Re: [Algorithms] RGB to XYZ conversion

[Jon O. <jon...@gm...>]

Hi Sam,

If you map out the Co Cg as a two dimensional 256x256 texture, you will see
that a linear interpolation between different Co and Cg points can sometimes
produce variations in color that are undesired, or at least different than
interpolating in RGB. This can produce minor visual glitches. You really
have to be staring at it to notice it though. We are just writing it off as
an acceptable loss for the improved DXT5 compression.

Best,
Jon

On Thu, Apr 24, 2008 at 10:41 AM, Sam Martin <sam...@ge...>
wrote:

> I believe the YCoCg-R version is detailed here:
> research.microsoft.com/~malvar/papers/JVT-I014r3.pdf<http://research.microsoft.com/%7Emalvar/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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