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[whatwg] question about canvas globalCompositeOperation

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Sat, 21 Aug 2010 20:04:46 +1000
Message-ID: <AANLkTimoag3q7qhOvLOdqs8x__WtnSGXXszfhXD0L6oK@mail.gmail.com>
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
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