- From: Rik Cabanier <cabanier@gmail.com>
- Date: Wed, 11 Dec 2013 11:04:16 -0800
- 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>
- Message-ID: <CAGN7qDBDjxZZdNT69B9poRE=TjZEtbXDYTO3ZYJheS95+z5GSA@mail.gmail.com>
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 á > 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 defined. As you noted, this was not the case originally and it caused confusion for implementors. > >>> (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