, And an interpolation step for performing color conversion by a predetermined method.
[0008]
In the color conversion method according to the second invention of the present application, a plurality of lookup tables are prepared, and a user switches between the plurality of lookup tables.
A color conversion method according to a third invention of the present application is to prepare a plurality of the look-up tables and automatically switch the plurality of look-up tables according to the statistic of the color density value of the input image .
[0009]
A color conversion method according to a fourth invention of the present application is a color conversion method for performing conversion from arbitrary coordinates on the first three-dimensional color space to predetermined coordinates on the second three-dimensional color space, In the first three-dimensional color space, a total of N + 3 axes are set by three axes Xa, Xb, Xc orthogonal to each other centered on the origin and N straight lines different from the three orthogonal axes. Then, a predetermined number of representative points are discretely arranged on the N + 3 axes, and each coordinate on the first three-dimensional color space is divided into a plurality of regions according to the distance from the N + 3 axes. A region determination step for specifying which of the plurality of regions the input color signal value belongs to, and the representative points that are close to the input color signal value on the boundary of the specified region . obtaining three representative points by selecting one each on the three axes, including at least one axis In the surface point selection step and the conversion from the first three-dimensional color space to the second three-dimensional color space, the input color signal value in the first three-dimensional color space is converted into the input color signal value. Using the corresponding three representative points and a mathematical formula that defines which coordinates in the second three-dimensional color space are converted into the representative points in the first three-dimensional color space, linear or predetermined And an interpolation step for performing color conversion by the method.
[0010]
A color conversion method according to a fifth invention of the present application is a method in which a plurality of the mathematical expressions are prepared, and a user switches the plurality of mathematical expressions.
A color conversion method according to a sixth invention of the present application prepares a plurality of the mathematical expressions, and automatically switches the mathematical expressions according to a statistic of the color density value of the input image .
[0011]
In the color conversion method according to the seventh aspect of the present invention, N = 1 and a single straight line that satisfies Xa = Xb = Xc is set apart from the three orthogonal axes Xa, Xb, and Xc .
[0012]
The color conversion method according to the eighth invention of the present application includes, as the straight lines set separately from the three orthogonal axes Xa, Xb, and Xc, Xa = Xb, Xb = Xc, and Xc = Xa. It is something to add .
[0013]
In the color conversion method according to the ninth aspect of the present application, as the straight line set separately from the three orthogonal axes Xa, Xb, and Xc, the coordinates at which the three components in the first three-dimensional color space are maximized are set. It includes or adds three straight lines that pass through as axes .
[0014]
The color conversion method according to the tenth invention of the present application is to make the discrete representative point arrangement density on the Xa = Xb = Xc axis higher than the other axes .
[0015]
In the color conversion method according to the eleventh aspect of the present application, the arrangement density of discrete representative points is increased at the intermediate density portion .
[0016]
In the color conversion method according to the twelfth aspect of the present invention, three new representative points are obtained by setting the legs perpendicular to the input color signal values for the three axes corresponding to the input color signal values, respectively. The new representative point in the first three-dimensional color space is converted to the coordinate in the second three-dimensional color space on the same axis as the new representative point and This is determined using two representative points arranged on both sides .
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of a color conversion apparatus according to the present invention will be described below with reference to the accompanying drawings.
[0018]
Embodiment 1 FIG. 1 is a block diagram illustrating a schematic configuration of a color conversion processing unit according to Embodiment 1, and an area determination unit that determines in which area in a three-dimensional color space an input value exists. 101, a representative point selection unit 102 for selecting representative points discretely arranged on each axis, a lookup table (LUT) 103 storing coordinates and color correction data of representative points, and based on the selected representative points The linear interpolation unit 104 performs color conversion by weighting.
[0019]
FIG. 2 schematically shows the arrangement of representative points. In FIG. 2, the representative points are discretely arranged on the four axes of C axis, M axis, Y axis, and C = M = Y axis. Here, the LUT 103 is arranged with respect to the coordinates of each representative point. Although the color correction data as shown in FIG. 3 is assumed, the number of representative points and the arrangement interval are not limited to this. The color correction data in FIG. 3 is determined according to the characteristics of the device to which the color conversion of the present invention is applied.
[0020]
The three-dimensional color space is divided into three regions depending on which axis is included when three axes are considered in the order of distance from the input value among the four axes shown in FIG. In each region, a triangular pyramid with the origin O as the apex, two of the C, M, and Y axes and the three buses with the C = M = Y axis was cut in a cube representing the color space. Is. This is a quadrangular pyramid OA1A2A3A6, OA4A5A1A6, OA3A7A4A6.
[0021]
The area determination unit 101 determines which of the three areas the input value belongs based on the magnitude relationship of the three components of the input value. Each region includes three of the four axes shown in FIG. 2, and the representative point selection unit 102 selects a representative point closest to the input value from each axis. Now, as shown in FIG. 4A, the input value is P (C, M, Y), and the representative point selection unit 102 has three points in the vicinity as points P1 (C1, M1, Y1), P2 (C2, M2, If Y2) and P3 (C3, M3, Y3) are selected, the output value P ′ (C ′, M ′, Y ′) is given by the following equation.
[0022]
[Expression 1]

[0023]
Here, P′1 (C′1, M′1, Y′1), P′2 (C′2, M′2, Y′2), P′3 (C′3, M′3, Y ′) 3) represents color correction data of points P1, P2, and P3, respectively, as shown in FIG.
[0024]
If the input value P is on any of the C-axis, M-axis, Y-axis, and C = M = Y-axis, the representative point selection unit 102 selects the two points closest to the input value on that axis. An output value P ′ is obtained by selection and linear interpolation using the two points.
[0025]
For example, as shown in FIG. 5, the input value P, the point P1, and the point P2 are on the same axis, and the point P1 and the point P2 are selected as the two points closest to the input value P, and the input value P selects the line segment P1P2. Assuming that L2 is internally divided into L1, the output value P ′ (C ′, M ′, Y ′) is given by the following equation.
[0026]
[Expression 2]

[0027]
When the input value exists on the CM plane, the MY plane, and the YC plane, representative points are selected from two adjacent axes, and linear interpolation is performed by weighting.
[0028]
[Embodiment 2] Next, a second embodiment of the present invention will be described. Although the schematic configuration of the present embodiment is shown in FIG. 1 as in the first embodiment, the number of axes on which representative points are arranged is increased here, and the arrangement of representative points in the present embodiment is shown in FIG. In FIG. 6, compared to FIG. 2, three axes on which representative points are arranged are added as follows. That is, representative points are C axis, M axis, Y axis, C = M = Y axis, C = M axis (Y = 0), M = Y axis (C = 0), Y = C axis (M = 0). The LUT 103 is assumed to have color correction data as shown in FIG. 7 for the coordinates of each representative point.
[0029]
The number of representative points and the arrangement interval are not limited to this. The color correction data in FIG. 7 is determined according to the characteristics of the device to which the color conversion of the present invention is applied. The three-dimensional color space is divided into six regions depending on which axis is included when three axes are considered from the seven axes shown in FIG. Each region has the origin O as a vertex, C axis, M axis, Y axis, C = M axis (Y = 0), M = Y axis (C = 0), Y = C axis (M = 0) ) Are triangular pyramids with two neighboring buses and C = M = Y axis as buses. This is triangular pyramid OA1A2A6, OA1A5A6, OA2A3A6, OA3A6A7, OA4A5A6, OA4A6A7.
[0030]
The region determination unit 101 in FIG. 1 determines which of the six regions the input value belongs to, and performs color conversion processing in the same procedure as in the first embodiment. If the input values are on the C = M plane, M = Y plane, and Y = C plane, representative points are selected from the two target axes in the vicinity, and linear interpolation is performed by weighting. Do. In this embodiment, since the number of axes on which the LUT representative points are arranged is increased, more accurate color conversion is possible.
[0031]
[Third Embodiment] Next, a third embodiment of the present invention will be described. Although the schematic configuration of the present embodiment is shown in FIG. 1 as in the second embodiment, the number of axes on which representative points are arranged is increased here, and the arrangement of representative points in the present embodiment is shown in FIG. In FIG. 8, compared with FIG. 2, three axes on which representative points are arranged are added as follows. That is, the representative point is newly centered on the point A6 (100, 100, 100), and three axes orthogonal to each other (an axis where C = M = 100, an axis where M = Y = 100, Y = C = It is assumed that the LUT 103 has the color correction data of FIG. 9 for the coordinates of each representative point. The number of representative points and the arrangement interval are not limited to this.
[0032]
The color correction data in FIG. 9 is determined according to the characteristics of the device to which the color conversion of the present invention is applied. The three-dimensional color space is divided into nine regions depending on which axis is included when three axes are considered in order of distance from the input value from among the seven axes shown in FIG.
[0033]
The area determination unit 101 in FIG. 1 determines which of the nine areas the input value belongs to, and performs color conversion in the same procedure as in the second embodiment. In the present embodiment, the accuracy of color conversion can be improved in the high density portion by newly adding the three arrangement axes of the representative points of the LUT.
[0034]
[Embodiment 4] Next, a fourth embodiment of the present invention will be described. Although the color conversion accuracy of the image varies greatly depending on the arrangement of the representative points, in the first embodiment, the C = M = Y axis is always selected, so by increasing the data arrangement density on the C = M = Y axis. Therefore, it is possible to perform color conversion with higher accuracy.
[0035]
[Embodiment 5] Next, a fifth embodiment of the present invention will be described. In regions where the density is very high or very low, the human eye is not very sensitive to color differences, so the conversion accuracy of these density regions may be relatively low. In the case of a natural image, the color data distribution in the three-dimensional color space tends to gather at a relatively intermediate density. For this reason, by arranging many representative points in the intermediate density portion in the three-dimensional color space, it is possible to perform preferable color conversion without increasing the amount of memory.
[0036]
[Sixth Embodiment] Next, a sixth embodiment of the present invention will be described. FIG. 10 shows a block diagram of the present embodiment, which includes LUT1 (402), LUT2 (403), and LUT3 (404) having different characteristics from the LUT selection unit 401. The number of LUTs is not limited to three. The user can adjust the color of the output image according to his / her preference by selecting the LUT using the LUT selection unit 401.
[0037]
[Embodiment 7] Next, a seventh embodiment of the present invention will be described. A block diagram of the present embodiment is shown in FIG. 11. A frame memory 501 and a histogram generation unit 502 are newly provided, and are provided with LUT1 (504), LUT2 (505), and LUT3 (506) having different characteristics. The number of LUTs is not limited to three. For example, it is assumed that LUT1 has a higher representative point arrangement density in the low density portion, LUT2 has a higher representative point arrangement density in the intermediate density portion, and LUT3 has a higher representative point arrangement density in the high density portion.
[0038]
First, an input image is read into the frame memory 501, and a histogram of image data is generated by the histogram generation unit 502. Based on the histogram thus obtained, LUT1 is selected when the distribution of the low density portion is large, LUT2 is selected when the distribution of the intermediate density portion is large, and LUT3 is selected when the distribution of the high density portion is large. Make a selection. In this way, more preferable color conversion can be performed by automatically selecting an LUT suitable for the input image.
[0039]
[Eighth Embodiment] Next, an eighth embodiment of the present invention will be described. A block diagram of this embodiment is shown in FIG. First, when the region where the input point exists is determined by the region determination unit 101, three axes close to the input point are obtained. Next, the perpendicular foot search unit 601 draws a perpendicular line from the input point to a nearby axis as shown in FIG. Points H1, H2, and H3 are the feet of the perpendicular when the perpendicular is drawn from the input point P to the three adjacent axes.
[0040]
Next, the new representative point generation unit 605 internally converts the color correction data at these points H1, H2, and H3 from two representative points that are on the same axis as these points and are located on both sides of the points. To find out. If the color correction data at the foot of the perpendicular line is obtained in this way, the color conversion processing is performed by the same procedure as in the first embodiment by linear interpolation using them. In the present invention, data closer to the input point can be generated and used for linear interpolation, so that highly accurate color conversion is possible.
[0041]
[Ninth Embodiment] Next, a ninth embodiment of the present invention will be described. A block diagram of the present embodiment is shown in FIG. In this method, the representative points are not discretely arranged on the axis, but a mathematical formula for color correction is provided for each axis. Therefore, no LUT is required. When the region where the input point exists is determined by the region determination unit 101, three axes close to the input point are obtained. Next, as shown in FIG. 13, the perpendicular foot search unit 601 lowers the perpendicular from the input point to the three adjacent axes, and obtains three perpendicular feet.
[0042]
Here, since a color conversion formula is set for each axis, the conversion data generation unit 602 determines the color conversion data at the coordinate value of the foot of the vertical line obtained previously. In this way, color conversion data of three neighboring points are obtained, and color conversion processing is performed by the same procedure as in the first embodiment by linear interpolation using them. Since this embodiment does not require an LUT, the memory capacity can be greatly reduced.
[0043]
The above embodiments have already confirmed by subjective evaluation that color conversion equivalent to the color conversion of the conventional example can be performed with a small amount of calculation, which is a feature of the embodiments, in the color printer. The color conversion target is not limited to a color printer, but can be similarly applied to various image input devices and image output devices such as when performing color conversion of a scanner or a CRT monitor.
[0044]
【The invention's effect】
As described above, according to the color conversion method of the present invention, representative points are discretely arranged in advance only on the respective axes in the three-dimensional color space, and color conversion can be performed with the minimum necessary LUT. Therefore , the memory capacity can be reduced, high-precision color conversion can be performed only with a 3 × 3 matrix operation and a small LUT, and the effect of reducing the amount of operation and the circuit scale is extremely remarkable.
[0045]
Further, according to the color conversion method of the present invention, it is possible to have no memory for LUT data at all by setting a mathematical expression for color correction instead of the LUT .
[0046]
Further, according to the color conversion method of the present invention, the user can adjust the hue of the output image according to his / her preference by selecting an appropriate LUT or mathematical expression from a plurality of LUTs or mathematical expressions. I can .
[0047]
Further, according to the color conversion method of the present invention, color conversion suitable for the properties of the input image can be performed automatically by selecting LUTs or mathematical expressions having different conversion characteristics according to the input image .
[0048]
Further, according to the color conversion method according to the present invention, it is possible to improve the color conversion accuracy by increasing the number of representative points .
[0049]
Further , according to the color conversion method of the present invention, it is possible to improve the accuracy of color conversion in the high density portion by arranging the representative points up to the high density region .
[0050]
Further, according to the color conversion method of the present invention, since the representative points on the Xa = Xb = Xc axis are always included in the region division, the arrangement density of the representative points on this axis is increased. The color conversion accuracy can be improved .
[0051]
In addition, according to the color conversion method of the present invention, it is not necessary to arrange the representative points at equal intervals on the axis, and the arrangement of the representative points can be freely changed, so that the representative points in the three-dimensional color space can be changed. Can be arranged in consideration of human visual characteristics .
[0052]
Further, according to the color conversion method of the present invention, the color reproducibility can be improved without increasing the memory capacity by partially collecting the arrangement of representative points .
[0053]
Further, according to the color conversion method of the present invention, the data of the point closest to the input point on the axis is newly generated from the two representative points on the same axis by the internal division, so that the color can be accurately reproduced. Can be converted .
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a schematic configuration of a color conversion unit according to a first embodiment.
FIG. 2 is a schematic diagram of arrangement of representative points of the LUT according to the first embodiment.
FIG. 3 is a LUT data correspondence diagram according to the first embodiment.
FIG. 4 is an explanatory diagram showing an outline of linear interpolation using three neighboring points.
FIG. 5 is an explanatory diagram showing an outline of linear interpolation on an axis.
FIG. 6 is a schematic diagram of LUT representative point arrangement according to the second embodiment.
FIG. 7 is a diagram corresponding to LUT data according to the second embodiment.
8 is a schematic diagram of LUT representative point arrangement according to Embodiment 3. FIG.
FIG. 9 is a diagram corresponding to LUT data according to the third embodiment.
FIG. 10 is a block diagram illustrating a schematic configuration of a color conversion unit according to a sixth embodiment.
FIG. 11 is a block diagram illustrating a schematic configuration of a color conversion unit according to a seventh embodiment.
FIG. 12 is a block diagram illustrating a schematic configuration of a color conversion unit according to an eighth embodiment.
FIG. 13 is an explanatory diagram of perpendicular legs obtained from an input point to a nearby axis in a three-dimensional color space in the eighth embodiment.
14 is a block diagram illustrating a schematic configuration of a color conversion unit according to Embodiment 9. FIG.
FIG. 15 is a schematic diagram of a typical representative point arrangement example.
[Explanation of symbols]
101 region determination unit 102 representative point selection unit 103 LUT
104

Linear interpolation unit

401, 503

LUT selection unit

402, 504 LUT1
403, 505 LUT2
404, 506 LUT3
501 Frame memory 502 Histogram generation unit 601 Perpendicular foot search unit 602 Conversion data generation unit 605 New representative point generation unit

Claims

A color conversion method for converting from arbitrary coordinates on a first three-dimensional color space to predetermined coordinates on a second three-dimensional color space,
In the first three-dimensional color space, a total of N + 3 axes are set by three axes Xa, Xb, Xc orthogonal to each other centered on the origin and N straight lines different from the three orthogonal axes. Then, a predetermined number of representative points are discretely arranged on the N + 3 axes, and each coordinate on the first three-dimensional color space is divided into a plurality of regions according to the distance from the N + 3 axes. A region determining step for specifying which of the plurality of regions the input color signal value belongs to,
The representative points close to the input color signal value are selected one by one on three axes including at least one of the N axes existing on the boundary of the specified region, and three representative points are selected. A representative point selection process to obtain
Upon conversion from the first three-dimensional color space to the second three-dimensional color space, the input color signal values on the first three-dimensional color space are converted into the three representatives corresponding to the input color signal values. Color conversion by a linear or predetermined method using a point and a look-up table indicating to which coordinate in the second three-dimensional color space the representative point in the first three-dimensional color space is converted A color conversion method comprising: an interpolation step for performing the step.

The color conversion method according to claim 1,
A color conversion method comprising preparing a plurality of the look-up tables and switching a plurality of look-up tables by a user.

The color conversion method according to claim 1,
A color conversion method comprising: preparing a plurality of lookup tables, and automatically switching the plurality of lookup tables according to a statistic of color density values of an input image.

A color conversion method for converting from arbitrary coordinates on a first three-dimensional color space to predetermined coordinates on a second three-dimensional color space,
In the first three-dimensional color space, a total of N + 3 axes are set by three axes Xa, Xb, Xc orthogonal to each other centered on the origin and N straight lines different from the three orthogonal axes. Then, a predetermined number of representative points are discretely arranged on the N + 3 axes, and each coordinate on the first three-dimensional color space is divided into a plurality of regions according to the distance from the N + 3 axes. A region determining step for specifying which of the plurality of regions the input color signal value belongs to,
The representative points close to the input color signal value are selected one by one on three axes including at least one of the N axes existing on the boundary of the specified region, and three representative points are selected. A representative point selection process to obtain
Upon conversion from the first three-dimensional color space to the second three-dimensional color space, the input color signal values on the first three-dimensional color space are converted into the three representatives corresponding to the input color signal values. Color conversion is performed linearly or by a predetermined method using a mathematical expression that defines a point and a coordinate in the second three-dimensional color space to which the representative point in the first three-dimensional color space is converted. And a color conversion method comprising an interpolation step.

The color conversion method according to claim 4, wherein:
A color conversion method characterized in that a plurality of the mathematical expressions are prepared, and a user switches the plurality of mathematical expressions.

The color conversion method according to claim 4, wherein:
A color conversion method comprising preparing a plurality of the mathematical expressions and automatically switching the plurality of mathematical expressions according to a statistic of a color density value of an input image.

In the color conversion method according to any one of claims 1 to 6,
A color conversion method characterized by setting N = 1 and setting one straight line satisfying Xa = Xb = Xc separately from the three orthogonal axes Xa, Xb, Xc.

In the color conversion method according to any one of claims 1 to 7,
A color conversion method including or adding a straight line of Xa = Xb, Xb = Xc, and Xc = Xa as a straight line set separately from the three orthogonal axes Xa, Xb, and Xc.

In the color conversion method according to any one of claims 1 to 8,
As a straight line set separately from the three orthogonal axes Xa, Xb, and Xc, including or adding three straight lines that pass through coordinates at which the three components in the first three-dimensional color space are maximum, respectively. A color conversion method characterized by the above.

In the color conversion method according to any one of claims 1 to 9,
A color conversion method characterized in that the discrete representative point arrangement density on the Xa = Xb = Xc axis is set higher than that of the other axes.

The color conversion method according to claim 1, wherein:
A color conversion method comprising increasing an arrangement density of discrete representative points at an intermediate density portion.

The color conversion method according to claim 1, wherein:
The three perpendicular points from the input color signal values corresponding to the three axes corresponding to the input color signal values are used as three new representative points, and the first three-dimensional color is set for each new representative point. To which coordinate in the second three-dimensional color space the new representative point in the space is converted is determined using two representative points arranged on the same axis as both sides of the new representative point. A color conversion method characterized by determining.