[whatwg] Proposal to remove a paragraph from canvas spec

[Robert O'Callahan]

On Wed, May 4, 2011 at 2:08 PM, Matthew Delaney <mdelaney at apple.com> wrote:

> Since:
> 1) This behavior is obviously not ideal (see the linked animation above)
> and is really just a bug
>

See the thread "<canvas> shadow compositing oddities" from July 2008. In
that thread, Eric Butler points out that always drawing shadows has problems
with operators other than dest-atop. For example, with source-in, always
drawing a fully transparent shadow would mean that source-in acts just like
clear!

So as well as being compatible with CG, this behavior was thought to be
desirable because it provides a simple way for shadows to be completely
disabled by default, without requiring authors having to explicitly enable
shadows.

So here are some options:
1) leave the spec as-is
2) introduce explicit API to enable/disable shadows (would break a lot of
Web content at this point, probably not realistic)
3) remove the no-shadow special case, as you suggest (breaks some operators,
probably not a good idea)
4) remove the no-shadow special case, but add a special case to not draw
shadows for operators other than source-over

I think I prefer #4. I have yet to hear of any use-case that needs shadows
with an operator other than source-over, and it would probably simplify the
spec and implementations a little bit.

Rob
-- 
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