Reshaping curve optimization in hdr coding
AssigneeDolby Laboratories Licensing Corporation
InventorsHarshad KADU, Guan-Ming SU
Life of this patent
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Abstract
In a system for coding high dynamic range (HDR) images using lower-dynamic range (LDR) images, a reshaping function allows for a more efficient distribution of the codewords in the lower dynamic range images for improved compression. A trim pass of the LDR images by a colorist may satisfy a director's intent for a given “look,” but may also result in unpleasant clipping artifacts in the reconstructed HDR images. Given an original forward reshaping function which maps HDR luminance values to LDR pixel values, a processor identifies areas of potential clipping and generates modified forward and backward reshaping functions to reduce the visibility of potential artifacts from the trim pass process while preserving the director's intent.
Claims21
In an encoder comprising one or more processors, a method for generating a reshaping function, the method comprising:
- receiving with the processor a first video frame in a first dynamic range;
- receiving a second video frame in a second dynamic range, wherein the first and the second frames represent the same scene;
- receiving a tone-mapping function mapping luminance pixel values from the first dynamic range to luminance pixel values in the second dynamic range;
- receiving a first cumulative density function (CDF) matching curve which maps luminance values from the first dynamic range to reshaped luminance values in the second dynamic range;
- generating a first histogram of luminance values based on luminance pixel values in the first video frame;
- generating a second histogram of luminance values based on luminance pixel values in the second video frame;
- generating one or more histogram peaks based on the first and second histograms of luminance values and the first CDF matching curve;
- generating a first luminance pixel range in the first dynamic range based on the one or more histogram peaks and the first CDF matching curve;
- generating a second luminance pixel range in the first dynamic range based on the first luminance pixel range, the first CDF matching curve, and the tone-mapping function;
- generating a second CDF matching curve based at least on the second luminance pixel range, the first CDF matching curve, and the tone-mapping function;
- and
- generating a backward reshaping function based on the second CDF matching curve.
In an encoder comprising one or more processors, a method for generating a reshaping function, the method comprising:
- receiving with the processor a first video frame in a first dynamic range;
- applying a tone-mapping function to the first video frame to generate a second video frame in a second dynamic range, wherein the tone-mapping function maps luminance pixel values from the first dynamic range to luminance pixel values in the second dynamic range;
- applying a trim pass to the second video frame to generate a third video frame in a target dynamic range;
- generating based on the first and the third video frames a first cumulative density function (CDF) matching curve which maps luminance values from the first dynamic range to reshaped luminance values in the target dynamic range;
- generating a second CDF matching curve based on the first and third video frames, the first CDF matching curve, and the tone-mapping function;
- and
- generating a backward reshaping function based on the second CDF matching curve.
Dependencies are inferred from the claim text.