IC
T
C
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.
[1]
It was developed by
Dolby Laboratories
[2]
from the
IPT
color space by Ebner and Fairchild.
[3]
[4]
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
T
C
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.
[2]
Ebner also used IPT as short for "Image Processing Transform".
[3]
The
IC
T
C
P
color representation scheme is conceptually related to the
LMS
color space
, as the color transformation from
RGB
to
IC
T
C
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
T
C
P
using another 3×3 matrix transformation.
[5]
IC
T
C
P
was defined as a
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).
[6]
Derivation
[
edit
]
IC
T
C
P
is defined by Rec. 2100 as being derived from linear
RGB
as follows:
[1]
- Calculate
LMS
from BT.2100 RGB:
- Normalize the LMS by a non-linearity:
- If the
PQ transfer function
is used:
- If the
HLG transfer function
is used:
- Calculate
IC
T
C
P
:
- for PQ:
- for
HLG
:
All three above mentioned matrixes were derived (only the first 2 are documented derivations
[2]
) 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
T
C
P
matrixes are specified in ITU-T Series H Supplement 18.
[7]
IC
T
C
P
is defined such that the entire BT.2020 space fits into the range [0, 1] for I and [-0.5, +0.5] for 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.
[8]
There is support for ICtCp in zimg (including zimg as part of FFmpeg) and color-science, for both HLG and PQ.
In IPT
[
edit
]
The preceder to
IC
T
C
P
, Ebner & Fairchild
IPT
color appearance model
(1998), has a mostly similar transformation pipeline of input → LMS → nonlinearity → IPT.
[3]
[9]
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
T
C
P
matrix, mainly in that is also considers S (blue cone) for intensity, but
IC
T
C
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:
[2]
[10]
- Calculate LMS (see
LMS color space § Hunt, RLAB
for D65, slightly different
[3]
):
- Nonlinearity (L'M'S'): For each of L, M, S components
apply a power function
:
IPTPQc2
[
edit
]
IPTPQc2
is another related colorspace used by
Dolby Vision
profile 5 BL+RPU (without EL).
[11]
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.
[12]
Documentation on this format is scarce due to its proprietary nature, but a patent
[13]
on the "IPT-PQ" (perceptually quantized IPT) color space
seems to describe how Dolby changed the domain to PQ by changing the traditional power function from 1998 IPT paper to PQ function for each of LMS components.
[
speculation?
]
The matrix is as follows:
Note the matrix inversion used and an error was made in patent in 1091 number
[
clarification needed
]
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.
[14]
The second step, the dynamic range adjustment modeling (reshaping
[15]
), is also defined in the patent.
It is used by
Disney+
,
Apple TV+
and
Netflix
.
[
citation needed
]
Decoder of IPTPQc2 with reshaping and MMR (but no NLQ and dynamic metadata) is available in libplacebo.
[16]
Support for decoding all stages was added in
mpv
.
Characteristics
[
edit
]
IC
T
C
P
has near constant luminance.
[17]
The
correlation coefficient
between encoded
I
and true luminance is 0.998, much higher than the 0.819 for
YC
B
C
R
. An improved constant luminance versus
YC
B
C
R
is an advantage for color processing operations such as
chroma subsampling
and
gamut mapping
where only the color difference information is changed.
[2]
IC
T
C
P
also improves hue linearity compared with
YC
B
C
R
, which helps with compression performance and color volume mapping.
[18]
[19]
Adaptive reshaping can further provide a 10% improvement on compression performance.
[20]
Improvement in luminance and hue uniformity make scaled
IC
T
C
P
a practical color space for calculating color differences (
ΔE
ITP
), as introduced by ITU-R Rec. BT.2124.
[8]
In terms of
CIEDE2000
color quantization error, 10-bit
IC
T
C
P
would be equivalent to 11.5 bit
YC
B
C
R
.
[2]
Uses
[
edit
]
IC
T
C
P
is supported in the
HEVC
video coding standard.
[21]
It is also a digital YCC format and can be signaled in
EDID
's Colorimetry block as part of CTA-861-H.
References
[
edit
]
- ^
a
b
"BT.2100-2: Image parameter values for high dynamic range television for use in production and international programme exchange"
.
ITU-R
. July 2018.
- ^
a
b
c
d
e
f
"What Is ICtCp ? Introduction?"
(PDF)
. Dolby
. Retrieved
2016-04-20
.
- ^
a
b
c
d
Ebner, Fritz (1998-07-01).
"Derivation and modelling hue uniformity and development of the IPT color space"
.
Theses
.
- ^
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
- ^
"ST 2084:2014"
.
Society of Motion Picture and Television Engineers
.
- ^
"A DTV Profile for Uncompressed High Speed Digital Interfaces (ANSI/CTA-861-H)"
.
Consumer Technology Association®
. Retrieved
2021-03-11
.
- ^
"ITU-T Recommendation database"
.
ITU
.
hdl
:
11.1002/1000/13441
. Retrieved
2020-11-14
.
- ^
a
b
"BT.2124: Objective metric for the assessment of the potential visibility of colour differences in television"
.
www.itu.int
. Retrieved
24 June
2020
.
- ^
Ebner, Fritz; Fairchild, Mark D. (1998-01-01).
"Development and Testing of a Color Space (IPT) with Improved Hue Uniformity"
.
Color and Imaging Conference
.
1998
(1): 8?13.
doi
:
10.2352/CIC.1998.6.1.art00003
.
S2CID
46137693
.
- ^
Xue, Yang (1 November 2008).
"Uniform color spaces based on CIECAM02 and IPT color difference equations"
.
RITTheses
: 7.
- ^
Dolby.
"Dolby Vision Profiles and Levels Version 1.3.2 - Specification"
(PDF)
. Archived from
the original
(PDF)
on 29 September 2020
. Retrieved
27 April
2021
.
- ^
"Dolby Vision with wrong colors · Issue #7326 · mpv-player/mpv"
.
GitHub
.
- ^
US patent 20180131938A1
, Lu, Taoran; Pu, Fangjun & Yin, Peng et al., "Signal reshaping and coding in the ipt-pq color space", published 2018-05-10, issued 2019-11-19, assigned to Dolby Laboratories Licensing Corp
- ^
"testing-av/testing-video: IPTPQc2.java"
.
GitHub
.
- ^
"Description of the reshaper parameters derivation process in ETM reference software"
.
phenix.it-sudparis.eu
. Retrieved
2020-11-14
.
- ^
"colorspace: add support for Dolby Vision (!207) · Merge requests · VideoLAN / libplacebo"
.
GitLab
. 29 November 2021
. Retrieved
2021-12-11
.
- ^
"Subsampling in ICtCp vs YCbCr"
(PDF)
. Dolby Laboratories, Inc. Archived from
the original
(PDF)
on 13 October 2018.
- ^
"ITP Colour Space and Its Compression Performance for High Dynamic Range and Wide Colour Gamut Video Distribution"
. ZTE.
- ^
Cotton, Andrew; Thompson, Simon (2018). "Scene-light conversions: the key to enabling live HDR production".
SMPTE 2018
. pp. 10?11.
doi
:
10.5594/M001822
.
ISBN
978-1-61482-960-7
.
S2CID
188363770
.
- ^
Perrin, Anne-Flore; Rerabek, Martin; Husak, Walt; Ebrahimi, Touradj (May 2018).
"ICtCp Versus Y'CbCr: Evaluation of ICtCp Color Space and an Adaptive Reshaper for HDR and WCG"
.
IEEE Consumer Electronics Magazine
.
7
(3): 38?47.
doi
:
10.1109/MCE.2017.2714696
.
S2CID
4800923
.
- ^
Peng Yin; Chad Fogg; Gary J. Sullivan; Alexis Michael Tourapis (2016-03-19).
"Draft text for ICtCp support in HEVC (Draft 1)"
. JCT-VC
. Retrieved
2016-04-20
.