Turn a pic in few shades of black and white

[rico]

Hello everyone
Does anyone know how to turn a photo into black and white but with very few intermediate shades?
I try to laser engrave a photo on wood and the problem is always the same: there are always too many variations of light which create a range of light too big and which results in the engraving either by white or black zones without no details.

whereas if I engrave a vector file, a logo, as there are only 2 shades, it works very well.

Would there be a way to homogenize the photos around only a few degrees of black or white?
Thank you in advance for any help you can give me.

[rich2005]

All depends on your photo. A vector is typically well defined shapes while a photo is complicated.

You could try the gimp_gmic_qt plugin http://www.gmic.eu  Two stages
Use the cutout filter to reduce the number of shades 
Destaurate the result. Use the gmic desaturate, it is very good.

   

[sallyanne]

[attachment=9591]Try desaturating your photo and posterizing it