i =imread('ef024e3cdf7d9c74731c919b119ef114.png');
i= rgb2gray(i);
i = im2bw(i,graythresh(i));
BW=~i;
se= strel('disk',4);
BW = imclose(BW,se);

CC = bwconncomp(BW);
numPixels = cellfun(@numel,CC.PixelIdxList);
[biggest,idx] = max(numPixels);
for ii=1:length(numPixels)
    if ii~=idx
        BW(CC.PixelIdxList{ii}) = 0;
    end
end
BW = imfill(BW,'holes');
subplot(1,2,1);
imshow(i);
subplot(1,2,2)
imshow(~(BW.*~i))