get the inner white circle

i wanted to get the inner white circle and blacken all the remaining portions....

please can someone help me solve this...

i am trying to get such an image.... but not getting... please help...

Answer 1

[Walter Roberson]

regionprops() 'BoundingBox' would give you the bounding box without any problems. You could also request the 'Eccentricity' and use that to choose between the two output regions. You might also want the 'Image' property.

[Elysi Cochin]

sir i used regionpropos i'm getting the outer circle... i needed the inner circle....

[Walter Roberson]

regionprops() is going to give you a structure array of length 2. You can examine the properties returned to decide which of the two each one refers to.

If you have a newer version of MATLAB, R2014b or later, you can use bwareafilt() and ask to keep the 1 'smallest'