gradientMag

PURPOSE ^

Compute gradient magnitude and orientation at each image location.

SYNOPSIS ^

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

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.
 Orientation is given modulo PI degrees. Highly optimized using mex.

 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] )

 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

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

 EXAMPLE
  I=rgb2gray(single(imread('peppers.png'))/255);
  tic, [M1,O1]=gradientMag(I); toc
  tic, [Gx,Gy]=gradient2(I); M2=sqrt(Gx.^2+Gy.^2);
  O2=mod(atan2(Gy,Gx),pi); toc, mean2(abs(M1-M2))
  d=abs(O1-O2); d(d>pi/2)=pi-d(d>pi/2); mean2(d)

 See also gradient, gradient2, gradientHist, convTri

 Piotr's Image&Video Toolbox      Version 3.00
 Copyright 2012 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]

CROSS-REFERENCE INFORMATION ^

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