jpx_source::get_num_layer_codestreams

This function is equivalent to calling access_layer, followed by jpx_layer_source::get_num_codestreams with one exception — if the layer cannot yet be opened due to the unavailability of some header box or the relevant codestream main headers, the present function may still succeed in reporting the actual number of codestreams used by the compositing layer. This is useful when the image is being served by a JPIP server which needs to be informed of the codestreams which are required before it will supply their main headers required to access the compositing layer via access_layer.

0 if the layer does not exist or if insufficient information is currently available concerning the layer's codestreams.

which_layer [int]