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Re: [css-compositing] How do 'alpha-compositing' and 'blend-mode' interact?

From: Rik Cabanier <cabanier@gmail.com>
Date: Fri, 3 Aug 2012 14:07:40 -0700
Message-ID: <CAGN7qDBx3WDWr-C+K5LL3wH87_KTxJ3Fa+TgN6UjOUaP+xBiCQ@mail.gmail.com>
To: Dirk Schulze <dschulze@adobe.com>
Cc: Maciej Stachowiak <mjs@apple.com>, www-style list <www-style@w3.org>
Hi Dirk,

It's unfortunate that these names were changed from their defaults, but I
agree that we have no choice but to follow them. Aliasing would just
increase confusion and I can't come up with an alternate solution.
I will make the following changes to the spec [1] so they match their
canvas counterparts [2]:
- 'source' becomes 'copy'
- 'plus' becomes 'lighter'

Please let me know if you object.

Rik

1:
https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#porterduffcompositingoperators
2:  http://www.w3.org/TR/2dcontext/#compositing

On Thu, Aug 2, 2012 at 9:22 PM, Dirk Schulze <dschulze@adobe.com> wrote:

>
> On Jul 26, 2012, at 10:05 AM, Rik Cabanier <cabanier@gmail.com> wrote:
>
> >
> >
> > On Wed, Jul 25, 2012 at 9:41 PM, Maciej Stachowiak <mjs@apple.com>
> wrote:
> >
> > On Jul 21, 2012, at 1:17 PM, Rik Cabanier <cabanier@gmail.com> wrote:
> >
> >> Hi Maciej,
> >>
> >>
> >> On Sat, Jul 21, 2012 at 11:21 AM, Maciej Stachowiak <mjs@apple.com>
> wrote:
> >>
> >> How do 'alpha-compositing' and 'blend-mode' interact if you have set
> both? I was unable to determine this from the spec. Does it actually make
> sense to specify both to non-default values at the same time? I was unable
> to understand what their relationship is supposed to be.
> >>
> >> blending and compositing are 2 separate operations.
> >> Blending takes the foreground and background pixels, mixes them
> together and produces a new image.
> >> This image is then composited with whatever compositing mode you have
> set.
> >>
> >> The 2 operations do not interact.
> >
> > They are both defined in terms of a function, a source color, a
> background color, and a result color. What happens when both are specified?
> Do you blend first, and then use that result to composite? It sounds like
> that's what you are saying, but I don't think that is spelled out in the
> spec.
> >
> > This needs to be clearly spelled out in the spec. Please consider this a
> comment on the spec.
> >
> > From the spec:
> > Blending is the aspect of compositing that calculates the mixing of
> colors where the source element and backdrop overlap. Blending takes the
> colors of the source element and mixes them with the backdrop in areas
> where the source element and backdrop overlap. Conceptually, the colors in
> the source element are blended in place with the backdrop. After blending,
> the modified source element is composited with the backdrop. [1]
> >
> >
> >>
> >> Most implementations (including Apple's core graphics and Adobe PDF)
> always assume 'src-over' after blending.
> >
> > Is that what the spec will require?
> >
> > yes, from the spec:
> > The description of the ‘alpha-compositing’ property is as follows:
> >
> > ‘alpha-compositing’
> > Value:        clear | src | dst | src-over | dst-over | src-in | dst-in
> | src-out | dst-out | src-atop | dst-atop | xor | plus
> > Initial:      src-over
> > Applies to:   All elements. In SVG, it applies to svg, g, use, image,
> path, rect, circle, ellipse, line, polyline, polygon, text, tspan, and
> marker.
> > Inherited:    no
> > Percentages:          N/A
> > Media:         visual
> > Animatable:   yes
> >
> >>
> >> Some other comments:
> >>
> >> - The property values for the 'alpha-compositing' operator are
> inconsistent with the values for HTML5 Canvas: <
> http://www.w3.org/TR/2dcontext/#dom-context-2d-globalcompositeoperation>.
> I suggest making them match if there is no compelling advantage to the new
> names, otherwise this will be needlessly confusing to authors.
> >>
> >> That's correct. We reused the names that were in the SVG compositing
> spec.
> >> It think we can make them longer, but I'd prefer to keep 'plus' instead
> of 'lighter' because 'lighter' sounds too much like the 'lighten' blend
> mode. 'clear' also seems absent in the canvas spec...
> >
> > I think consistency with the HTML5 Canvas spec outweighs consistency
> with the old SVG compositing spec (which I don't think was ever implemented
> in browsers). I also think consistency outweighs fine-tuning the names to
> be more clear. Adding a new mode is fine, but I'd suggest proposing it for
> canvas as well. I think it would be confusing for authors if the names are
> different. Let's not make the newly introduced names different.
> >
> > I will update the spec so the 'source' and 'destination' are spelled out.
> > I've made a proposal to extend canvas with the blending operators and I
> could add the missing compositing modes to it.
> >
> > It's unfortunate that 'plus' was renamed to 'lighten'. I'm a bit
> hesitant to change it…
>
> I would like to come to a conclusion sooner than later since we are about
> to implement parts in WebKit already. I think the names should not differ
> between CSS Compositing and HTML Canvas compositing. That is more logical
> for authors and easier for implementors. But given the fact that we have
> Canvas for such a long time, I don't think that we can change the names
> there anymore. Adding new modes to Canvas  on the other hand doesn't seem
> to be a problem. I suggest renaming the compositing modes in the
> compositing spec that are the same as for Canvas so that we have a unique
> naming schema. As far as I can see this is 'copy','lighten','darken' on
> HTML Canvas. What are the corresponding modes on CSS Compositing?
>
>
> Dirk
>
> >
> >
> >>
> >> - Consider renaming 'alpha-compositing' to 'composite-operator' for
> similar reasons.
> >>
> >>
> >> 'alpha-compositing' was a suggestion from Chris Marrin and people on
> the mailing list liked it so I added it to the spec. (It was originally
> 'comp-op')
> >
> > I think alpha-compositing is a bad name because:
> >
> > 1) "alpha" is redundant, we don't have any non-alpha compositing.
> >
> > There was a discussion last year on this [3].
> > Blending can be seen as color-compositing so we chose the
> alpha-compositing to make it clear that it is based upon the alpha of the
> pixels and not the color.
> >
> > 2) "alpha-compositing" or "compositing" is ambiguous without a value -
> does this turn compositing on/off, does it specify the alpha, or does it
> specify the operator? It's not clear from the name. "composite-operator" or
> "compositing-operator" are not ambiguous in this way.
> >
> > I'm not sure what you mean.
> > These are CSS so they have a default value which is listed in the spec
> [2]
> >
> > 3) Consistency with HTML5 Canvas seems valuable to me, more so than
> crafting the most aesthetically pleasing name.
> >
> > I agree. If the 'lighten' name can't be changed in Canvas (or 'plus'
> added), I will be happy to update the spec.
> >
> >
> > Is there a way for me to make sure the CSS WG considers these spec
> comments? Should I file bugs?
> >
> > The spec is not a WD yet so I will make the changes.
> >
> > Thanks!
> >    Rik
> >
> > 1:
> https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#blending
> > 2:
> https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#alpha-compositing
> > 3: http://lists.w3.org/Archives/Public/public-fx/2011OctDec/0109.html
>
>
Received on Friday, 3 August 2012 21:08:08 GMT

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