SDR to HDR Image Conversion via Luminance Expansion and Chroma Correction
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Solution Overview
Problem
High dynamic range (HDR) images captured by modern imaging devices are often converted to standard dynamic range (SDR) for compatibility with household television receivers, resulting in loss of luminance information and degraded image quality, as these receivers do not support HDR.
Innovation Solution
An image processing device and method that includes a luminance signal expansion unit and a chroma signal correction unit, which corrects and expands the chroma signal in a manner adjusted to the luminance signal expansion, ensuring the ratio of luminance to chroma signals remains fixed and within the display color gamut, effectively converting SDR images back to HDR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If HDR images are converted to SDR for compatibility with household television receivers, then compatibility with standard displays is improved, but luminance information is lost and image quality deteriorates
Solution Approach 1:
Instead of compressing HDR to SDR for display, the patent inverts the process by expanding SDR luminance signals back to HDR range. The luminance signal expansion unit applies inverse Knee compression to restore the original high luminance values that were compressed during SDR conversion, thereby recovering lost luminance information while maintaining compatibility with standard broadcast signals.
Solution Approach 2:
The patent changes the luminance parameter range by expanding the SDR luminance signal (0-255) to HDR luminance range (0-1023 or higher). The luminance signal expansion unit transforms the compressed luminance values back to their original dynamic range, restoring the contrast ratio from 1000:1 (SDR) to 10000:1 or greater (HDR), thereby recovering the lost luminance information.
2Loss of information
If the luminance signal is expanded to restore HDR, then luminance information is recovered, but the chroma signal becomes mismatched and color accuracy deteriorates
Solution Approach 1:
The patent implements feedback by using the expanded luminance signal to dynamically adjust the chroma signal. The chroma signal correction unit calculates the ratio between original and expanded luminance values and applies this ratio to scale the chroma signal accordingly, ensuring that color information remains accurately matched to the restored luminance levels.
Solution Approach 2:
The patent coordinates changes in both luminance and chroma parameters. When the luminance signal is expanded from SDR to HDR range, the chroma signal is simultaneously scaled by the same expansion ratio to maintain the Y/C relationship. This coordinated parameter change ensures that color accuracy is preserved while luminance information is recovered.
3Loss of information
If the chroma signal is expanded along with luminance, then HDR color information is restored, but the output may fall outside the display color gamut
Solution Approach 1:
The patent applies preliminary anti-action by predicting and preventing color gamut violations before they occur. The chroma signal correction unit calculates the expanded chroma values and checks whether they exceed the display color gamut boundaries. If violations are predicted, the chroma signal is pre-adjusted to remain within the valid gamut range while preserving as much HDR color information as possible.
Solution Approach 2:
The patent dynamically adjusts the chroma signal parameters based on the expanded luminance values and display gamut constraints. When luminance is expanded to HDR range, the chroma signal is scaled proportionally but then clamped or compressed to ensure the final RGB values remain within the displayable color gamut, thereby preventing color overflow while maintaining HDR color fidelity.
Data Source
AI summary
An image processing device includes a luminance signal expansion unit which expands a luminance signal, and a chroma signal correction unit which corrects a chroma signal in a manner adjusted to the expansion of the luminance signal.