how to fill the black regions inside the white region? imfill() is not working.

this is the image.

Answer 1

[Hamoon]

imfill does not work because it thinks that your image is like this:

Actually this algorithm is not as smart as you are, and it doesn't know, that it's a retina image an it should be circular. So if you just want to fill the area for this image, you can write this:

 img = imread('mm.png');     % your image

 img2 = false(size(img)+2);
 img2(2:end-1,2:end-1)=img;  % your image with 1 pixel padding from each side
                             % you can also use padarray function 

 img2(1,:)=true;   
 img2(end,:)=true; % let first and end rows of your new image be "1"  

 img3 = imfill(img2,'holes'); % use imfill for holes

 outputImage = img3(2:end-1,2:end-1); % eliminate padding pixels

 imshow(outputImage)

so img2 will be considered something like this:

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and the output image is like this:

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But it only works for this image or images like this, I just wanted to show you what the problem is.

[Pradeep Gowda]

Thank you! for your reply. Is there a way to generalize this to work for all images? some papers talk about using morphological operators like erode and dilate. Is there a way to do it?

[Hamoon]

It is hard to talk about generalization when you are dealing with image processing problems, but yes you can use dilation ( using imdilate in Matlab) to extend your image and then use erosion ( using imerode in Matlab) to erode it. dilation extend your object to all directions and this will fill the holes, but other areas of your object will extend too, so you need to erode it to have your object in the same size as it was in the first place. If you choose proper morphological structuring element ( strel) with proper size, then you will get an acceptable output.

[Image Analyst]

Erosion, dilation, opening, and closing will all change the shape of the outline(s), unlike hole filling. You could use bwconvhull() but that would change the shape of that little nub sticking out of the -45 degrees location.