Finding pixels in connected component

2012 9 27
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Hi
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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Image Analyst
Image Analyst 2012-9-27
编辑：Image Analyst 2012-9-27

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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: http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862

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Muffin
Muffin 2012-9-27
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?
Image Analyst
Image Analyst 2012-9-27
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.
Muffin
Muffin 2012-10-10
Hey, thanks a lot, your tutorial was of help! I managed to draw my bounding boxes!

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Ryan
Ryan 2012-9-27
编辑：Ryan 2012-9-27

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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

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Muffin
Muffin 2012-9-28
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...
Image Analyst
Image Analyst 2012-9-28
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.
Ryan
Ryan 2012-9-28
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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