1. with RGB Reversibility
YCoCg:
A Color Space
with RGB Reversibility
(FRExt)
1st Feb 2006

2. Profiles H.264 has four profiles:
Baseline (IP Video phone, Simple streaming)
Main Profile (Digital Storage Media, Television Broadcasting)
Extended Profile (Streaming video)
High Profile (Content contribution, Content distribution, Studio editing, post processing)

3. High Profile
There are four High Profiles (Fidelity range extensions-FRExt)
High Profile is to support the 8-bit video with 4:2:0 sampling for applications using high resolution.
High 10 Profile is to support the 4:2:0 sampling with up to 10 bits of representation accuracy per sample.
High 4:2:2 Profile is to support up to 4:2:2 chroma sampling and up to 10 bits per sample.
High 4:4:4 Profile is to support up to 4:4:4 chroma sampling, up to 12 bits per sample, and integer residual color transform for coding RGB signal.
Mainly targeting the HD DVD, Broadcast, Editing, Digital still camera market

4. Specific coding parts for the Profiles

5. MacroBlock (MB) structure in 4:4:4 formatY Cb Cr 16
In 4:4:4 video, luma and both the chroma components
will have same spatial resolution

6. High Profile Applications using High Profile
Use more than 8 bits per sample of source video accuracy
Use higher resolution for color representation than what is typical in consumer applications (i.e., 4:2:2 or 4:4:4 sampling as opposed to 4:2:0 chroma sampling format)
Perform source editing functions such as alpha blending (a process for blending of multiple video scenes, best known for use in weather reporting where it is used to super- impose video of a newscaster over video of a map or weather-radar scene)
Use very high bit rates
Use very high resolution
Achieve very high fidelity – even representing some parts of the video losslessly
Avoid color-space transformation rounding error
Use RGB color representation

7. H.264 Encoder

8. In 4:4:4 video, FRExt has Residual Color Transform.
Keep RGB domain (same depth) for input, output and stored reference pictures and use the forward and inverse color transformations inside the encoder and decoder for processing of the residual data only.
Eliminates color-space conversion error without significantly increasing the overall complexity of the system.

9. H.264 Encoder
= Residual Color Transform RGB to YCoCg

10. Residual color transform exploits the redundancy among the residual data of each RGB component after intra/inter prediction. The correlation among the color components still exists even after the intra/inter prediction. To decorrelate this redundancy, we apply the YCoCg color transform to the residual data.
This scheme can be easily incorporated into the existing JVT specification.
This technique achieves better coding efficiency than applying the color transform outside the coding loop as manifested in the simulation results of [1], [2].
YCoCg color space has improved coding gain relative to both RGB and YCbCr.
Y = Luminance component
Co = Orange Chroma component
Cg = Green Chroma component

11. YCbCr Color Space There are two problems with this approach.
Since the samples are actually represented using integers, rounding error is introduced in both the forward and inverse color transformations.
The second is that, because the above transformation was not originally designed for digital video compression, it uses a sub-optimal trade-off between the complexity of the transformation (with difficult-to-implement coefficient values such as and ) and coding efficiency.

12. YCoCg Color Space Forward Color Space Transform
Inverse Color Space Transform

13. So encoder/decoder needs only additions and shifts to RGB YCoCg

14. R, G, and B are the residual data and
Y, Co, and Cg are the residue transformed data

15. Simulation Results that show the coding gain for Y component in dB
Seq.
bit rate (Mbps) 10 40
100
Analog TV NA 1
1.5
Trees and Bicycle
0.5
0.6
0.8
Man in Restaurant
0.7
Rolling Tomatoes
1.2
Card Toss
Dinner
The coding gain using residual color transform for Y component in YCoCg color domain is about 0.5~1.5 db depending on the bit rate.
Note: These results are taken from [2]

16. Conclusion
YCoCg color space transformation has three important features:
1. Exactly reversible in integer arithmetic. (i.e MSE=0)
2. Minimal increase in dynamic range; no increase for Y and only 1 bit increase for Co and Cg.
3. Higher coding gain than other color spaces.

17. RGB Component

18. YCoCg Component

19. Reconstructed RGB Component

20. References
[1] JVT document - JVT-I014r3.doc (ftp://standards.polycom.com)
[2] JVT document - JVT-L025.doc
[3] JVT document - JVT-Q059_L.doc & JVT-Q059_L.xls
[4] Soon-kak Kwon, A. Tamhankar, K. R. Rao, “Overview of H.264/MPEG-4 Part10”, Special issue on “ Emerging H.264/AVC video coding standard”, J. Visual Communication and Image Representation, vol.17, 2006
[5] G. Sullivan, P. Topiwala and A. Luthra, “The H.264/AVC Advanced Video Coding Standard: Overview and Introduction to the Fidelity Range Extensions,” SPIE Conference on Applications of Digital Image Processing XXVII, vol. 5558, pp , Aug
[6] H.264 : International Telecommunication Union, “Recommendation ITU-T H.264: Advanced Video Coding for Generic Audiovisual Services,” ITU-T, 2003

21. Thank You