YCgCo color model

The YCgCo color model (also YCoCg ) describes the color space which is formed by the luminance  Y, chrominance green Cg and chrominance orange Co. It is used, for example, in the video compression methods H.264 / MPEG-4 AVC and Dirac , as it leads to a stronger decorrelation of the color planes compared to other color models .

Original picture above and representation of the individual components Y, chrominance green Cg and chrominance orange Co.

Comparison with other color models

RGB color model

The three values ​​of the YCgCo color model can be calculated from the three color values ​​of the RGB color model as follows:

${\displaystyle {\begin{bmatrix}Y\\Cg\\Co\end{bmatrix}}={\begin{bmatrix}1/4&1/2&1/4\\-1/4&1/2&-1/4\\1/2&0&-1/2\end{bmatrix}}\cdot {\begin{bmatrix}R\\G\\B\end{bmatrix}}}$

The values ​​of the luminance Y are in the range from 0 to 1, chrominance green Cg and chrominance orange Co in the range from −0.5 to 0.5. A pure red corresponds, for example, in the RGB system (1,0,0) and in the YCgCo system (1/4, -1 / 4.1 / 2).

The conversion from the YCgCo color model to the RGB color model results from the inverted matrix to:

${\displaystyle {\begin{bmatrix}R\\G\\B\end{bmatrix}}={\begin{bmatrix}1&-1&1\\1&1&0\\1&-1&-1\end{bmatrix}}\cdot {\begin{bmatrix}Y\\Cg\\Co\end{bmatrix}}}$

In order to realize this conversion, only 2 additions and 2 subtractions are necessary. Furthermore, no real coefficients are necessary, which means that the transformation can be implemented efficiently as integer additions and subtractions:

tmp := Y   − Cg;
R   := tmp + Co;
G   := Y   + Cg;
B   := tmp − Co;

YCbCr color model

Compared to the YCbCr color model, the YCgCo color model has the advantage of a simpler and faster calculation as well as a stronger decorrelation of the color planes.

literature

• Tilo Strutz: Image data compression. Basics, coding, wavelets, JPEG, MPEG, H.264 4th edition, Vieweg + Teubner 2009, ISBN 978-3-8348-0472-3 (print), ISBN 978-3-8348-9986-6 (online)

Research on the YCgCo color model:

• H. Malvar, G. Sullivan: YCoCg-R: A color space with RGB reversibility and low dynamic range. ISO / IEC JTC1 / SC29 / WG11 and ITU-T SG16 Q.6, Document JVT-I014, 2003.
• S. Sun: Residual Color Transform Using YCoCg-R. ISO / IEC JTC1 / SC29 / WG11 and ITU-T Q6 / SG16, Document JVT-L014, March 2004.
• Woo-Shik Kim, Dmitry Birinov, Dae-Sung Cho, Hyun Mun Kim (Multimedia Lab, Samsung AIT); Video Coding Experts Group (VCEG): Enhancements to RGB coding in H.264 / MPEG-4 AVC FRExt . Proposal, 26th Meeting: Busan, KR, 16–22. April 2005, ITU Document VCEG-Z16 ( MS Word )
• P. Agawane, KR Rao (Multimedia Processing Lab, University of Arlington): Implementation and evaluation of residual color transform for 4: 4: 4 lossless RGB coding. International Conference on Recent Advances in Communication Engineering, Hyderabad, India. December 20-23, 2008, uta.edu ( ppt )

Individual evidence

1. ^ Dirac Specification. (PDF; 891 kB) (No longer available online.) P. 136 , archived from the original on May 3, 2015 ; accessed on May 4, 2010 (English). Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.   
2. ↑ ^{a b} YCoCg: A Color Space with RGB Reversibility. ( ppt ; 583 kB) University of Texas at Arlington, accessed on May 2, 2010 (English).  
3. ↑ ^{a b} Yair Moshe: H.264 Amendment: Fidelity Range Extensions. (PDF) (No longer available online.) Signal and Image processing Lab (SIPL), p. 15 , archived from the original on October 6, 2014 ; accessed on May 2, 2010 (English). Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.