Oh I'm so Sorry, I already has this idea but I gave up on it. Yes, for small angles this might be kind of inaccurate. Anyway, I could not figure out how to really code it. Thank you so much! Going to test it right when I am home. Is there maybe some kind of formula to calculate the real distance between image center and intersection coordinate? My shell is just a rectangle with a half circle on the front. By the way it's only like 10×4 pixels which makes it questionable if more precise calculation is needed, since Greenfoot is not about perfection but learning things. Thanks for your kind help! So glad to have this forum!