Date: Apr 5, 2013 9:34 AM
Author: Luca
Subject: speeding up assigning values to a matrix

Hello everybody.
I have a code which I found to be very slow, compared to what I expect.
I have two variables: mask (512x512x97 uint8) and maskC (512x512x97x3, uint8)

(actually the values in mask can be only between 0 and 6.)
I would like to give values to maskC according to the values in "mask".
Now I do this:
ts=tic;
maskC = zeros (512,512,97,3,'uint8');
toc(ts);
ts=tic;
maskC(:,:,:,1) = (mask>1 &mask<3)*255;
toc(ts);
ts=tic;
maskC(:,:,:,2) = (mask>3 & mask<5)*255;
toc(ts);
ts=tic;
mask(:,:,:,3) = (mask>4 &mask<7)*255;
toc(ts);

The output is:
0.0003
0.29
0.31
0.29
It takes me 1 second (!!!!) to set the values of maskC in this way. On a top-speed PC.
What am I doing wrong? I would expect there should be some way to do this in less than 10ms! (like the creation of maskC in 0.3 ms!!)
I think I should be use logical indexing. But I don't see how in this case, where I add another dimension.