The values of globalCompositeOperation are very poorly defined: http://www.whatwg.org/specs/web-apps/current-work/#globalcompositeoperation Some math is probably needed; prose alone seems unlikely to be sufficient.
I made a quick test at: http://dbaron.org/tests/canvas/composite-operations Trunk Mozilla and Opera 9 do not interoperate on at least the following (ignoring differences that could be attributed to rounding): source-atop source-over (the default!) destination-atop lighter darker xor (Safari seems to show the colors as either random noise (latest for 10.3/PPC) or blank (latest for 10.4/Intel).) Looking at Mozilla's code, it implements "copy" and "source-over" as the same thing; from the code I'd guess that "source-over" was the incorrect one, and this test seems to agree. -David -- L. David Baron <URL: http://dbaron.org/ > Technical Lead, Layout & CSS, Mozilla Corporation
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