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Re: [css-compositing] Request to move Compositing and Blending spec to CR

From: James Robinson <jamesr@google.com>
Date: Wed, 11 Dec 2013 10:55:20 -0800
Message-ID: <CAD73mdJ4p+rRzNnZOEXiZw7jmu75kLbOPKXQKh2TxsDDH=p39w@mail.gmail.com>
To: Rik Cabanier <cabanier@gmail.com>
Cc: www-style list <www-style@w3.org>, "public-fx@w3.org" <public-fx@w3.org>, www-svg <www-svg@w3.org>
On Wed, Dec 11, 2013 at 9:11 AM, Rik Cabanier <cabanier@gmail.com> wrote:

> Hi James,
> thanks for the review!
> On Tue, Dec 10, 2013 at 11:40 PM, James Robinson <jamesr@google.com>wrote:
>> I have an issue with the way the spec defines clip-to-self:
>> http://dev.w3.org/fxtf/compositing-1/#groupcompositingcliptoself
>> "
>> When compositing, the areas of the composite that may be modified by the
>> compositing operation, must fall within the shape of the element being
>> composited (i.e. where  > 0). This is known as "clip to self" in some
>> graphics libraries. The alternative is to not clip the compositing
>> operation at all. The results can be seen in the figure below. Some of the
>> Porter Duff operators are unchanged, because they normally have no effect
>> outside the source region. The changes can be seen in the clear, source,
>> source-in, destination-in, source-out and destination-atop.
>> "
>> If I understand correctly, this is defining that compositing only occurs
>> when source pixels have alpha > 0.  There are three problems with this
>> proposal:
>> 1.) This introduces a sharp discontinuity between near-zero and zero
>> alpha values
>> 2.) Due to (1), this is highly susceptible to precision issues in
>> implementations
>> 3.) This is inconsistent with other web technologies like Canvas
> Note that this is for operations that are implemented with 'clip-to-self'.
> Currently, there are none.
> Compositing for HTML/SVG originally had this feature and this is why it
> was cut from the specification.

OK, if it's not used by any operations let's remove that text from the spec
(or clarify what it means).

>> (1) This introduces a sharp discontinuity between near-zero alpha values
>> and zero alpha values.  An alpha value of 256 and 255 render very much the
>> same, same with a red channel value of 0 vs 1 or any other values.  With
>> this clip behavior, an alpha value of zero means "do not apply composite
>> operation" whereas one of very nearly but not quite zero means "apply the
>> operation" which could result in the final color being entirely different.
>>  This can produce unexpected results in cases where the alpha value is
>> naturally close to zero, such as with gradiants or low opacity values, but
>> especially in combination with (2) - this is highly susceptible to
>> precision issues.  Depending on how implementations store alpha values in
>> intermediate steps, how they perform blending operations, and the render
>> other effects like gradients, filters, text etc two implementations could
>> end up with vastly different areas with alpha==0 vs alpha < epsilon on the
>> same content.  With this compositing definition, the final output would be
>> completely different.  This is a really difficult thing to nail down
>> especially as implementations consider using more or fewer bits for alpha -
>> for instance doing 10 bit/channel, using per-channel alpha for text AA, or
>> using fewer bits for intermediate results.  This has been a continuing
>> concrete problem for our implementation in tests that are over-eager about
>> checking the alpha values.  Often the results will be perceptually
>> identical but have minor differences in low bits of the alpha or color
>> channels.
>> (3) This is inconsistent with canvas.  If you will remember, several
>> years ago different implementations of the CanvasRenderingContext2D
>> interface had different behaviors when compositing for non-default
>> compositing modes.  Firefox applied the compositing operation to the entire
>> canvas, respecting the current clip, and WebKit applied the compositing
>> operation only to the "bounds" of the draw.  The issue was there was no
>> reasonable definition of the "bounds" of the draw.  The implementation
>> didn't use a alpha=0 test and had surprising behavior in some cases.  After
>> much discussion we decided to unify on the whole-canvas-respecting-clip
>> behavior.  You can find the discussion in the archives.  If CSS compositing
>> behaves differently, it both reintroduces the problems we had with canvas
>> and introduces another model for web authors to try to deal with an
>> understand.
> Canvas compositing specifies the following: [1]
> Compositing and blending in canvas 2D must always done with clip-to-self<http://dev.w3.org/fxtf/compositing-1/#groupcompositingcliptoself> assumed
> false. This means that a compositing operation may affect the entire canvas
> and not just be limited to the shape that is being composited. However, the clipping
> region <http://www.w3.org/TR/2dcontext/#clipping-region> will still be in
> effect and limit the affected area.
>> I think we should change this to the canvas behavior and add a way for
>> authors to define the region they wish compositing to apply in, perhaps by
>> using CSS shapes.  If that's not considered desirable for this level of the
>> spec, we should drop the compositing operations that depend on this and
>> reintroduce them in a future level with better clipping behavior.  From the
>> limited discussions I can find on the mailing list it seems that these
>> cases are considered rather rare for now, so maybe deferring is the way to
>> go.
> Yes, compositing for CSS was deferred but will be put back in for level 2.
> Limiting it to CSS shapes is interesting!
>> On Tue, Dec 3, 2013 at 1:12 PM, Rik Cabanier <cabanier@gmail.com> wrote:
>>> All,
>>> We would like to request that the CSS and SVG WG approve the compositing
>>> and blending spec to Candidate Recommendation level. [1]
>>> The deadline for comments for Last Call was on November 8 2013 and no
>>> changes were requested.
>>> The 'isolation' [2] property as mark at-risk since there is only 1
>>> partial implementation at this point.
>>> The deadline for the earliest progress to PR would be 4 months after CR
>>> is published,
>>> 1: http://www.w3.org/2005/10/Process-20051014/tr#cfi
>>> 2: http://dev.w3.org/fxtf/compositing-1/#isolation
> 1: http://dev.w3.org/fxtf/compositing-1/#canvascompositingandblending
Received on Wednesday, 11 December 2013 18:55:50 UTC

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