Substituting particular matrix values

I have four 512x512 matrices: X1a and X1b, X2a and X2b

I first want to find the coordinates in X1a and X2a where an element equals 255. For this I have been using:

[r,c,v] = find(X1a==255);

a= [r, c, v];

[r,c,v] = find(X2a==255);

b= [r, c, v];

c= [a; b];

Then I want to use ALL the coordinates that I have found (matrix c) to replace the elements in X1a AND X2a with the elements at the same coordinates from X1b and X2b respectively. However- I am unsure how to do this.

Any help would be most appreciated.

Answer 1

[Andrei Bobrov]

t1 = X1a == 255; 
X1a(t1) = X1b(t1);
t2 = X2a == 255;
X2a(t2) = X2b(t2);

or

Xa = cat(3,X1a,X2a);
Xb = cat(3,X1b,X2b);
t = Xa == 255;
Xa(t) = Xb(t);
X1a = Xa(:,:,1);
X2a = Xa(:,:,2);

[Charlotte]

Thanks for your response

I was just wondering how I would edit this code so that it changed all the elements that equalled 255 in both matrices

As a simple example:

X1a= [ 123 123 123; 123 255 123; 255 123 123];

X1b= [ 4 4 4; 4 4 4; 4 4 4];

X2a= [255 111 111; 111 111 111; 111 111 111];

X2b= [5 5 5; 5 5 5; 5 5 5];

In X1a there are 2 elements that equal 255, and in X2a there is 1. I would like to change the 3 coordinates in both matrices so I ended up with:

X1c = [4 123 123; 123 4 123; 4 123 123]

X2c= [5 111 111; 111 5 111; 5 111 111]

Thanks again for your help