The compositing canvas

If req=img, the size of the compositing canvas is determined entirely by the size of layer 0.

If the size= for layer 0 is not specified explicitly, the layer transforms are used to calculate the size of the compositing canvas (see below).

If req=tmb, the size of the compositing canvas is determined by the size= specified for layer 0, or, if size is not specified, the compositing canvas size is set to the view rect (see below).

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