Global Tone Mapping Using Luminance Chrominance Gain Separation
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Solution Overview
Problem
Current global tone mapping techniques in digital imaging devices often result in oversaturation, unnatural color saturation, and damaged brightness ratios between chromatically saturated and grey colors, particularly in high dynamic range (HDR) to standard dynamic range (SDR) conversions, due to limitations in calculating output color coordinates based solely on RGB color channels and luminance gains.
Innovation Solution
The proposed solution involves separating a pixel's color vector into luminance and chrominance components and applying distinct gains to each, with a luminance gain calculated using existing algorithms and a chromatic gain determined using the formula ℊch=(ℊl+c)*(1+b-ℊl*l)a/(1+c)*(1+b-l)a, where ℊl is the luminance gain, l is the pixel's luminance, and a, b, and c are configurable parameters, to maintain natural color reproduction and saturation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a single luminance gain is applied to all color channels based on luminance component, then the processing is simple and fast, but the output image suffers from oversaturation, unnatural color saturation, and damaged brightness ratios
Solution Approach 1:
The invention segments the color processing into two independent parts: luminance processing and chrominance processing. The luminance gain is calculated separately from the chromatic gain, allowing each to be optimized independently. This segmentation enables complex color processing without significantly increasing computational burden, as the two processing streams can be computed in parallel.
Solution Approach 2:
The invention extracts the chromatic information from the RGB color space by converting to a color space with separate luminance and chrominance components. This extraction allows the chromatic gain to be calculated and applied independently, removing the harmful coupling between luminance and chrominance that causes oversaturation and brightness ratio damage in conventional approaches.
2Adaptability or versatility
If high gain values are applied during HDR to SDR conversion, then the dynamic range compression is effective, but the color saturation becomes abnormal and hue mistakes occur
Solution Approach 1:
The invention applies different gain characteristics to different parts of the color space. The chromatic gain formula incorporates the luminance component and luminance gain to create a localized adjustment that adapts to the specific pixel's characteristics. This allows high gain values to be applied where needed for dynamic range compression while preventing abnormal saturation and hue mistakes in regions where they would be harmful.
Solution Approach 2:
The invention changes the parameters used to calculate chromatic gain based on the luminance and luminance gain. The formula gch = (gl + c) * (1 + b - gl * l)^a / (1 + c) * (1 + b - l)^a dynamically adjusts the chromatic gain based on the pixel's luminance and the applied luminance gain, allowing the system to handle high gain values during HDR to SDR conversion while maintaining color fidelity.
3Device complexity
If only luminance gain is calculated without separate chromatic gain, then the computational complexity is low, but the brightness ratios between chromatically saturated and grey colors are damaged
Solution Approach 1:
The invention introduces an intermediary chromatic gain calculation that bridges the luminance processing and the final color output. This intermediary step uses the luminance gain and luminance component as inputs to generate a chromatic gain that preserves brightness ratios while allowing the luminance processing to proceed independently. The chromatic gain acts as a mediator that corrects the brightness ratio damage caused by luminance-only processing.
Data Source
AI summary
Example methods, apparatus, systems and articles of manufacture (e.g., physical storage media) to implement global tone mapping of images based on luminance and chrominance are disclosed. Examples disclosed herein determine a chromatic gain to apply to input chrominance components corresponding to an input color of a pixel of the input image, the chromatic gain based on an input luminance component corresponding to the input color of the pixel and a luminance gain to be applied to the input luminance component of the pixel to determine an output luminance component of the pixel. Disclosed examples also apply the chromatic gain to the input chrominance components of the pixel to determine output chrominance components of the pixel. Disclosed examples further combine the output luminance component and the output chrominance components to determine an output color of the pixel.