MATLAB: finding values in one array and using those locations to change values in one channel of a separate 3 channel array

Question

So I have a labeled array (array1) with connected regions of interest (background is all zeros, connected regions are all 1's for the first region, all 2's for the second, 3's for the 3rd etc.) I also have a vector (vector1) of the region labels I find important (ex. 1,6,9). I want to find the locations of these values in the labeled array and then change values in one channel of a separate 3 channel array at the same locations (want to color certain parts of an image green based regions of interest found in another image).

I can use the below code to change all channels, but don't know how to specify (img(y,x,1=0), img(y,x,2=0), img(y,x,3=255)).

for i=1:1:length(vector1)
    img(array1==vector1(i))=255;
end

rayryeng rayryeng · Accepted Answer · 2015-04-12T04:04:19

If I understand you correctly, you have a label map of objects - each with an associated ID with 0 as the background. You also have a vector of important IDs and a colour image that is associated with this label map.

You wish to set all locations that have an important ID to 1 colour. I would first create a logical mask where true means that the pixel is important and false otherwise. What I mean by important is that the pixel is either 1, 6 or 9 if we go with your example. You can use bsxfun combined with any to create this mask, then we can use this to index into your image and set the right colour to these locations.

Therefore, do this:

%// Create your logical mask
mask3D = bsxfun(@eq, array1, permute(vector(:), [3 2 1]));
mask = any(mask3D, 3);

%// Set the image pixels to blue at these locations
red = img(:,:,1);
green = img(:,:,2);
blue = img(:,:,3);
red(mask) = 0;
green(mask) = 0;
blue(mask) = 255;

img = cat(3, red, green, blue);

Here's a quick example run. Suppose we have this image with squares:

We can see that there are three squares. Let's change object 1 and object 3 to blue. Before we do that, we need to get a label map:

%// Originally a binary image
im = imread('http://i.stack.imgur.com/DnYQS.png');

%// Get label map 
array1 = bwlabel(im);

%// Create colour version out of this binary image
img = 255*uint8(im);
img = cat(3, img, img, img);

array1 is our label map as you have also mentioned in your question and img is the colour version of the input image. Now, vector = [1 3] so we can change those objects. The labelling is such that the top left square is label 1, the middle is label 2 and the bottom right is label 3.

Once I do this and I run the above code, this is the image I get:

enter image description here

MATLAB: finding values in one array and using those locations to change values in one channel of a separate 3 channel array

2 Answers