Dump Image Pixels into vector from coordinates

Is there a way to use 2 vectors of pixel coordinates to quickly dump the pixel values into a 3rd vector?

Currently, I am using the following for loop, but it is very slow:

for j=1:length(x)
      pixel_vals(j) = img(x(j),y(j));
end

where, 'x' and 'y' are the x,y coordinates (1xN vector form).

Many thanks!

Answer 1

[Walter Roberson]

pixel_vals = img(subs2ind(size(x), x, y));

[Philip]

Thanks! That worked perfectly!!

Answer 2

[Image Analyst]

Philip, in response to the new question in your "Answer" (not your original question), the preferred way to do this is with ismember(). Your way would take forever for an image with lots of pixels that have labels. See this code snippet from my image segmentation tutorial, BlobsDemo: http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862

% Extract only those blobs that meet our criteria, and 
% eliminate those blobs that don't meet our criteria. 
% Note how we use ismember() to do this.
keeperBlobsImage = ismember(labeledImage, keeperIndexes); 
% Re-label with only the keeper blobs kept.
labeledDimeImage = bwlabel(keeperBlobsImage, 8);     % Label each blob so we can make measurements of it

Answer 3

[Philip]

On a similar note, is there any way that I can replace the following loop:

label_ids = [1 3 11 24 55];
map = zeros(height_img, width_img);
for i=1:length(label_ids)
    map = BWlabel_img==label_ids(i) + map;
end

The label_ids correspond to line segments in a 'bwlabel' image that I want to keep. Essentially, I want to create a new image 'map' that contains only the line segments with these id numbers...

[Image Analyst]

This should have been a new question, not an answer. Comments can't have formatted code, so see my answer below.