MATLAB Answers


Finding pixels in connected component

Asked by Muffin
on 27 Sep 2012
I need to know how to get access to any white pixel in a connected component. Are there any functions to get the coordinates of white pixels? With that I have to start a traversal algorithm to draw a bounding box around the connected component in the image. Please help me out. Thank you very much!


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2 Answers

Answer by Image Analyst
on 27 Sep 2012
Edited by Image Analyst
on 27 Sep 2012
 Accepted Answer

Call regionprops(). You can ask for PixelIdxList which will give you the index (location) of every pixel in the blob. But if you just want the bounding box, no need to do some traversal algorithm, just ask regionprops() for the 'BoundingBox'. See my image segmentation tutorial for a demo:


Hi, thanks for ur reply! I found the PixelIdxList function but i'm lost as how to retrieve the index coordinate of the pixel, as in the code...
As for the bounding box by regionprops, would it bound using a rectangle just around the component with the white pixels?
OK, so we now know that you don't need PixelIdxList so let's forget about that. I can tell you didn't look at my BlobsDemo program or you'd know that that's just what BoundingBox does. For each blobs, it gives that blob's bounding box, as shown in my demo.
Hey, thanks a lot, your tutorial was of help! I managed to draw my bounding boxes!

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Answer by Ryan
on 27 Sep 2012
Edited by Ryan
on 27 Sep 2012

It sounds like you are looking for a perimeter of a shape as the "bounding box." If so, use bwboundaries to index the perimeter points. It pics the top most and then left most pixel as a starting point for the perimeter I believe.
[P C] = bwboundaries(A,'noholes'); % Records perimeter points
Perimeter = cat(2,P{1,1}); % Convert it to a vector


hey is this perimeter precisely around the component or in a form of a rectangle. Because i'm looking for a rectangle to bound a connected component using the corner coordinates...
No, bwboundaries gives you the actual tortuous shape of the boundary. If you want the bounding box, ask regionprops for BoundingBox, like I illustrate in my demo.
on 28 Sep 2012
If you are looking for the rectangular bounding box, then use Image Analyst approach. Utilize regionprops(), it's a fantastic analysis tool. If you are looking for the actual free-form perimeter of a shape, then bwboundaries() would be better.

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