[rbayer]

Technically you should use NOT instead of ~ for things like this. Also, you should use AND instead of &. & will calculate the bitwise "and" of your two expressions, meaning it will apply the "and" to each individual set of bits. If it so happens that both p1_y > 127 as well as p2_y > 127 evaluate to exactly 1, it will work. Otherwise it may not. ~ Will compute the bitwise NOT of your expression. Assuming the inner part evaluates to 1, ~ will make it 1111111111111110 (65534).

If you ever wonder what an expression will evaluate to, try popping it in RoboEmu and doing a debug. For example, using:

myVar= ~1
debug? myVar

will give you:
myVar=254

-and-

myVar=0-1
debug? myVar

will give you:
myVar=255