W3C home > Mailing lists > Public > www-style@w3.org > October 2011

Re: [css-shaders] subdivision for transparency

From: Dean Jackson <dino@apple.com>
Date: Tue, 04 Oct 2011 14:18:06 +1100
Cc: www-style list <www-style@w3.org>
Message-id: <B71E700A-2B29-4E3F-BE8B-96DCF9602B5B@apple.com>
To: Gregg Tavares <gman@google.com> (wrk)

On 04/10/2011, at 11:10 AM, Gregg Tavares (wrk) wrote:

> Looking at the CSS shader proposal I wonder what if any story there is about subdivision of polygons to correctly display content.
> 
> The current 3D CSS spec does not list subdivision as far as I know but Safari implemented it first and as the implied reference implementation it subdivides polygons where necessary. Chrome currently does not. 

It's unfortunate that the specification does not describe what to do here. In fact, that was deliberate at the time because we didn't know what was best. Safari originally did not subdivide the polygons, and still doesn't on OS X 10.5. The fact that we do now isn't really something we specify (even at a platform level)

> What do I mean? Well, in order to correctly display 2 or more semi-trasparent elements that intersect each other in 3d space it's necessary to subdivide the polygons that are used to draw the elements with so that the part of the elements that are behind others are drawn first, then the parts in front are drawn last.
> 
> Here are 2 screenshots. The first shows Safari which correctly subdivides the polygons. The second shows Chrome which currently does not.
> 
> <correct-3d-css-polygon-sorting-subdivisions-safari.png> <incorrect-3d-css-polygon-sorting-subdivisions-chrome.png>
> 
> Arguably Chrome should update its renderer to handle this case so that web developers can count on correct displaying of 3d css content.
> 
> 
> 
> But, that brings up the question, that's the sorting spec going to be for CSS shaders?

I'm not sure I understand the question completely. CSS filters (let's not call them shaders - that's just a proposed extension to the current draft) are always a 2d effect, even if they appear 3d. In other words, the effect might operate locally in a 3d or pseudo-3d environment, but the result is always composited in 2d onto a flat plane. Elements should sort the way they do for regular HTML (with CSS 3D transforms, which is probably less-than-well-defined). A filter does not make an element 3d.

Dean

> 
> 
> 
> ps: the live html/css is here
> http://greggman.com/downloads/examples/intersecting-elements-3d-css.html
> 
> pps: if you care about Chrome status on this issue you can follow it here
> 
> 
Received on Tuesday, 4 October 2011 03:18:42 GMT

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