[CREATE] ORA spec: adding additional layer effects

[Andrew Chadwick]

On 28 February 2012 14:35, Chris Lilley <chris at w3.org> wrote:
> On Sun, Feb 26, 2012 at 1:10 AM, Alexandre Prokoudine wrote:
> > [newer version of the GIMP use "CIE LCH" for the Hue, Saturation, Value and Color layer modes]
>
> Assuming that means LCHab (as opposed to LCHuv) then SVG2 will have that one, as well.
>
> The advantage over HSV or HSL is much better perceptual uniformity. The disadvantage is that interpolation can produce out of gamut colours, which need to be clipped or, better, gamut mapped.

I'm interested only in perceptual relevance here, so not supporting
HSV or HSL in OpenRaster is no big deal. I want to drive the
OpenRaster spec towards having Colour and Lightness layer modes that
are relatively simple to implement, and don't require too many levels
of transformation.

Linear YUV (a.k.a. YCbCr) space seems to me to be a good candidate for
a codified pair of set-color and set-lightness operators as well,
being a perceptual if not absolute colour space. Perhaps this could be
used for a "simple" profile, for use by apps which do not use colour
management? The results of the obvious transform in Y require scaling
or clipping of U, V to the corresponding RGB space's gamut envelope,
just as LCHab manipulations do, but it turns out that computing this
for YUV is extremely simple and efficient[1][2]. I'll document this a
bit further on the MyPaint wiki and put up some test images for
discussion shortly.


An interesting open question is whether such a simple profile could be
used as an approximation to the LCHab method used by the GIMP. If so,
then the same "composite-op" attribute value could be used in
conformant ORA files, and the user would be guaranteed similar if not
identical views and continuity of workflow in both simple and complex
apps.


[1] http://dx.doi.org/10.1109/ICIP.2010.5652000
[2] https://gitorious.org/~achadwick/mypaint/achadwick-mypaint/commits/layer-colorize

-- 
Andrew Chadwick