Lightness mapping in two steps

Patent metadata

Field	Value
Publication number	US-10284750-B2
Application number	US-201715406714-A
Country	US
Kind code	B2
Filing date	Jan 14, 2017
Priority date	Jan 15, 2016
Publication date	May 7, 2019
Grant date	May 7, 2019

Abstract

Said method comprises a first cusp-oriented lightness-mapping step and a second lightness-mapping step in which a source color (N) is mapped into a mapped color (N″) having the same hue (H″=H) and the same chroma (C″=C) as the source color, where the second mapping step is defined such as to bring more colors in a target color gamut and more colors closer to the boundary of this target gamut.

First claim

The invention claimed is:

1. Method of color gamut mapping source colors into targets colors from a source color gamut towards a target color gamut in a gamut mapping color space that can be partitioned into 2D constant-hue leaves having axes for lightness and chroma, said method comprising:
first lightness-mapping each source color having a lightness L into a first lightness-mapped color having a lightness L′ according to a first lightness-mapping function ƒ 1 (C,L) associated with the constant-hue leaf comprising said source color, wherein said lightness-mapping function is defined such that a source cusp color defined as a color having a highest color chroma C C SOURCE in said constant-hue leaf and in said source color gamut is lightness-mapped according to said function into a target cusp color defined as a color having a highest color chroma C C TARGET in said constant-hue leaf and in said target color gamut, and,

when said first lightness-mapped color is located out of said target color gamut:
second lightness-mapping said first lightness-mapped color into a second lightness-mapped color such that this second lightness-mapped color is located inside said target color gamut or outside said target color gamut but closer to the boundary of said target color gamut than before said second lightness-mapping, and such that any two different first mapped colors that are second lightness-mapped are second lightness-mapped into different second lightness-mapped colors.

2. Method of color gamut mapping according to claim 1 wherein said second lightness mapping is defined such that it maps lightness in opposite lightness direction of said first lightness mapping.

3. Method of color gamut mapping according to claim 1 comprising third mapping said second lightness-mapped color or said first lightness-mapped color when it is not second lightness-mapped into a final target color, wherein said first and second lightness-mapping keep unchanged hue and chroma, wherein said third mapping is defined such that said target color is within said target color gamut and keeps unchanged hue and lightness.

4. Method of color gamut mapping according to claim 2 wherein said second lightness-mapping is defined by a second lightness mapping function that correspond to said first lightness mapping function multiplied by a negative number in the interval]−1, 0[.

5. Method of color gamut mapping according to claim 1 wherein said first lightness-mapping function depends on a parameter t representative of the difference between the lightness L of the source color to map and the lightness of the black point of the source color gamut, or between the lightness L of the source color to map and the lightness of the white point of the source color gamut.

6. Method of color gamut mapping according to claim 1 wherein said first lightness mapping function depends on a parameter u representative of the ratio between the chroma C of the source color to map and the chroma of said source cusp color of the leaf of said constant hue h.

7. Color processing device for mapping source colors of a content into targets colors from a source color gamut towards a target color gamut, comprising at least one processor configured to:
define, for source colors located in a same leaf of constant-hue h, a first lightness-mapping function ƒ 1 (C,L) adapted to lightness-map any of these source colors such that a source cusp color defined as a color having a highest color chroma in said constant-hue leaf and in said source color gamut is lightness-mapped according to said function into a target cusp color defined as a color having a highest color chroma in said constant-hue leaf and in said target color gamut,

wherein said at least one processor is further configured to define, for colors located in a same leaf of constant-hue h, a second lightness-mapping function ƒ 2 (C′,L′) adapted to lightness-map these colors inside said target color gamut or outside said target color gamut but closer to the boundary of said target color gamut than before said second lightness-mapping, and such that any two different first mapped colors that are second lightness-mapped are second lightness-mapped into different second lightness-mapped colors,
apply said first lightness-mapping function ƒ 1 (C,L) then said second lightness-mapping function ƒ 2 (C′,L′) to said source colors located in said leaf of constant-hue h such as to output second lightness-mapped colors.

8. Color processing device according to claim 7 , wherein said second lightness-mapping function ƒ 2 (C′,L′) is further adapted to map colors in opposite lightness direction of said first lightness mapping.

9. Color processing device according to claim 7 , wherein said at least one processor is further configured to define a third chroma-mapping function keeping unchanged hue and lightness, and wherein at least one processor is further configured to apply said third lightness-mapping function to colors obtained by the application of said second lightness-mapping function.

10. Color processing device according to claim 7 , wherein said at least one processor is further configured to obtain, from said source color gamut, a source cusp line and, from said target color gamut, a target cusp line, and to define said source cusp color as the intersection of said constant-hue leaf with said source cusp line; and to define said target cusp color as the intersection of said constant-hue leaf with said target cusp line.

11. A non-transitory processor readable medium having stored therein instructions for causing a processor to perform the method of color gamut mapping according to claim 1 .

Assignees

• Thomson Licensing
• Interdigital Ce Patent Holdings

Inventors

• Morvan Patrick
• Stauder Jurgen
• Poree Corinne

Classifications

• H04N1/6058 Primary

  Reduction of colour to a range of reproducible colours, e.g. to ink- reproducible colour gamut · CPC title

• H04N1/6097 Primary

  depending on the characteristics of the output medium, e.g. glossy paper, matt paper, transparency or fabrics · CPC title