or the given channel.
 If full==1 orientation is computed in [0,2*pi) else it is in [0,pi).

 If normRad&gt;0, normalization is performed by first computing S, a smoothed
 version of the gradient magnitude, then setting: M = M./(S + normConst).
 S is computed by S = convTri( M, normRad ).

 This code requires SSE2 to compile and run (most modern Intel and AMD
 processors support SSE2). Please see: http://en.wikipedia.org/wiki/SSE2.

 USAGE
  [M,O] = gradientMag( I, [channel], [normRad], [normCons