Remove unwanted region in image by matlab

Question

I have a image that includes object and some unwanted region (small dots). I want to remove it. Hence, I use some morphological operator example 'close' to remove. But it is not perfect. Do you have other way to remove more clear? You can download example image at raw image

This is my code

load Image.mat %load Img value
Img= bwmorph(Img,'close');
imshow(Img);

Answer 1

You might prefer a faster and vectorized approach using bsxfun along with the information obtained from bwlabel itself.

Note: bsxfun is memory intensive, but that's precisely what makes it faster. Therefore, watch out for the size of B1 in the code below. This method will get slower once it reaches the memory constraints set by the system, but until then it provides good speedup over the regionprops method.

Code

[L,num] = bwlabel( Img );
counts = sum(bsxfun(@eq,L(:),1:num));
B1 = bsxfun(@eq,L,permute(find(counts>threshold),[1 3 2]));
NewImg = sum(B1,3)>0;

EDIT 1: Few benchmarks for comparisons between bsxfun and regionprops approaches are discussed next.

Case 1

Benchmark Code

Img = imread('coins.png');%%// This one is chosen as it is available in MATLAB image library
Img = im2bw(Img,0.4); %%// 0.4 seemed good to make enough blobs for this image

lb = bwlabel( Img );
threshold = 2000;

disp('--- With regionprops method:');
tic,out1 = regionprops_method1(Img,lb,threshold);toc
clear out1

disp('---- With bsxfun method:');
tic,out2 = bsxfun_method1(Img,lb,threshold);toc

%%// For demo, that we have rejected enough unwanted blobs
figure,
subplot(211),imshow(Img);
subplot(212),imshow(out2);

Output

Benchmark Results

--- With regionprops method:
Elapsed time is 0.108301 seconds.
---- With bsxfun method:
Elapsed time is 0.006021 seconds.

Case 2

Benchmark Code (Only the changes from Case 1 are listed)

Img = imread('snowflakes.png');%%// This one is chosen as it is available in MATLAB image library
Img = im2bw(Img,0.2); %%// 0.2 seemed good to make enough blobs for this image
threshold = 20;

Output

Benchmark Results

--- With regionprops method:
Elapsed time is 0.116706 seconds.
---- With bsxfun method:
Elapsed time is 0.012406 seconds.

As pointed out earlier, I have tested with other bigger images and with a lot of unwanted blobs, for which bsxfun method doesn't provide any improvement over regionprops method. Due to the unavailability of any such bigger images in MATLAB library, they couldn't be discussed here. To sum up, it could be suggested to use either of these two approaches based on the input features. It would be interesting to see how these two approaches perform for your input images.

Answer 2

You can use regionprops and bwlabel to select all regions that are smaller than a certain area (=number of pixels)

lb = bwlabel( Img );
st = regionprops( lb, 'Area', 'PixelIdxList' );
toRemove = [st.Area] < threshold; % fix your threshold here
newImg = Img;
newImg( vertcat( st(toRemove).PixelIdxList ) ) = 0; % remove