Signal reshaping for high dynamic range signals
In a method to improve backwards compatibility when decoding high-dynamic range images coded in a wide color gamut (WCG) space which may not be compatible with legacy color spaces, hue and/or saturation values of images in an image database are computed for both a legacy color space (say, YCbCr-gamma) and a preferred WCG color space (say, IPT-PQ). Based on a cost function, a reshaped color space is computed so that the distance between the hue values in the legacy color space and rotated hue values in the preferred color space is minimized. HDR images are coded in the reshaped color space. Legacy devices can still decode standard dynamic range images assuming they are coded in the legacy color space, while updated devices can use color reshaping information to decode HDR images in the preferred color space at full dynamic range.
1 . A method for transmitting a bitstream comprising image data and metadata, the method comprising:
transmitting the bitstream which is encoded by an encoding method, wherein the encoding method comprises signaling metadata with the bitstream, wherein the metadata indicate an IPT-PQ color transformation matrix to decode the image data in an IPT-PQ color space, wherein the IPT-PQ color transformation matrix is
(
0.4
0.4
0.2
4.455
-
4.851
0.396
0.8056
0.3572
-
1.1628
)
,
wherein generating an input image in the image data from a linear RGB color space to the IPT-PQ color space comprises:
generating a first image in a first-LMS color space by applying a first color transformation to the input image;
applying a non-linear function to each color component of the first image to generate color components of a second image in a second-LMS color space; and
generating an output image in the IPT-PQ color space by applying the IPT-PQ color transformation matrix to the color components of the second image.
2 . The method of claim 1 , wherein the non-linear function comprises an inverse of an electro-optical transfer function.
3 . The method of claim 2 , wherein the electro-optical transfer function is determined according to the SMPTE ST 2084 specification.
4 . The method of claim 1 , wherein the non-linear function is based on a Hybrid Log-Gamma (HLG) function.