Chroma reshaping of HDR video signals
AssigneeDolby Laboratories Licensing Corporation
InventorsTaoran Lu, Fangjun Pu, Peng Yin, Tao Chen
Life of this patent
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Abstract
Methods for the chroma reshaping of high-dynamic range (HDR) signals are presented. For each input pixel, a first scaler is generated based on the pixel luminance and a luma-based chroma reshaping (LCR) function. A second scaler is generated based on a saturation-based chroma reshaping (SCR) function and the saturation value of the pixel. A cascaded scaler is generated by multiplying the first scaler with the second scaler. Reshaped chroma values are generated by multiplying the input chroma values with the cascaded scaler. The characteristics of preferred LCR and SCR functions are discussed. Inverse decoding methods based on chroma-reshaping metadata that define the LCR and SCR functions are also described.
Claims13
In a video encoder, a method to reshape chroma components of a high dynamic range image, the method comprising:
- receiving an input high dynamic range image comprising pixels, wherein each pixel is coded in the ICTCP color format and comprises a luma component I, a first chroma component CT and a second chroma component Cp;
- and
- for a pixel in the input high dynamic range image:
- generating a first scaler as a function of the luma component of the pixel, wherein the function of the luma component comprises a bell-shaped function;
- generating the colour saturation of the pixel as a function of the first and second chroma components according to the function Sat=√{square root over (CT2+CP2)};
- generating a second scaler as a piecewise, monotonically decreasing function of the colour saturation of the pixel;
- and
- generating, as first and second reshaped chroma components for the pixel, i) the multiplicative product of the first chroma component, the first scaler and the second scaler and ii) the multiplicative product of the second chroma component, the first scaler and the second scaler.
In a decoder, a method to perform inverse reshaping of chroma components of a high dynamic range image, the method comprising:
- receiving a reshaped high dynamic range image comprising pixels, wherein each pixel comprises a luminance component I, a first reshaped chroma component CTccr and a second chroma reshaped component CPccr;
- and
- for a pixel in the reshaped high dynamic range image:
- generating a first inverse scaler as a function of the luma component of the pixel, by applying, to the luma component of the pixel, the inverse of a function that generates values of a first forward scaler;
- generating the reshaped colour saturation of the pixel as a function of the first and second reshaped chroma components according to the function Satccr=√{square root over (CTccr2+CPccr2)};
- generating a second inverse scaler as a function of the reshaped colour saturation of the pixel;
- and
- generating, as first and second inverse reshaped chroma components in the ICTCP color format for the pixel, i) the multiplicative product of the first reshaped chroma component, the first inverse scaler and the second inverse scaler and ii) the multiplicative product of the second reshaped chroma component, the first inverse scaler and the second inverse scaler.
Dependencies are inferred from the claim text.