How can I make my code run faster? - matlab

Question

I'm currently working on a piece of code which blurs an image by taking a circular sample of pixels around each pixel, finding their mean and then applying that to the central pixel. It works, but it takes ages, especially with large images and large radii.
Can anyone give me some tips on how to speed this up?
function [] = immean(IMAGE, r)
%Find the maximum width, nx, and maximum height, ny, of the image - so that
%it can be used to end the for-loop at the appropriate positions.
[nx, ny] = size(IMAGE);
%Create a completely black image of the same size as the subject image,
%into which the appropriate pixel values can be fed.
average = uint8(zeros(size(IMAGE)));
%Loop through all the pixels of the image.
for x = 1:nx
for y = 1:ny
%This next code takes a square sample of pixels, with dimensions of
%r x r.
%First, set the boundaries of this square, from which the circular
%sample of pixels will be taken.
if x-r <= 0
startx = 1;
else
startx = x-r;
end
if x+r > nx
endx = nx;
else
endx = x+r;
end
if y-r <= 0
starty = 1;
else
starty = y-r;
end
if y+r > ny
endy = ny;
else
endy = y+r;
end
%Loop through this square sample and, if the pixel is within the
%range of the circle, add its intensity to the total.
total = 0;
pixelcount = 0;
for xp = startx : endx
for yp = starty : endy
if (x-xp)^2 + (y-yp)^2 <= r^2
total = total + uint32(IMAGE(xp, yp));
pixelcount = pixelcount + 1;
end
end
end
mean = total / pixelcount;
average(x,y) = mean;
end
end
imshow(average)
I've tried changing things like uint32 but that hasn't worked. Beyond that, I'm a bit of a beginner, so I'm not sure what the best tricks are in this situation. Thanks for your time.

Answer 1

Loops are extremely slow in MATLAB. As a rule, you should always vectorize your code where possible. This is one of those cases. Looping through every pixel is painfully slow.
MATLAB has a function imfilter which does basically what you want. Since you are just taking the average intensity, a simple filter function will do this quite well and very fast. You can define the filter coefficients as a matrix:
% Define a 2D Filter with radius r:
d = 2*r+1;
h = zeros(d);
% Now make it a "circular" filter (keeping it square would be much easier
% and probably look the same but whatever):
[x, y] = meshgrid(1:d,1:d);
distance = sqrt((x-(r+1)).^2 + (y-(r+1)).^2);
h(distance<=r) = 1;
h = h / sum(h(:))
% Now pump it into imfilter and youre done:
average = imfilter(uint32(IMAGE), h);
Also, there are a TON of MATLAB image processing tools, so search around a bit and you'll probably find helpful tools for whatever your working on, no need to reinvent the wheel. I don't have an image in front of me to test this one but let me know if it works.