Chroma reshaping for high dynamic range images

Patent metadata

Field	Value
Publication number	US-10645403-B2
Application number	US-201716303028-A
Country	US
Kind code	B2
Filing date	May 17, 2017
Priority date	May 19, 2016
Publication date	May 5, 2020
Grant date	May 5, 2020

Abstract

Methods and systems for adaptive chroma reshaping are discussed. Given an input image, a luma-reshaped image is first generated based on its luma component. For each chroma component of the input image, the range of the pixel values in the luma reshaped image is divided into bins, and for each bin a maximal scale factor is generated based on the chroma pixel values in the input image corresponding to the pixels of the luma reshaped image in the bin. A forward reshaping function is generated based on a reference reshaped function and the maximal scale factors, and reshaped chroma pixel values for the chroma component are generated based on the forward reshaping function and the corresponding pixel values in the luma reshaped image. Implementations options using look-up tables for mobile platforms with limited computational resources are also described.

First claim

The invention claimed is:

1. A method for adaptive chroma reshaping of a high dynamic range (HDR) image with a processor, the method comprising:
accessing with the processor an HDR input image comprising pixel values in a luma component and one or more chroma components;
for a chroma component of the input image:
partitioning the pixel values of either the luma component of the HDR input image or a reshaped luma component of the HDR input image into two or more bins;
generating for each bin a maximal scale factor based on the chroma pixel values corresponding to the respective luma or reshaped luma pixel values in each bin;
generating a forward reshaping function based on a reference reshaping function and the maximal scale factors, the forward reshaping function mapping either luma pixel values or reshaped luma pixel values to forward chroma scale factors and correspondingly the reference reshaping function mapping luma pixel values or reshaped luma pixel values to reference chroma scale factors; and
generating reshaped chroma pixel values for the chroma component based on the chroma pixel values, the forward reshaping function, and the corresponding luma pixel values or reshaped luma pixel values of the HDR input image.

2. The method of claim 1 , further comprising:
generating an inverse reshaping function based on the forward reshaping function;
generating parameters characterizing the inverse reshaping function; and
generating and outputting metadata based on the parameters characterizing the inverse reshaping function.

3. The method of any preceding claim 1 , wherein for each bin, for respective luma pixel values or reshaped luma pixel values in the bin, the maximal scale factor is computed based on a t th -percentile value of a function of the corresponding chroma values in the bin; wherein for pixel values normalized in [0 1), generating the maximal scale factor for the m-th bin comprises computing

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where M is the number of bins, Ψ j,m denotes the m-th bin for the HDR input image j, ŝ ji y denotes the i-th pixel of the respective luma component or the reshaped luma component of the HDR input image j, v ji u denotes the corresponding chroma pixel for the chroma component of the HDR input image j, η is a weighting factor, q max is a fixed value, and p t (x) returns the t th -percentile value of the values in vector x, wherein optionally: t=100 and η is between 0.85 and 0.95, or η=1 and tis between 95 and 99.

4. The method of claim 1 , wherein generating the forward reshaping function {tilde over (ƒ)} j ( )for the HDR input image j comprises computing

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Assignees

• Dolby Laboratories Licensing Corp

Inventors

• Song Qing
• Su Guan-Ming
• Chen Qian
• Chen Tao

Classifications

• H04N19/136

  Incoming video signal characteristics or properties · CPC title

• H04N19/30 Primary

  using hierarchical techniques, e.g. scalability (H04N19/63 takes precedence) · CPC title

• H04N19/98

  Adaptive-dynamic-range coding [ADRC] · CPC title

• H04N19/124 Primary

  Quantisation · CPC title

• H04N19/85

  using pre-processing or post-processing specially adapted for video compression · CPC title