# xmin, xmax, xinterval, ymin, ymax, yinterval
# -15, 15, 6, -3, 3, 7
# p2_slant_angle = [10 35 60] at y = [-15 0 15]
# ====================================================
# x = -15~15
# v0 = 35
# p2 = 35 + (-25~25)*v1
# if v1 = 1, p2 = 10~60
# if v1 = -2, p2 = 85~-15
# p2 is available between -75~75 and interval is 5.
# outside of -75~75, the ray is rejected.
p2 = v0 + x * v1; -75, 75, 1

