[CREATE] W3C Compositing and Blending 1.0 draft is up: implications for OpenRaster?

[Andrew Chadwick]

On 9 November 2012 19:38, Guillermo Espertino (Gez)
<gespertino at gmail.com> wrote:
> El 09/11/12 14:39, Andrew Chadwick escribió:
> I'm not an expert about this, but having seen the differences between layer
> blending/compositing in linear space and gamma corrected spaces I wonder if
> this specifications take color management in consideration or simply assume
> the sRGB TRC for the inputs.

The PDF specs these W3C ones are based on say:

"A separable blend mode can be used with any color space, since it
applies independently to any number of components. Only separable
blend modes can be used for blending spot colors." -- "PDF Blend
Modes: Addendum" (to the PDF specification v1.6, Jan 23 2006)

"Since the nonseparable blend modes consider all color components in
combination, their computation
depends on the blending color space in which the components are
interpreted. They may be applied to all multiple-component color
spaces that are allowed as blending color spaces (see Section 7.2.3,
“Blending Color Space”)." -- Ibid.

"The blend functions for the various blend modes assume that the range
for each color component is 0.0 to 1.0 and that the color space is
additive." -- PDF Reference 1.6 , §7.2.3.

"The compositing computations and blend functions generally compute
linear combinations of color component values, on the assumption that
the component values themselves are linear. For this reason, it is
usually best to choose a group color space that has a linear gamma
function. If a nonlinear color space is chosen, the results are still
well-defined, but the appearance may not match the user’s
expectations. Note, in particular, that the CIE-based sRGB color space
(see page 256) is nonlinear and hence may be unsuitable for use as a
group color space." -- Ibid, §7.6.1

That's pretty a clear-cut recommendation. You can blend and composite
in nonlinear space if you like, but the results "might be" strange.

> Perhaps this question is out of place and it doesn't have anything to do
> with this specification, but when I asked Pippin about why not doing always
> compositing in linear space in GIMP and just ditch the legacy blending in
> gamma-corrected space, one of the reasons he offered was that compositing in
> web browsers is done in non-linear space and designs for the web with linear
> blending would look different than the actual rendering in a browser.
> That's of course a good reason, but it looks like the way web browsers do
> compositing is keeping us in legacyland.
>
> Since blending in linear space seems mandatory for high quality compositing
> and painting, free software applications should use it.

Agreed, though perhaps with a fallback mode for earlier free software apps.

> I did a quick search looking for gamma/linear compositing in w3c but I
> couldn't find anything that suggests that other than sRGB is used.
> Was this issue ever discussed?

I've not been following the W3C discussions that closely. Sorry.

-- 
Andrew Chadwick