The limit is the maximum size of a renderbuffer image when using
an FBO. A canvas backing store is a renderbuffer image. The
required minimum maximum size for ES is, er.., 1x1. I don't know
if any implementations are actually that limited. In GL 3.2 it is
1024x1024. Again I have no idea if any implementations are
restricted to this minimum. In GL 4.1 it is 16384x16384.
MAX_VIEWPORT_DIMS must always be greater than or equal to
max(display size, max renderbuffer image size) so a browser can
always use the GL for compositing to a full screen window surface.
Isn't there a deeper problem here? If the underlying hardware has a limit on the size of an OpenGL/Direct3D rendering context - and the new generation of browsers are using the GPU to do compositing, isn't it the case that the browser window (or at least the canvas) will also be limited in size? If so, then wouldn't the idea of rendering at less-than-requested resolution then resizing in the compositor also be impossible?
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