cant seem to make the background fully black

I have uploaded two images, one is the original image and the other one is a processed image. i am wondering how can i make the background fully black because i cant seem to make the bottom right fully black in the processed image. i have processed the image by thresholding, binarising and inverting it.

Answer 1

[Matt J]

A=imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1141370/original.png');

B=imquantize(A,multithresh(A,3));

C=1;
for i=1:4
    C=C & ~bwareafilt(B==i,[80000,inf]);
end

imshow(C,[])

[Pravindkumaran Baskaran]

thank you very much !

[Image Analyst]

If you want the shapes to have their original gray level and class (like uint8 or uint16), instead of being a binary (logical) image, you can use that mask to set the background to zero and leave the original gray levels untouched:

% Blacken background:
A(~C) = 0;

Answer 2

[Image Analyst]

Looks a lot like homework. Is it?

What I'd try first is to create a background and then subtract it from the image. You can construct a background by getting the values along the edges, or the corners and then using biliear interpolation, like with interp1 to build the background. Should be easy - try it.

[Pravindkumaran Baskaran]

Thank you so much, managed to do this way aswell!

Answer 3

[Matt J]

A=imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1141370/original.png');

B=imclose(edge(A),strel('disk',5));

C=imfill(B,'holes');

imshow(C,[])