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 decode video data, the apparatus comprising:
an input to receive an input bitstream comprising coded data and signal parameters, wherein the signal parameters comprise information indicating whether adaptive reshaping is enabled or not in the input bitstream; and a processor, wherein the processor:
parses the input bitstream; and upon detecting that adaptive reshaping is enabled in the input bitstream:
extracts adaptive reshaping parameters from the input bitstream, wherein the adaptive reshaping parameters comprise at least parameters related to a piece-wise polynomial representation of a reshaping function; and
decodes the input bitstream to generate an output decoded signal based on the coded data and the adaptive reshaping parameters.
2. The apparatus 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 adaptive reshaping parameters; and
generating the output decoded signal based on the preliminary output decoded signal and the adaptive reshaping parameters.
3. The apparatus of claim 1 , wherein the adaptive reshaping parameters comprise:
a first parameter based on a total number of polynomial functions used to define the reshaping function, and for each (pi) 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, wherein the order of the polynomial function can't exceed a value of two; and
one or more non-zero coefficients for the polynomial function.
4. The apparatus 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 (ci), if the order of the first polynomial function is two; and
a first order coefficient (bi), 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 apparatus of claim 4 , wherein if the coefficients for the first polynomial function do not include the first order coefficient (bi), then if the order of the first polynomial function 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.
6. The apparatus 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 apparatus of claim 5 , wherein the zero order coefficient (ai) 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.
8. The apparatus of claim 7 , wherein the zero order coefficient (ai) 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{circumflex over ( )} 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 apparatus of claim 1 , wherein the processor further parses slice header data in the input bitstream and extracts slice-related reshaping parameters.
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.
12. The apparatus of claim 1 , wherein the signal parameters comprise sequence parameter set (SPS) data, picture parameter set (PPS) data, or supplemental enhancement information (SEI) data.