- From: James Robinson <jamesr@google.com>
- Date: Wed, 11 Dec 2013 10:55:20 -0800
- 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>
- Message-ID: <CAD73mdJ4p+rRzNnZOEXiZw7jmu75kLbOPKXQKh2TxsDDH=p39w@mail.gmail.com>
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). > >> (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 18:55:49 UTC