Gamut Mapping Using Lightness and Chroma Vector Scaling
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Solution Overview
Problem
Existing gamut mapping methods, such as gamut clipping, often result in undesirable hue shifts and posterization when mapping out-of-gamut RGB colors to a color gamut, while more complex algorithms increase computational complexity.
Innovation Solution
A novel method operating in RGB space uses a pseudo four-channel perceptual approximation to lightness and chroma, with lightness-preserving and chroma-preserving mappings blended via linear interpolation, and adjusts chroma vectors based on user-defined saturation parameters to constrain the hue, ensuring efficient gamut mapping.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If gamut clipping is used to map out-of-gamut RGB colors, then computational simplicity is improved, but hue accuracy and image quality deteriorate due to hue shifts and posterization
Solution Approach 1:
The patent transforms the gamut mapping problem from simple RGB clipping to a parameter-based approach using lightness (L) and chroma (C) parameters. By representing colors in terms of L and C components and independently adjusting these parameters, the method achieves accurate hue preservation while maintaining computational efficiency. The chroma vector is scaled based on saturation parameters to constrain the mapped point within the gamut surface without causing posterization.
2Manufacturing precision
If elaborate gamut mapping algorithms are used to avoid posterization, then image quality is improved, but computational complexity increases
Solution Approach 1:
The patent segments the color mapping process into distinct computational stages: (1) determining lightness and chroma parameters from input RGB values, (2) clipping the lightness parameter to the valid range, (3) calculating minimum and maximum saturation values, (4) scaling the chroma vector based on saturation constraints, and (5) converting the mapped L and C values back to RGB. This segmentation allows each stage to be computationally efficient while collectively achieving high image quality without posterization.
3Manufacturing precision
If chroma vector scaling is applied to constrain hue, then hue fidelity is improved, but color saturation may be lost
Solution Approach 1:
The patent implements dynamic chroma vector scaling where the scaling factor is determined by the saturation parameter σ and the relationship between lightness γ and saturation bounds a0, a2. The chroma vector is scaled adaptively: C' = C × min(1, σ × (a2 - a0) / (2 × max(γ, 1-γ))) when saturation constraints are active, and C' = C otherwise. This dynamic approach preserves maximum color saturation while ensuring hue fidelity by only applying scaling when necessary to maintain the mapped point within the gamut surface.
Data Source
AI summary
What is disclosed is a system and method for mapping an out of gamut RGB color to a surface of a color gamut in RGB space. A point xc having {Rc Gc Bc} values in RGB space is identified which is outside a surface of a color gamut of a target device. A lightness value γ and a chroma vector {right arrow over (C)}=[Cr, Cg, Cb] are determined from the {Rc Gc Bc} values of the identified point. A minimum and a maximum saturation are determined for point xc. A new lightness value γ′ and a new chroma vector {right arrow over (C)} are calculated based on the minimum and maximum saturation values, in a manner as is described herein in detail. The new lightness γ′ and the new chroma vector {right arrow over (C)} are converted back into {R′c G′c B′c} values. The {R′c G′c B′c} values are provided to a gamut mapping function.