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).