Date: Apr 5, 2013 11:17 AM
Author: kinor
Subject: Re: speeding up assigning values to a matrix

"Luca " <l.presottoRE@MOVE.campus.unimib.NOTit> wrote in message <kjmjsi$amg$1@newscl01ah.mathworks.com>...
> 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.



Hi Luca,
i guess your mask(:,:,:,3) = (mask>4 &mask<7)*255;
should be maskC(:,:,:,3) = (mask>4 &mask<7)*255; ?

this is faster but slower than your achieved 10 ms, i wonder if you get it that fast.
ts=tic;
maskC(:,:,:,1) = (mask==2);
toc(ts);
ts=tic;
maskC(:,:,:,2) = (mask==4);
toc(ts);
ts=tic;
maskC(:,:,:,3) = (mask==6);
toc(ts);
tic
maskC = maskC*255;
toc

hth
kinor