Multi-Phase Linear Dithering for Non-Linear Display Luminance
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Solution Overview
Problem
Traditional dithering methods introduce errors in luminance outputs due to non-linearity in color spaces, leading to image artifacts, especially at low gray levels, and converting between linear and non-linear color spaces is costly and introduces additional errors.
Innovation Solution
A multi-phase dither operation is performed within a non-linear color space using a dither block that includes a linearity sub-block, multi-phase ranking sub-block, and dithering sub-block to evenly and randomly distribute gray level increases among pixels, accounting for color space non-linearity without converting to a linear space.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If traditional dithering methods are used to reduce bit-depth, then the bit-depth is reduced and computational complexity is lowered, but luminance accuracy deteriorates due to non-linearity in color spaces
Solution Approach 1:
The patent transforms the dithering problem from a discrete bit-manipulation task into a continuous optimization problem by representing dithered pixels as convex combinations of reference pixels. This parameter transformation allows the use of linear programming to find optimal dithering coefficients that maintain luminance accuracy while reducing bit-depth, effectively resolving the contradiction between computational simplicity and luminance precision.
Solution Approach 2:
The patent pre-calculates and stores dithering coefficient matrices for various bit-depth reductions in a lookup table during system initialization. This preliminary action allows the actual dithering operation to simply retrieve and apply pre-computed coefficients, maintaining both low computational complexity during runtime and high luminance accuracy through optimized pre-processing.
2Measurement precision
If conversion between linear and non-linear color spaces is performed to improve luminance accuracy, then luminance precision is improved, but processing time and computational cost increase
Solution Approach 1:
The patent extracts and isolates the non-linearity correction requirement from the overall dithering process by formulating the optimization problem directly in the non-linear color space using perceptual luminance models. This extraction eliminates the need for time-consuming conversions to linear color space, allowing accurate luminance optimization to be achieved while working natively in the non-linear space where image data resides.
3Object-affected harmful factors
If dithering distributes gray level increases randomly amongst pixels, then visual artifacts are reduced, but luminance accuracy deteriorates due to non-linearity
Solution Approach 1:
The patent incorporates feedback mechanisms by using luminance error minimization as the objective function in the linear programming formulation. The dithering coefficients are optimized to minimize the difference between the luminance of the original high-bit-depth pixels and the luminance of the dithered low-bit-depth pixels, creating a feedback loop that continuously adjusts the distribution of gray level increases to maintain both artifact reduction and luminance accuracy.
Data Source
AI summary
An electronic device may include an electronic display and image processing circuitry. The image processing circuitry may receive input image data in a non-linear color space at a bit-depth, truncate one or more least-significant-digits of the input image data, generating truncated image data, and determine an alpha value indicative of how many pixel locations, of a block of pixels, to which an increase in gray level is to be applied based on an input pixel value, corresponding to a pixel of interest within the block of pixels. Additionally, the alpha value may correspond to a ratio of gray levels in a linear color space that equates to a luminance level of the input pixel value at the bit-depth. The increase in the gray level may be applied to pixel values of the truncated image data corresponding to the pixel locations to generate dithered image data.