Comments on: Logical indexing

By: Steve

Steve — Thu, 29 Apr 2010 11:45:49 +0000

Morten—I misinterpreted your last comment. I thought you were asking for a comparison of performance between linear and logical indexing because you used a syntax that looked like linear indexing in your question. Anyway, you need to use linear indexing, like this:

pixels = image(sub2ind(size(image), rows, cols));

By: Morten

Morten — Thu, 29 Apr 2010 08:28:29 +0000

Hi Steve
As it is now I use

pixels = zeros(size(rows));
for i=1:2000
   pixels(i) = image(rows(i),cols(i));
end

because that was the straight to use my input data (rows and cols) for indexing. And the indexing is the bottleneck of my algorithm – I have of cause checked this through profiling. It will take some extensive code modifications to get this right, so I was just asking you, in order to do it right from the start – you might have had a quick answer. But I guess I’ll just go with one of them and a new profiling will show whether the indexing is still the bottleneck.

Thanks,

Morten

By: Steve

Steve — Thu, 29 Apr 2010 01:27:07 +0000

Morten—I don’t know. I wouldn’t even bother trying to find out unless detailed profiling demonstrated that it was a true bottleneck in the context of the rest of your algorithm.

By: Morten

Morten — Wed, 28 Apr 2010 14:40:34 +0000

Hi Steve
I need to index a number of pixels in an image, say 2000 locations in a 1024×1024 uint8 image. What is the fastest way to get the pixel values at these locations?

pixels = image(index); % size(index) = [1,2000]

pixels = image(binary); % size(binary) = [1024,1024]

Or maybe some smarter way? And does the choice of method depend on the ‘pixels_indexed/image_size’-ratio (a method might be the fastest when indexing a few pixels in a large picture but the slowest when indexing many pixels in a small picture)?
Hope you have an answer for this.

Morten

By: Ronny

Ronny — Fri, 05 Mar 2010 18:45:48 +0000

Hii Steve, I found the solution, I used inpolygon(X,Y,xv,yv) as in the example:
L = linspace(0,2.*pi,6); xv = cos(L)’;yv = sin(L)’;
xv = [xv ; xv(1)]; yv = [yv ; yv(1)];
x = randn(250,1); y = randn(250,1);
in = inpolygon(x,y,xv,yv);
plot(xv,yv,x(in),y(in),’r+’,x(~in),y(~in),’bo’)

This function is useful,

Thank you,

Ronny

By: Ronny

Ronny — Fri, 05 Mar 2010 13:42:04 +0000

Hi, Steve I think that I didn’t explain me very well, I am sorry for my English. I have the following problem, I have a matrix (3000×3), the points in the matrix are coordinates X,Y,Z, I need to divide my map in 9 similar portions or little windows:
w1 w2 w3
w4 w5 w6
w7 w8 w9
I defined my limits for every window, but I can’t to find the right operation, recently i apply logical operations and the result it doesn’t match with operation with filter on excel. I think that if I use find operator may be work, but I don’t know how can I to create the matrix from index.
If you know the best way please let me know,

Thank a lot,

Ronny

By: Steve

Steve — Wed, 03 Mar 2010 20:42:07 +0000

Ronny—Sounds like basic MATLAB array indexing:

B = A(xmin:xmax, ymin:ymax, zmin:zmax);

By: Ronny

Ronny — Wed, 03 Mar 2010 19:29:24 +0000

Hi Steve,
I have a question, I am trying to do a function for to create a submatrix (X,Y,Z) from another bigger (X,Y,Z), I need to extract from a range (xmin,xmax; ymin,ymax), I tried for several ways, for example with “for cicle”, “find”, “logical”, but I can’t find the right code, please let me know if you know a best way.
Thank you so much,

By: Steve

Steve — Sat, 12 Jul 2008 16:21:27 +0000

Oliver—I’m glad you found it useful.

By: Oliver A. Chapman, P.E.

Oliver A. Chapman, P.E. — Fri, 11 Jul 2008 18:24:05 +0000

Steve,

Thanks for directing me to this. It is very well written.