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. An apparatus to encode video data, the apparatus comprising:
an input to receive a sequence of input video pictures; and
a processor, wherein the processor:
generates an output comprising a coded bitstream and signal parameters, wherein the coded bitstream comprises coded video pictures generated based on the sequence of input video pictures, and the signal parameters comprise a flag indicating whether adaptive reshaping is enabled or not in the output, and if adaptive reshaping is enabled in the output, then the signal parameters further comprise at least parameters related to a piece-wise polynomial representation of a reshaping function to be used in decoding the output.
2. The apparatus of claim 1 , wherein the signal parameters are part of at least one of a slice header, a sequence parameter set, a picture parameter set, a network abstraction layer unit header, or a supplemental enhancement information message.
3. The apparatus of claim 1 , wherein generating the coded video pictures comprises:
generating reshaped video pictures by applying a representation of the reshaping function to the input sequence of video pictures; and
encoding the reshaped video pictures to generate the coded video pictures.
4. The apparatus of claim 1 , wherein if adaptive reshaping is enabled, then the signal parameters 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 polynomial function's order, wherein the polynomial function's order does not exceed a value of two; and
one or more non-zero coefficients for the polynomial function.
5. The apparatus of claim 4 , 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 first polynomial function's order is two; and
a first order coefficient (b i ), if the first polynomial function's order is two and the second adjoining polynomial function's order is not two, or if the first polynomial function's order is one.
6. The apparatus of claim 5 , wherein if the coefficients for the first polynomial function do not include the first order coefficient (b i ), then if the first polynomial function's order is two, the first order coefficient is computed based on the first order and 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.
7. The apparatus of claim 6 , 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.
8. The apparatus of claim 6 , wherein the 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 third order coefficients of the first polynomial function, and the starting pivot point for the first polynomial function.
9. The apparatus of claim 8 , 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.
10. The apparatus of claim 1 , wherein the piece-wise polynomial representation comprises N linear (first order) segments, wherein N is an integer larger than 0.
11. The apparatus of claim 1 , wherein the piece-wise polynomial representation comprises N linear or second order segments, wherein N is an integer larger than 0.