test values of matrix without loop to optimize my function

Hi evbdy,

I would like to optimize my code, to test a values of matrix (without loop for) and store result in a vector to find index of nonzero element : (it's a code for image processing, image : 2160*1434)

    for k=1:a %a=2160
         for t = 1:b %b=1434
        for l=1:numOfImages
            if S(k,t,l)<Sombre
                 mat(l,1)=S(k,t,l);
            end
        end
    F=find(mat);
    if length(F)>=3
    .......Continuation of code

Think you for help

[Michelangelo Ricciulli]

Is it correct that you overwrite many times the matrix mat during your for loop? I don't get the rationale... Maybe you want the matrix mat to be different from zero only when the l-th image satisfies at least once that S(k,t,l)<Sombre. Am I correct?

[telkab01]

Yes i overwite vector mat during my loop for, i used it uniquely to caluclate a combination matrix to calculate a normal surface for each illumination. (photometric stereo problem). I want to test a 3D matrix S(k,t,1:6) and get the index for values wich respect the condition (Sombre<90), to use the vector of index later.. it's like a binary test but i dont see how i can do it with a fast maneer.. thank you

Answer 1

[Jan]

Perhaps you want something like this:

for k = 1:a
  for t = 1:b
    M = S(k, t, :);
    if sum(S(:) < Sombre) >= 3
      ...

Actually I assume that a vectorization might be useful:

S2 = (sum(S < Sombre, 3) >= 3);

Now S2 contains a TRUE on all positions, which match the criterion.

[telkab01]

Thank you for answer, for each k,t (pixels) i have 6 differents values and i want to test them and return a vector with true or false for each position to use it later in my code. then i can't test all of values of S in one time beacause it's not useful for my case.

Answer 2

[John BG]

Hi Telkab01

 a=2160;  b=1434;  % image size
 ni=10  % number of images
S   % matrix containing the images with format S(a(i),b(j),ni(k))

no need for any loop, look:

S2=S(:)
v=find(S2==0)
[a0,b0,ni0]=ind2sub(size(S2),v)

a0 b0 are vector containing the coordinates of the zeros in each image, and ni0 contains the image numeral where zeros are located.

if you find this answer useful would you please be so kind to mark my answer as Accepted Answer?

To any other reader, please if you find this answer of any help solving your question,

please click on the thumbs-up vote link,

thanks in advance

John BG

<mailto:[e-mail] [e-mail]>

[telkab01]

Thank you for your answer, I want to test a 3D matrix S(k,t,1:6) and get the index for values wich respect the condition (Sombre<90), to use the vector of index later.. it's like a binary test but i dont see how i can do it with a fast maneer.. S It's a matrix values and not a image, and for each pixels (k,t) i do my test. thank you