Image treshold

[Johny]

Can anyone give me an example how to apply a threshold value to an
image?

(everything above a certain brightness level becomes white, and
everything below the
 level becomes black.)

I was told I should use 256-item mapping table and pass it to the
"point" method.

But how should the 256-item mapping table  look like, if the threshold
is 18( found from the histogram)


Thank you for any hint.
L.