[webkit-dev] Adding blending mode to WebKit canvas

Dirk Schulze dschulze at adobe.com
Mon Nov 12 07:39:22 PST 2012

On Nov 11, 2012, at 10:09 PM, "Rik Cabanier" <cabanier at gmail.com<mailto:cabanier at gmail.com>> wrote:

On Sun, Nov 11, 2012 at 9:52 PM, Dirk Schulze <krit at webkit.org<mailto:krit at webkit.org>> wrote:

On Sunday, November 11, 2012, Rik Cabanier wrote:

On Sun, Nov 11, 2012 at 8:43 PM, Maciej Stachowiak <mjs at apple.com> wrote:

On Nov 11, 2012, at 6:59 PM, Rik Cabanier <cabanier at gmail.com> wrote:

Wouldn't it be better to add a new property to canvas for blending? At the beginning, implementations are just require to use different blend modes in combination with 'source-over'.

That could work too.
There was a mailing list conversation about this a couple of months ago, and people were evenly split on the subject.

The vast majority of cases will use 'source-over' in combination with blending so maybe it's best to keep it simple...

It doesn't make sense to me for blend mode and composite operator to be separate in CSS, but combined in Canvas. Either there are valid use cases for specifying them separately or there are not. I cannot imagine how this could differ between Canvas and CSS.

To be fair, the 'globalCompositOperator' property mixed the compositing modes with some blend modes already. Which is the fault of the WebKit implementation. IIRC they have been removed from the Canvas part of the HTML spec for some time, but were added later again. Now we have multiple independent implementations that support all currently specified operators.

Is this the 'darker' compositing mode or are there others?

Lighter and darker, yes.


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