Re: Why channel decorrelation can improve compression?

smithxjohn@xxxxxxxxx wrote:
On Jun 17, 8:39 pm, Thomas Richter <t...@xxxxxxxxxxxxxxxxx> wrote:
danilobrambi...@xxxxxxxxxx wrote:
Hi,
I am trying to understand why channel decorrelation can improve
compression in RGB24 images. Why in general (G, R-G, B-G) would
compress better than (R,G,B)? Can you suggest me any book/web site
where I can read about this?
One can give two reasons, depending on your definition of "compression".
One reason is that it is a property of natural images, i.e. color
channels are highly correlated because typically colors are not very
extreme, and most pixels lie inside the color gammut, showing mostly
variation in luminance, but not very extreme variation in color. One
might say that this is probably because the dyes of nature are never
"perfect" and do not generate pure colors.

Color differencing doesn't always works nicely. Are there better
methods when goal is just lossless compression?

Sure. One option is the RCT of JPEG2000 which uses better "color weights", namely

(r + 2g + b) / 4 -> y

b - y -> cb

r - y -> cr

which can be seen to be reversible. MS proposes a "YCbCo" transformation, which works AFAIK as follows:

b - r -> v
g - r - (v+1)/2 -> u
g - (u/2) -> y


Here the "u" coordinate is approximately g - (r + b) / 2, and thus y is again approximately g/2 + r/4 + b/4 as above, but u and v are "rotated" compared to the standard YCbCr in the sense that v is approximately the difference between Cb and Cr, and u is approximately its average.

I haven't tried to how well that works compared to the above, or if it works substantially better. You may want to try yourself. I'm certainly interested in your findings.

So long,
Thomas


.

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