Hi Simon,
On 20/01/2015 7:29 am, Simon Thum wrote:
Hi Nikos,
thank you for your kind answer. Please see inline for my comments.
On 01/19/2015 05:28 AM, Nikos Andronikos wrote:
Hi Simon,
On 16/01/2015 8:21 am, Simon Thum wrote:
Dear all,
I just reviewed the latest draft of the compositing spec [0], and I
would like to ask if an explicit clarification about the color
interpolation space could be included in the compositing spec.
So, to confirm, you would ideally like the compositing spec to state
that all blending and compositing operations must be performed in a
linear colour space?
I *would like*, yes, but I understand the limitations. My order of
preference is:
1) A correct and unambiguous standard
2) An intentionally incorrect but unambiguous standard
3) An incorrect but unambiguous standard
4) an ambiguous standard
I really would avoid 4, which is where compositing is at now (if I'm
not missing something).
The compositing spec isn't assuming sRGB. It is defining blending and
compositing operations that can be performed in any RGBA colour space -
with varying levels of quality (and perhaps this should be stated - see
below). SVG and CSS provide mechanisms for setting the colour space for
those operations, but these aren't implemented anywhere, which is
unfortunate.
I don't think that's possible as it would not be backwards compatible,
but perhaps the next best thing would be to add a note for authors
stating that compositing and blending operations should always be done
in a linear colour space, which is controllable with
color-interpolation?
However I fear that wouldn't achieve very much, as the browser vendors
have shown (from what I can see) little interest in allowing
transformations to be performed in any alternate colour spaces (probably
because of limitations in the underlying graphics libraries that they
use).
e.g. color-interpolation isn't implemented anywhere (although is still
present in SVG 2 so maybe there's hope?).
If I'm not mistaken composite specifies a bunch of stuff that has not
been available broadly yet. E.g. SVG used the vehicle of
color-interpolation-filter to put a sane default on newly-specified
stuff (at least that's my interpretation). I'm not sure if that's an
option in compositing but it should be given some thought.
The compositing specification builds on what already existed - that was
called simple alpha compositing, which uses the src-over Porter Duff
operator and the Normal blend mode.
So, when an author selects these values, the result must be the same as
it appeared in SVG 1.1.
I don't think it's performance that's blocking this - it's support in
the underlying libraries.
I also don't think we can make linearRGB the default due to backwards
compatibility issues.
Bugs tracking color-interpolation in browsers:
WebKit: bugzilla.opendarwin.org/show_bug.cgi?id=5972
Mozilla: https://bugzilla.mozilla.org/show_bug.cgi?id=298281
Thanks for those bugs! But whatever is going on implementation-wise,
the document should be unambiguous. Right now, as mentioned, it's in
the terrain but not the spirit of PNG and SVG. This will bring up
implementation issues that could be avoided with explicitly specifying
the relation to PNG compositing, the color-interpolation CSS/SVG
property etc.
For example, when alpha-compositing a PNG on a page, which algorithm
applies? PNG or compositing?
My understanding is that the PNG spec only says 'should'. Therefore PNG
only recommends alpha compositing in a linear space, but this is not
strictly required, and browsers ignore it and do all PNG composing in sRGB.
A test: http://jsfiddle.net/dodgeyhack/ffzjtrwz/1/
I know it's really hard to find correct implementations of color
algorithms, but one cannot expect that to improve if something exposed
like W3C compositing implies it's a non-issue by silently assuming
sRGB compositing, even in conflict with some of SVG and PNG.
So, whatever the actual choice to go with [which is probably the
simple but wrong choice as the default or only option], please make it
explicit and well-documented, thus climbing up to 2 in the above
order. I volunteer to provide input if that's what is missing.
Cheers,
Simon
My thoughts are that the compositing spec should include a note for
authors stating:
- to achieve high quality/accurate colour output, all operations need to
be performed in a linear colour space
- the default colour space is non-linear sRGB
- the colour space for compositing operations can be controlled with the
color-interpolation property
Then it is up to implementations to implement color-interpolation.
I would like to add this wording to the next version of the spec - level
2. I don't think it's worth revising the current CR since the advice
cannot be acted upon currently.
Would you be happy with this? And what do others think?
Cheers,
Nikos
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