bwareaopen not removing noise as expected

Hi,

I have been using bwareaopen to remove noise from a binary image without having any problem. But after i changed my thresholding function, the noise reduction stopped working as expected.

The code that worked:

gray = rgb2gray(my_image);
bw = imbinarize(gray);
bw = bwareaopen(bw,100);

The code that did not work:

   ballImg = my_image(:,:,1)/2 + my_image(:,:,2)/2 - my_image(:,:,3)/2;
   greenThreshold = ballImg < 80;
   ball=zeros(size(ballImg));   
   ball(greenThreshold)=1;
   bw = bwareaopen(ball,100);

Original image:

After modified code (that did not work):

Can anybody see the problem here?

Thanks in advance!

[Mirac Chen]

I found the solution myself: the image needs to be reversed for bwareaopen to work properly :)

Fix:

ball=ones(size(ballImg));   
ball(greenThreshold)=0;
bw = bwareaopen(ball,100);

Answer 1

[Fanos Christodoulou]

Just a comment on the above that could benefit others having a similar problem. My understanding is that the function bwareaopen deletes the white areas smaller than P pixels.

In your example above you need to reverse the black and white pixels first using the function imcomplement and then apply bwareaopen.

Best,

Fanos