Question

My question has two parts. First one is:

How can I include the background as a component in the bwconncomp function, because it’s default behavior doesn’t include it.

Also, and this is my other question is, how can I select the n-th largest component based on what I get by using bwconncomp.

Currently I was thinking about something like this, but that doesn’t work 😛

function out = getComponent(im,n)
CC = bwconncomp(im,4);
%image is an binary image here

numPixels = cellfun(@numel,CC.PixelIdxList);
sortedPixels = sort(numPixels,'descend');
w = sortedPixels(n);
[largest, index] = find(numPixels==w);
im(CC.PixelIdxList{index}) = 0;
out = im;

But that doesn’t work at all. But im not too sure what the CC.PixelIdxList{index} does, is it just changing elements in the array. I also find it kinda vague what exactly PixelIdxList is.

Answer 1

1. To find the background, you can use ‘not’ operation on the image
2. ‘PixelIdxList’ is not what you need. You need the ‘Area’ property.

function FindBackgroundAndLargestBlob
x = imread(‘peppers.png’);
I = x(:,:,2);
level = graythresh(I);
bw = im2bw(I,level);
b = bwlabel(bw,8);
rp = regionprops(b,’Area’,’PixelIdxList’);
areas = [rp.Area];
[unused,indexOfMax] = max(areas);
disp(indexOfMax);
end

Update:
You can do it with bwconncomp as well:

function FindBackgroundAndLargestBlob
x = imread(‘peppers.png’);
I = x(:,:,2);
level = graythresh(I);
bw = im2bw(I,level);
c = bwconncomp(bw,4);
numOfPixels = cellfun(@numel,c.PixelIdxList);
[unused,indexOfMax] = max(numOfPixels);
figure;imshow(bw);
bw( c.PixelIdxList{indexOfMax} ) = 0;
figure;imshow(bw);
end

Which will give the following results: