Hello,
I'm not sure if there is any existing .Net methods that can do this for you
but from an image processing point of view it's not so difficult.
Basically you choose a threshold value. Any pixel intensity below that value
on your original image, you convert the pixel to black. Everything else will
be white.
A good choice for the threshold is the "dominant pixel value". Basically you
can determine it by finding what pixel is predominant on your original image
and you choose the intensity of that pixel as your threshold.
Hope this helps.
Sitar.