[whatwg] Canvas: clarification of compositing operations needed

[Gregg Tavares (wrk)]

Even Firefox's implementation is inconsistent.

drawShape uses the "infinite transparent black bitmap" but drawImage does
not.

I believe even many at Mozilla would like Firefox to switch to the
Chrome/Safari method because it's more easily GPU accelerated.

In that direction it would be nice if 2 things in the spec changed

#1) Get rid of the "infinite transparent black bitmap" stuff and change it
to something that say only pixels inside the shape/image are effected

#2) Change the globalCompositingOperation spec from referencing PORTER-DUFF
to referencing OpenGL

source-over
   glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

source-in
   glBlendFunc(GL_DST_ALPHA, GL_ZERO);

source-out
   glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_ZERO);

source-atop
   glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

destination-over
   glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_ONE);

destination-in
   glBlendFunc(GL_ZERO, GL_SRC_ALPHA)

destination-out
   glBlendFunc(GL_ZERO, GL_ONE_MINUS_SRC_ALPHA);

destination-atop
   glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_SRC_ALPHA);

lighter
   glBlendFunc(GL_ONE, GL_ONE);

darker
   deprecated

copy
   glBlendFunc(GL_ONE, GL_ZERO);

xor
   glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
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