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Subject: Re: 3D matrix operation speed up
Date: Tue, 17 Feb 2009 10:15:03 +0000 (UTC)
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Thanks again for the answer.
Sorry, but I didn't want someone to do my job. :)
I am quite new to Matlab and for me the Matlab help seems not so easy/clear.
I tried the arrayfun solution and it works, but not as i would like to.
It was my fault because I couldn't explain the problem very well!
I'll try again.

I have a MxNx3 matrix which is an image.
I have to compute the following formula for each pixel and store the result in a MxN matrix.
Singleton(row,col) = (px(row,col)-m(class)) * invS(class) * (px(row,col)-m(class))';

px=RGB_matrix(row,col,:):
px=reshape(px,1,3); %I have the 3 RGB color components of one pixel in one row
m=[mean(R) mean(G) mean(B)]; %the mean vector contains the mean value of the color components
invS %is the inverse of the covariance matrix

The mean vector and inverse matrix change depending on the class which the pixel belongs to. So I also wrote a for loop to compute all the m and invS belonging to each class.

I hope the problem is more clear now.
I have to compute that formula over all the pixels trying to avoid for loops. More over I have to store the result in a new MxN matrix in the same position of the original pixel.
Now I am using 3 for loops and for big images it is very slow, of course.