how to set black (pixel=0 or <20) to NaN in an image

Hi there, can anyone tell me how to convert black regions on grayscale image to NaN value? I appreciate any answers.

Answer 1

[KSSV]

Let I be your matrix....read using imread

If you want to make pixels [0 20] to NaN..do this:

I(I>=0 & I<= 20) = NaN

[Tian Tian]

Thanks!