how to vectorize this code?

Program to perform contrast stretching on an Image

clc;
close all;
clear all;
%%Input section
I= imread('pout.tif');
%%Calculation section
[r,c]=size(I);
P=I;
a=85;
b=170;
v=65;
w=195;
for i=1:r
    for j=1:c
        k=I(i,j);
        if(k<a)
            l=(v/a);
            I(i,j)=l*k;
        elseif(a<k<b)
            m=(w-v)/(b-a);
            I(i,j)=m*k;
        else
            n=(r-w)/(c-b);
            I(i,j)=n*k;
        end
    end
end
subplot(1,2,1)
gpuArray(imshow(P))
subplot(1,2,2)
imshow(I)
%%End of contrast.m

Answer 1

[Roy Kadesh]

Use meshgrid and logical indexing.

Answer 2

[Jan]

Note that "elseif(a<k<b)" will not do what you expect. The condition is evaluated from left to right: At first "a<k", which replies either false (which is 0) or true (which is 1). Afterwards you get "0<b" or "1<b" respectively. You need this instead:

 elseif a<k && k<b

Before caring about a vectorization, create correctly working code.

You can replace the loops by logical indexing:

m    = I < a;
I(m) = I(m) * (v / a);
m    = (a < I) & (I < b);
I(m) = I(m) * (w-v) / (b-a);
m    = I >= b;
I(m) = I(m) * (r-w) / (c-b);

imshow replies the graphics handle. Then

gpuArray(imshow(P))

converts the value of the handle to a gpuarray, but you do not store the result. This does not seems to be meaningful.

I cannot reconsider, why so many beginners like the brute clearing header "clc; close all; clear all;". Better use functions to keep your workspace clean. Especially "clear all" removes all functions from the memory and reloading them from the slow hard disk wastes a lot of time.

[Stephen23]

+1 clearly explained and good advice.