How to shrink a binary image of an irregular object?

I have a binary image of an irregular object. Any clue on how to shrink this object, for example, by 20pixel from all directions?

I tried tracing the boundary and edit the coordinates but it was not practical.

Answer 1

[Sean de Wolski]

A few ways:

doc imerode

Or if you need to maintain objects:

doc bwmorph

Look at the skel and thin options.

[Yun Inn]

Hi Sean, I tested IM2=bwmorph(IM,'thin',19) and it works perfect! in shaving 19pixels off the perimeter of my square phantom (the binary object). I am preparing the code for patient images. Does it work as well for body contour? I don't have any proper image to work with yet.

[Sean de Wolski]

You'll have to try it :)

[Yun Inn]

I hope it works! Anyway, thank you very much for pointing me to the right function :)