There is a RPi demo "drawing_letters_with_mouse"
If you modify code at line 168
from
gh_gpu_program.uniform4f(texture_prog, "color", p._r, p._g, p._b, 1)
to
gh_gpu_program.uniform4f(texture_prog, "color", p._r, p._g, p._b, 0.5)
When you click your left buttom, You will find a magical situation.
You can watch your desktop through letter whatever your bg color in GLSLHacker
