How to efficiently loop through matrix elements

Question

I have a matlab script for 8-bit image analysis and I'm trying to enhance objects in the image by subtracting the background with no objects present. What I want to do at a pixel level is:

if B-I>50 then E=I
else E=255-B-I

Where, B is the background, I the image and E my enhanced image. I know I can do this by looping through each element of the image matrix by the following:

diff=imsubtract(B,I);
nrows=1024;
ncols=1360;
for r=1:nrows
     for c=1:ncols
         if diff(r,c)>50
             E=I(r,c);
         else
             E=255-diff(r,c);
         end
     end
 end

But is this rather slow when going multiple images. I've also tried the follow:

 E=255-diff;
 E(diff>50)=I;

But receive the following error:

In an assignment A(I) = B, the number of elements in B and I must be the same.

Any tips on optimizing this would be greatly apprenticed!

Answer 1

In an assignment A(I) = B, the number of elements in B and I must be the same.

The reason for this error is that you are trying to assign all the content of I to a subset of E (those pixels where diff>50). You have to specifically tell MATLAB that you want those pixels set to the matching pixels in I.

E(diff>50)=I(diff>50);

Incidentally you should be careful using imsubtract here. For pixels where I has a higher value than B, that will result in zeros (if your values are uint8). It may be okay (not 100% clear if you're looking for the absolute difference or really just where B is larger than I)

Answer 2

What if you use use find()

ind = find(B-I>50)
E(ind) = I(ind)

% And then the ones that are not `B-I>50`
E(~ind) = 255-B(~ind)-I(~ind)

Answer 3

Try this vectorized approach that uses logical indexing. I could not test it out on images though, so would be great if that's taken care of.

Code

diff1=double(imsubtract(B,I));
E = double(I).*(diff1>50) + (255-diff1).*(diff1<=50);

You might be needed to convert the datatype back to unsigned integer formats as used for images.