How to optimize two for loops in MATLAB

Question

I have two images which i would like to compare and do different operations depending on what value the specific pixel has. The problem is that is is really slow and i need to speed the operations up alot, what could be done with the code?

currentFrame = rgbimage; %rgbimage is an 800x450x3 matrix

for i = 1:size(currentFrame, 1)

   for j = 1 : size(currentFrame,2) 

       if currentFrame(i,j) > backgroundImage(i,j) %backgroundimage is an equally sized image which i would like to compare with
          backgroundImage(i,j, :) = double(backgroundImage(i,j, :) +1); 

       elseif currentFrame(i,j) < backgroundImage(i,j)
          backgroundImage(i,j, :) = double(backgroundImage(i,j, :) -1);          
       end


   end

end

diff = abs(double(currentFrame) - double(backgroundImage)); %difference between my backgroundimage and my current frame
fusion = zeros(size(currentFrame)); % A fusion image

for i=1:size(backgroundImage,1)
    for j = 1:size(backgroundImage,2)

           if diff(i,j) > 20

            fusion(i,j, :) = double(currentFrame(i,j, :));

           else
             fusion(i,j, :) = 0;  

           end
    end 
end

Thanks for any help!

bdecaf · Accepted Answer · 2012-06-19 09:00:35Z

1

You don't need to loop - you can do stuff like:

indexes = currentFrame > backgroundImage;
backgroundImage(indexes) = backgroundImage(indexes) + 1;

btw. in your code using currentFrame(i,j) > backgroundImage(i,j) you are just comparing the first of the three color dimensions. Is this intended?

answered Jun 19, 2012 at 9:00

bdecaf

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4 Comments

Jonas Over a year ago

No it was not intended, can I just do currentFrame(i,j, :) > backgroundImage(i,j, :) instead?

bdecaf Over a year ago

That works - but gives you a 1x1x3 dimensional logical vector. You'd then need to clarify the if. (is it enough when just one color is larger?)

Jonas Over a year ago

Ye it works, but i get a problem when im doing the same thing when i'm computing the fusion image. The colors gets really messed up on some regions (it is a pretty dark image from the beginning but now I get colors like clear blue/green/pink. Here is the code: fusion = zeros(size(currentFrame)); indexes = diff > 20; fusion(indexes) = double(rgbimage(indexes));

bdecaf Over a year ago

Can only guess here - but I believe Matlab uses RGB values in range 0-1, and it sounds yours are between 0-255.

Ran · Accepted Answer · 2012-06-19 08:59:30Z

1

You can compare matrices in one operation. for example,

D = diff > 20;

matrix D will containe D(i,j) = 1 where diff(i,j) > 20, otherwise zero.

Then you can use it to set other matrices:

fusion = zeros(size(currentFrame));
fusion(diff > 20) = double(currentFrame(diff > 20));

and the same with the first loop.

answered Jun 19, 2012 at 8:59

Ran

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1 Comment

Jonas Over a year ago

Thanks alot, got much faster, I'm using your code and 'bdecaf' but he posted first so he gets the "correct answer"

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