Difference between revisions of "YCoCg"

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Some modern video codecs (e.g. [[dirac]], [[h264|h264 FRExt]]) support the YCoCg colorspace. More correctly written this is YC<sub>o</sub>C<sub>g</sub>, with the 'o' and 'g' characters as subscripts.
Some modern video codecs (e.g. [[Dirac]], [[H.264|H.264 FRExt]]) support the YCoCg colorspace. The correct written expression for this colorspace is YC<sub>o</sub>C<sub>g</sub>, with the 'o' and 'g' characters as subscripts.


[[Category:Compression Theory]]
== Technical Details ==
 
The various components of the name represent:
 
== technical Details ==
This is what the components represent:


     * Y  = pseudo luminance, or intensity
     * Y  = pseudo luminance, or intensity
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     * C<sub>g</sub> = "green chrominance"
     * C<sub>g</sub> = "green chrominance"


In contrast to [[YCbCr]] this doesn't represent the human kind of viewing.
In contrast to [[YCbCr]], this colorspace isn't based on the human vision model. This colorspace was invented to use similar encoding techniques as [[YCbCr|YC<sub>b</sub>C<sub>r</sub>]] but with frames in [[RGB]] colorspace. It is possible to losslessly transform from [[RGB]] to YCoCg when using 2 more bits for YCoCg representation than for RGB. E.g., it is possible to losslessly transform a pixel from a 30-bit RGB frame into a pixel in a 32-bit YCoCg 4:4:4 frame and back. This assumes that each R, G, and B component will have 10 bits of information which Y will have 10 bits and Co and Cg will each have 11 bits.
 
This colorspace is invented to use similar encoding techniques as for [[YCbCr|YC<sub>b</sub>C<sub>r</sub>]] but with frames in [[RGB]] colorspace.
It is possible to losslessly transform from [[RGB]] to YCoCg when two more bits than used for RGB are available for the YCoCg representation. This way it is possible to losslessly transform a 30-bit RGB frame into 32 bits in YCoCg 4:4:4 and back.
 
lossless: 10 bits for each color RGB <=> 10 bits for Y and 11 bits for each chrominance
 
Sometimes this colorspace is called YCoCg-R because of the lossless reversable transformation. The original and outdated algorithm could not restore the RGB value exactly but used only as much bits for the YCoCg version as the RGB version. The latter algorithm isn't used anymore.


Like with [[:Category:YCbCr Formats|YCbCr]] it is also possible to use different sized planes for each component. Thus, every pixel in an image of a YCoCg encoded frame is associated with one Y sample, but possibly groups of pixels share C<sub>o</sub> and C<sub>g</sub> samples.
Sometimes this colorspace is called YCoCg-R because of the lossless reversible transformation. The original and outdated algorithm could not restore the RGB value exactly but used as many bits for the YCoCg version as the RGB version. The latter algorithm isn't widely used anymore.


Like with [[:Category:YCbCr Formats|YCbCr]] it is also possible to use different plane sizes for each component. Thus, every pixel in an image of a YCoCg encoded frame is associated with one Y sample, but possibly groups of pixels share C<sub>o</sub> and C<sub>g</sub> samples.


== possible Formats ==
== Possible Formats ==


So this encodings are possible for instance
Possible encodings include:
* YCoCg 4:4:4
* YCoCg 4:4:4, analogous to [[YCbCr 4:4:4]]
* YCoCg 4:2:2
* YCoCg 4:2:2, analogous to [[YCbCr 4:2:2]]
* ...
* ...


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   B = t  - (Co >> 1)
   B = t  - (Co >> 1)
   R = Co + B
   R = Co + B
[[Category:Compression Theory]]

Latest revision as of 14:59, 16 March 2009

Some modern video codecs (e.g. Dirac, H.264 FRExt) support the YCoCg colorspace. The correct written expression for this colorspace is YCoCg, with the 'o' and 'g' characters as subscripts.

Technical Details

The various components of the name represent:

   * Y  = pseudo luminance, or intensity
   * Co = "orange chrominance"
   * Cg = "green chrominance"

In contrast to YCbCr, this colorspace isn't based on the human vision model. This colorspace was invented to use similar encoding techniques as YCbCr but with frames in RGB colorspace. It is possible to losslessly transform from RGB to YCoCg when using 2 more bits for YCoCg representation than for RGB. E.g., it is possible to losslessly transform a pixel from a 30-bit RGB frame into a pixel in a 32-bit YCoCg 4:4:4 frame and back. This assumes that each R, G, and B component will have 10 bits of information which Y will have 10 bits and Co and Cg will each have 11 bits.

Sometimes this colorspace is called YCoCg-R because of the lossless reversible transformation. The original and outdated algorithm could not restore the RGB value exactly but used as many bits for the YCoCg version as the RGB version. The latter algorithm isn't widely used anymore.

Like with YCbCr it is also possible to use different plane sizes for each component. Thus, every pixel in an image of a YCoCg encoded frame is associated with one Y sample, but possibly groups of pixels share Co and Cg samples.

Possible Formats

Possible encodings include:

Algorithm

The algorithm is described in this paper: YCoCg(-R) Color Space Conversion on the GPU.

And in the dirac spec in section F.1.5.2.

From RGB to YCoCg:

Co = R - B
 t = B + (Co >> 1)
Cg = G - t
 Y = t + (Cg >> 1)

and back from YCoCg to RGB:

 t = Y  - (Cg >> 1)
 G = Cg + t
 B = t  - (Co >> 1)
 R = Co + B