HiSoUR: ICtCp

IC_TC_P, ICtCp, or ITP is a color space used as a part of the color image pipeline in video and digital photography systems for high dynamic range (HDR) and wide color gamut (WCG) imagery and is specified in Rec. 2100. I is the intensity luma component encoded using the Perceptual Quantizer (PQ) or Hybrid Log-Gamma (HLG) non-linearity. CT and CP are the blue-yellow (named from tritanopia) and red-green (named from protanopia) chroma components.

ICtCp is defined from Rec. 2100 RGB as follows:

Calculate LMS from BT.2100 RGB:

{\begin{aligned}L&=(1688\cdot R+2146\cdot G+262\cdot B)/4096\\M&=(683\cdot R+2951\cdot G+462\cdot B)/4096\\S&=(99\cdot R+309\cdot G+3688\cdot B)/4096\\\end{aligned}}

Calculate L'M'S' using the PQ inverse EOTF (or the HLG OETF):

{\begin{aligned}L'&=EOTF_{PQ}^{-1}(L)\\M'&=EOTF_{PQ}^{-1}(M)\\S'&=EOTF_{PQ}^{-1}(S)\\\end{aligned}}

Calculate ICtCp:

{\begin{aligned}I&=(0.5\cdot L'+0.5\cdot M')\\C_{T}&=(6610\cdot L'-13613\cdot M'+7003\cdot S')/4096\\C_{P}&=(17933\cdot L'-17390\cdot M'-543\cdot S')/4096\\\end{aligned}}

ICTCP is near constant luminance, which improves chroma subsampling versus YCbCr. ICTCP also improves hue linearity compared with YCbCr which helps with compression performance and color volume mapping. When combined with adaptive reshaping ICTCP can improve compression performance by 10%. For CIEDE2000 color quantization errors 10-bit ICtCp would be equal to 11.5 bit YCbCr. Constant luminance is also improved with ICtCp which has a luminance relationship of 0.998 between the luma and encoded brightness while YCbCr has a luminance relationship of 0.819. An improved constant luminance is an advantage for color processing operations such as chroma subsampling and gamut mapping where only color information is changed. A draft document for ICtCp has been released by the JCT-VC.

HiSoUR

2019年4月6日星期六

ICtCp

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Objective abstraction

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