Tonal Correction Apparatus for Natural Skin Tone Preservation
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Solution Overview
Problem
Conventional tonal correction methods in digital photography often result in excessive saturation of skin tones, leading to unnatural appearances, and fail to preserve pixel saturation while adjusting exposure, causing loss of detail in images with varying lighting conditions.
Innovation Solution
A tonal correction method that applies a specific function to each pixel's luminance, using a processor to calculate and adjust luminance gain without altering saturation, by separating luminance from color differences and applying a piece-wise linear tonal correction function with knee points to stretch or compress low-key, mid-key, and high-key tones, preserving original saturation and hue.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If conventional tonal correction methods combine gamma correction with tonal correction to adjust exposure, then image exposure is improved, but pixel saturation is adversely impacted causing excessive color saturation
Solution Approach 1:
The patent segments the tonal correction operation into two independent parts: luminance adjustment and saturation preservation. By separating the luminance correction from the saturation channel, the invention adjusts exposure through luminance gain while explicitly preserving original saturation values, thus avoiding the excessive color saturation that occurs when gamma correction is combined with tonal correction
Solution Approach 2:
The patent changes the parameter being adjusted from combined gamma-tonal correction to pure luminance gain adjustment. By modifying only the luminance parameter while keeping saturation unchanged, the system achieves proper exposure correction without the harmful side effect of excessive color saturation that plagues conventional methods
2Illumination intensity
If conventional tonal correction stretches low-key tones to improve exposure, then image contrast is improved, but detail is lost due to clipping
Solution Approach 1:
The patent applies dynamic, piece-wise linear tonal correction with knee points that adaptively stretch different tonal regions. The knee points allow the correction to be gentle in critical regions (preserving detail) while still achieving contrast improvement in other areas, making the stretching process dynamic rather than uniform across all tones
3Illumination intensity
If tonal correction is applied to correct exposure in images with varying lighting conditions, then exposure balance is improved, but skin tones become unnatural due to saturation changes
Solution Approach 1:
The patent applies local quality preservation by specifically protecting saturation in regions where it matters most (skin tones) while still achieving global exposure correction. The piece-wise linear function with knee points allows different treatment of different tonal regions, preserving natural appearance in critical areas while correcting exposure overall
Data Source
AI summary
A method and apparatus for applying tonal correction to images to obtain a more pleasing photographic image by redistributing low-key, mid-tone and high-key tones. Luminance is calculated by using formulas appropriate for the color space or directly inputted. Two color-difference components are computed for the original image. Luminance is subjected to a tonal correction function to obtain a tonal corrected luminance. Luminance gain is calculated and applied to the color-difference components to obtain two tonal corrected color-difference components. The tonal corrected luminance and two tonal corrected color-difference components can be directly output or used to calculate three color component signals for the desired color space.