Re: [Algorithms] RGB to XYZ conversion

[Sam M. <sam...@ge...>]

I believe the YCoCg-R version is detailed here:
research.microsoft.com/~malvar/papers/JVT-I014r3.pdf

I haven't really looked into it though.

What sort of errors did you get from bilinear interpolation? I had been expecting interpolation to produce the correct result as it's just a linear operation and doesn't matter if you take linear combinations, ie:

If a and b are scalars (your bilinear interpolation values), x any y are colours, and R is the RBG->YUV transform with R' it's inverse:

a*x + b*y = R' (a*Rx + b*Ry)
	    = a*R'Rx + b*R'Ry
     	    = a*x + b*y

However, I fear that I'm missing something ridiculously 'obvious' here and about to make a fool of myself!

Cheers,
Sam


From: gda...@li... [mailto:gda...@li...] On Behalf Of Jon Olick
Sent: 24 April 2008 15:50
To: Game Development Algorithms
Subject: Re: [Algorithms] RGB to XYZ conversion

Hi Sam,

I agree, the loss of 1 bit is usually nothing to worry about. Surely we don't here. Just wanted to point it out for correctness. However, I am interested in the variant Simon alluded to which does not require the additional bit.

The only artifacts I've seen from using YCoCg as a production format is that when interpolating between different colors the wrong colors can show up in the bilinear interpolation. However, this is usually minor and barely noticeable.

Best,
Jon Olick

On Thu, Apr 24, 2008 at 4:39 AM, Sam Martin <sam...@ge...> wrote:
>He missed a few signs:
>> [y,Co,Cg]' = [1/4, 1/2, 1/4].[r,g,b]'
>>              [1/2, 0,   -1/2]
>>              [-1/4, 1/2, -1/4]
Yeah, cheers - I was reformatting it from a copied pdf. The signs must
have been eaten somewhere in the process.

> Out of curiosity, what did you mean by "had any issues with it"? YCoCg
> is primarily a replacement for YUV/YCbCr that allows cheaper (and
> lossless) transforms between the two colour spaces and typically does
a
> good job of extracting the correlation in the RGB. If you want to use
it
> for image compression, then it's a good first step.
As other people have commented, I'm also not bothered by the 1-bit
'lossy' precision aspect. That's well below the radar :)

I'm actually interested in how this luminance+chroma variant stands up
against its equivalents from an art and data 'reduction' point of view.
Can you sensibly work as an artist in YCoCg space? How well does it
represent the rgb correlation in comparison to its alternatives?

I suspect a fair amount of effort went into defining the balance of
luminance/chroma in other variants of this and (at least on the surface)
YCoCg appears to just throw all that away!

Cheers,
Sam

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