[whatwg] Fwd: RE: Inconsistent behaviour of globalCompositeOperation property - Drawing model discussion

Robert O'Callahan robert at ocallahan.org
Mon Jan 17 13:47:07 PST 2011

On Wed, Jan 5, 2011 at 12:05 PM, <carol.szabo at nokia.com> wrote:

> Given the statements above I no longer think that changing the spec in this
> regard is a good thing, but I still believe that the disappearance of
> shadows for the source-in and copy modes and the strange result when shadows
> are drawn and the composite operation is source-out should be corrected. To
> do this I suggested the following in my previous e-mail, but I got no
> comments about my suggestion so I repeat it here (please excuse my
> insistence):
> Replace steps 3 to 6 of the drawing model, with:
> 3. When shadows are drawn<http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#when-shadows-are-drawn>,
> composite B (source) with A (destination) using destination-over operation.
> 4. Multiply the alpha component of every pixel in A by globalAlpha<http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-globalalpha>
> .
> 5. Composite A within the clipping region<http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#clipping-region>over the current canvas bitmap using the current composition operator.

That might work. However, I have an alternative proposal: if we don't have
good use cases for using shadows with non-"source-over" operators (I don't),
let's just say that shadows don't draw for non-"source-over" operators. That
would reduce spec and implementation complexity.

"Now the Bereans were of more noble character than the Thessalonians, for
they received the message with great eagerness and examined the Scriptures
every day to see if what Paul said was true." [Acts 17:11]

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