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

From: Rik Cabanier <cabanier@gmail.com>
Date: Wed, 11 Dec 2013 11:04:16 -0800
Message-ID: <CAGN7qDBDjxZZdNT69B9poRE=TjZEtbXDYTO3ZYJheS95+z5GSA@mail.gmail.com>
To: James Robinson <jamesr@google.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 10:55 AM, James Robinson <jamesr@google.com> wrote:

> 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).

The definition is still needed so the canvas' non-clip-to-self behavior is
As you noted, this was not the case originally and it caused confusion for

>>> (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 19:04:47 UTC

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