Re: [w3ctag/design-reviews] Review OffscreenCanvas, including ImageBitmapRenderingContext (#141)

Resources are basically textures and geometry data. As it is now if you load a 2048x2048 texture (16meg) into WebGL it can only be used directly on a single canvas. If you want to use the same texture in another canvas you have to load that texture again into the context for the 2nd canvas because textures (and geometry) can not be shared across contexts (one context per canvas, all resources belong to a single context.

The creative workaround is to make a canvas offscreen, render to that, then draw that canvas into the other canvases. That's slow. It can be optimized but no amount of optimization will erase the large copy that's happening to copy from one canvas to another.

Why one context per canvas? Because honestly I think it was just assumed to be the right thing to do (if other ways were discussed I'm unaware of them). Why can't you share resources across contexts? Because OpenGL has some very strict rules on how 2 contexts share resources and when changes to a resource made in one context are visible in another context. Those rules are extremely hard to enforce so as to make sure that sharing will work correctly everywhere across all platforms, something important for WebGL.

So, sharing was never enabled but that's all based on the idea that there should be one context per canvas. If instead contexts and canvases were disconnected and realizing that canvases are really just themselves textures the entire problem of sharing disappears. If you want to share make one context and use it with as many canvases as you want. If you don't want to share make a new context.

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