[whatwg] question about canvas globalCompositeOperation

Silvia Pfeiffer silviapfeiffer1 at gmail.com
Sat Aug 21 03:04:46 PDT 2010

Sorry, forget about this question.
I have managed to get it working after all.

On Sat, Aug 21, 2010 at 5:54 PM, Silvia Pfeiffer
<silviapfeiffer1 at gmail.com>wrote:

> Hi all,
> I've tried to work with the globalCompositeOperation [1] over the last days
> for a particular use case where I am trying to use a gradient to merge with
> a canvas as a mask. I now wonder if the globalCompositeOperation is indeed
> meant to be used this way and what transparency means.
> For example, "source-in" is defined as:
> "A in B": Display the source image wherever both the source image and
> destination image are opaque. Display transparency elsewhere.
> What about partial transparency? I would expect the use of the canvas
> bitmap B as a mask for the object or image A, where A becomes partially
> transparent where-ever B is partially transparent. As it stands, browsers do
> not implement it this way - only completely transparent pixels will create
> transparency in A, which means when using a gradient that nothing become
> partially transparent - it's either there or gone.
> Should partial transparency in B not be dealt with as partial transparency
> in the resulting display, too?
> Apologies if this has been dealt with before. I'm looking at it with video
> as the input medium, i.e. through drawImage(), trying to make it partially
> transparent.
> Cheers,
> Silvia.
> [1]
> http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-globalcompositeoperation
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.whatwg.org/pipermail/whatwg-whatwg.org/attachments/20100821/319335bb/attachment-0002.htm>

More information about the whatwg mailing list