# gradientMag

## PURPOSE

Compute gradient magnitude and orientation at each image location.

## SYNOPSIS

function [M,O] = gradientMag( I, channel, normRad, normConst, full )

## DESCRIPTION

     Compute gradient magnitude and orientation at each image location.

     If input image has k>1 channels and channel=0, keeps gradient with
     maximum magnitude (over all channels) at each location. Otherwise if
     channel is between 1 and k computes gradient for the given channel.
     If full==1 orientation is computed in [0,2*pi) else it is in [0,pi).

     If normRad>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], [normConst], [full] )

     INPUTS
      I          - [hxwxk] input k channel single image
      channel    - [0] if>0 color channel to use for gradient computation
      normRad    - [0] normalization radius (no normalization if 0)
      normConst  - [.005] normalization constant
      full       - [0] if true compute angles in [0,2*pi) else in [0,pi)

     OUTPUTS
      M          - [hxw] gradient magnitude at each location
      O          - [hxw] approximate gradient orientation modulo PI

     EXAMPLE
      I=rgbConvert(imread('peppers.png'),'gray');
      [Gx,Gy]=gradient2(I); M=sqrt(Gx.^2+Gy.^2); O=atan2(Gy,Gx);
      full=0; [M1,O1]=gradientMag(I,0,0,0,full);
      D=abs(M-M1); mean2(D), if(full), o=pi*2; else o=pi; end
      D=abs(O-O1); D(~M)=0; D(D>o*.99)=o-D(D>o*.99); mean2(abs(D))

     See also gradient, gradient2, gradientHist, convTri

     Piotr's Image&Video Toolbox      Version 3.23
     Copyright 2013 Piotr Dollar & Ron Appel.  [pdollar-at-caltech.edu]
     Please email me if you find bugs, or have suggestions or questions!
     Licensed under the Simplified BSD License [see external/bsd.txt]