Compute()
  chns = chnsCompute( I, pChns )

 INPUTS
  I           - [hxwx3] input image (uint8 or single/double in [0,1])
  pChns       - parameters (struct or name/value pairs)
   .shrink       - [4] integer downsampling amount for channels
   .pColor       - parameters for color space:
     .enabled      - [1] if true enable color channels
     .smooth       - [1] radius for image smoothing (using convTri)
     .colorSpace   - ['luv'] choices are: 'gray', 'rgb', 'hsv', 'orig'
   .pGradMag     - parameters