   .hFunc        - ['REQ'] function handle for computing custom channels
     .pFunc        - [{}] additional params for chns=hFunc(I,pFunc{:})
     .padWith      - [0] how channel should be padded (e.g. 0,'replicate')
   .complete     - [] if true does not check/set default vals in pChns

 OUTPUTS
  chns       - output struct
   .pChns      - exact input parameters used
   .nTypes     - number of channel types
   .data       - [nTypes x 1] cell [h/shrink x w/shrink x nChns] channels
   .info       - [nType