Signal reshaping and coding for HDR and wide color gamut signals
In a method to improve the coding efficiency of high-dynamic range (HDR) images, a decoder parses sequence processing set (SPS) data from an input coded bitstream to detect that an HDR extension syntax structure is present in the parsed SPS data. It extracts from the HDR extension syntax structure post-processing information that includes one or more of a color space enabled flag, a color enhancement enabled flag, an adaptive reshaping enabled flag, a dynamic range conversion flag, a color correction enabled flag, or an SDR viewable flag. It decodes the input bitstream to generate a preliminary output decoded signal, and generates a second output signal based on the preliminary output signal and the post-processing information.
1. A method to generate video data in a decoder, the method comprising:
receiving an input bitstream comprising a sequence parameter set (SPS) data, wherein the SPS data comprises information indicating whether adaptive reshaping is enabled or not in the input bitstream;
parsing the SPS data; and upon detecting that adaptive reshaping is enabled in the input bitstream:
extracting adaptive reshaping metadata from the input bitstream, wherein the adaptive reshaping metadata comprise at least parameters related to a piece-wise polynomial representation of a reshaping function;
generating the reshaping function based on the adaptive reshaping metadata; and
decoding the input bitstream to generate an output decoded signal based on the reshaping function.
2. The method of claim 1 , wherein generating the output decoded signal comprises:
decoding the input bitstream to generate a preliminary output decoded signal without taking into consideration the reshaping function; and
generating the output decoded signal based on the preliminary output decoded signal and the reshaping function.
3. The method of claim 1 , wherein the adaptive reshaping metadata comprise:
a first parameter based on a total number of polynomial functions used to define the reshaping function, and for each (p i ) polynomial function in the reshaping function, further comprising: a starting pivot point for the polynomial function, a second parameter based on the order of the polynomial function, and one or more non-zero coefficients for the polynomial function, wherein the order of the polynomial function is lower than 3.
4. The method of claim 3 , wherein, for a first polynomial function in the reshaping function following a second adjoining polynomial function, the coefficients for the first polynomial function comprise:
a second order coefficient (c i ), if the order of the first polynomial function is two, and
a first order coefficient (b i ), if the order of the first polynomial function is two and the order of the second adjoining polynomial function is not two, or if the order of the first polynomial function is one.
5. The method of claim 4 , wherein if the coefficients for the first polynomial function do not include the first order coefficient (b i ), then if the order of the first polynomial function is two, the first order coefficient is computed based on the first order and the second order coefficients of the second adjoining polynomial function, a third order coefficient of the first polynomial function, and the starting pivot point for the first polynomial function.
6. The method of claim 5 , wherein the first order coefficient (b i ) for the first polynomial is computed as:
b i =b i−1 +2( c i−1 −c i ) x i ,
wherein b i−1 and c i−1 denote the first order and second order coefficients of the second adjoining polynomial function, c i denotes the third order coefficient of the first polynomial function, and x i denotes the starting pivot point for the first polynomial function.
7. The method of claim 5 , wherein a zero order coefficient (a i ) for the first polynomial function is computed based on the coefficients of the second adjoining polynomial function, the second order and the third order coefficients of the first polynomial function, and the starting pivot point for the first polynomial function.
8. The method of claim 7 , wherein the zero order coefficient (a i ) for the first polynomial function is computed as:
a i =a i−1 +( b i−1 −b i ) x i +( c i−1 −c ) x i 2 ,
wherein a i−1 , b i−1 , and c i−1 denote the zero order, first order, and second order coefficients of the second adjoining polynomial function, b i and c i denotes the second order and third order coefficients of the first polynomial function, and x i denotes the starting pivot point for the first polynomial function.
9. The method of claim 1 , further comprising:
parsing slice header data in the input bitstream and extracting slice-related reshaping metadata.
10. The method of claim 1 , wherein the piece-wise polynomial representation comprises N linear (first order) segments.
11. The method of claim 1 , wherein the piece-wise polynomial representation comprises N linear or second order segments.
12. A non-transitory computer-readable storage medium having stored thereon computer-executable instructions for executing a method with one or more processors in accordance with claim 1 .