ICtCp

''IC_TC_P'', ''ICtCp'', or ''ITP'' is a color representation format specified in the Rec. ITU-R BT.2100 standard that is 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. It was developed by Dolby Laboratories from the ''IPT'' color space by Ebner and Fairchild.F.Ebner, M.D.Fairchild, Development and testing of a color space (IPT) with improved hue uniformity. In: Proceedings of The Sixth Color Imaging Conference, 8-13, 1998 The format is derived from an associated ''RGB'' color space by a coordinate transformation that includes two matrix transformations and an intermediate nonlinear transfer function that is informally known as gamma pre-correction. The transformation produces three signals called ''I'', ''C_T'', and ''C_P''. The ''IC_TC_P'' transformation can be used with ''RGB'' signals derived from either the perceptual quantizer (PQ) or hybrid log–gamma (HLG) nonlinearity functions, but is most commonly associated with the PQ function (which was also developed by Dolby).

The ''I'' ("intensity") component is a luma component that represents the brightness of the video, and ''C_T'' and ''C_P'' are blue-yellow (named from tritanopia) and red-green (named from protanopia) chroma components. Ebner also used IPT as short for "Image Processing Transform".

The ''IC_TC_P'' color representation scheme is conceptually related to the ''LMS'' color space, as the color transformation from ''RGB'' to ''IC_TC_P'' is defined by first converting ''RGB'' to ''LMS'' with a 3×3 matrix transformation, then applying the nonlinearity function, and then converting the nonlinear signals to ''IC_TC_P'' using another 3×3 matrix transformation. IC_TC_P was defined as YCC digital format with support for 4:4:4, 4:2:2 and 4:2:0 chroma subsampling in CTA-861-H (that means that in limited range 10 bit mode 0, 1, 2, 3, 1020, 1021, 1022, 1023 values are reserved).

Derivation

''IC_TC_P'' is defined by Rec. 2100 as being derived from linear ''RGB'' as follows:

# Calculate ''LMS'' from BT.2100 RGB:$\begin L\\M\\S \end = \frac \begin 1688 & 2146 & 262\\ 683 & 2951 & 462\\ 99 & 309 & 3688 \end \begin R\\G\\B \end$
# Normalize the LMS by a non-linearity:
#* If the PQ transfer function is used:$\begin L'\\M'\\S' \end = EOTF_^ \left( \begin L\\M\\S \end \right)$
#* If the HLG transfer function is used:$\begin L'\\M'\\S' \end = OETF_ \left( \begin L\\M\\S \end \right)$
#Calculate ''IC_TC_P'':
#*for PQ:$\begin I\\C_T\\C_P \end = \frac \begin 2048 & 2048 & 0\\ 6610 & -13613 & 7003\\ 17933 & -17390 & -543 \end \begin L'\\M'\\S' \end$
#*for HLG: $\begin I\\C_T\\C_P \end = \frac \begin 2048 & 2048 & 0\\ 3625 & -7465 & 3840\\ 9500 & -9212 & -288 \end \begin L'\\M'\\S' \end$
All three above mentioned matrixes were derived (only the first 2 are documented derivations) from the matrixes in IPT. The HLG matrix can be derived the same way as the PQ matrix, with the only difference being the scaling of the chroma rows. The inverted decoding ''IC_TC_P'' matrixes are specified in ITU-T Series H Supplement 18.

IC_TC_P is defined such that the entire BT.2020 space fits into the range , 1for I and 0.5, +0.5for the two chroma components. The related uniform color space ITP used in ΔE_ITP (Rec. 2124) scales C_T by 0.5 to restore uniformity. There is support for ICtCp in zimg (including zimg as part of FFmpeg) and color-science, for both HLG and PQ.

In IPT

The preceder to ''IC_TC_P'', Ebner & Fairchild IPT color appearance model (1998), has a mostly similar transformation pipeline of input → LMS → nonlinearity → IPT. The differences are that it defines its input to the more general CIEXYZ tristimulus color space and as a result has a more conventional Hunt-Pointer-Estevez (for D65) matrix for LMS. The nonlinearity is a fixed gamma of 0.43, quite close to the one used by RLAB. The second matrix here is slightly different from the ''IC_TC_P'' matrix, mainly in that is also considers S (blue cone) for intensity, but ''IC_TC_P'' has also Rotation matrix (to align skin tones) and Scalar matrix (scaled to fit the full BT.2020 gamut inside the -0.5 to 0.5 region) multiplied with this matrix:

# Calculate LMS (see for D65, slightly different): $\begin L\\M\\S \end = \begin 0.4002 & 0.7075 & -0.0807 \\ -0.2280 & 1.1500 & 0.0612 \\ 0 & 0 & 0.9184 \end \begin X\\Y\\Z \end$
# Nonlinearity (L'M'S'): For each of L, M, S components apply a power function: $X' = \begin X^ &\text X \ge 0\\ -(-X)^ &\text X < 0 \end$
# $\begin I\\P\\T \end = \begin 0.4000 & 0.4000 & 0.2000\\ 4.4550 & -4.8510 & 0.3960\\ 0.8056 & 0.3572 & -1.1628 \end \begin L'\\M'\\S' \end$

IPTPQc2

IPTPQc2 is another related colorspace used by Dolby Vision profile 5 BL+RPU (without EL). The "c2" in the name means a cross talk matrix is used with c = 2%. It uses full range quantization (0-1023 for 10 bit video, no values reserved). It is also often referred to as IPTPQc2/IPT, as the matrix is in fact the same as in the 1998 IPT paper, just in inverse representation. Documentation on this format is scarce due to its proprietary nature, but a patent on the "IPT-PQ" (perceptually quantized IPT) color space The matrix is as follows:

$\begin I\\P\\T \end_ = \left( \frac \begin 8192 & 799 & 1681\\ 8192 & -933 & 1091\\ 8192 & 267 & -5545 \end \right)^ \begin L'\\M'\\S' \end$

Note the matrix inversion used and an error was made in patent in 1091 number of the matrix (the matrix after inversion is correct in patent). In addition, this format has no nonlinearity, and is assumed to be BT.2020-based.

The second step, the dynamic range adjustment modeling (reshaping), is also defined in the patent.

It is used by Disney+, Apple TV+ and Netflix.

Decoder of IPTPQc2 with reshaping and MMR (but no NLQ and dynamic metadata) is available in libplacebo.

Support for decoding all stages was added in mpv.

Characteristics

''IC_TC_P'' has near constant luminance, which improves chroma subsampling versus ''YC_BC_R''. ''IC_TC_P'' also improves hue linearity compared with ''YC_BC_R'', which helps with compression performance and color volume mapping. When combined with adaptive reshaping ''IC_TC_P'' can improve compression performance by 10%. For CIEDE2000 color quantization errors, 10-bit ''IC_TC_P'' would be equivalent to 11.5 bit ''YC_BC_R'', that is why ΔE_ITP standard was introduced as ITU-R Rec. BT.2124 and is already used in Calman. Luminance constancy is improved with ''IC_TC_P'', which has a luminance relationship of 0.998 between the luma and encoded brightness while ''YC_BC_R'' 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 the color difference information is changed.

Uses

''IC_TC_P'' is supported in the HEVC video coding standard. It is also a digital YCC format and can be signaled in EDID's Colorimetry block as part of CTA-861-H.

References

{{Color space

Color space
High dynamic range
Ultra-high-definition television