bulletml/collision.py

   1 """Simple collision check.
   2
   3 This module provides simple collision checking appropriate for
   4 shmups. It provides a routine to check whether two moving circles
   5 collided during the past frame.
   6
   7 Basic Usage:
   8
   9     from bulletml.collision import collides
  10
  11     for bullet in bullets:
  12         if collides(player, bullet): ... # Kill the player.
  13 """
  14
  15 from __future__ import division
  16
  17 def overlaps(a, b):
  18     """Return true if two circles are overlapping.
  19
  20     Usually, you'll want to use the 'collides' method instead, but
  21     this one can be useful for just checking to see if the player has
  22     entered an area or hit a stationary oject.
  23     """
  24
  25     dx = a.x - b.x
  26     dy = a.y - b.y
  27     radius = getattr(a, 'radius', 0.5) + getattr(b, 'radius', 0.5)
  28
  29     return dx * dx + dy * dy <= radius * radius
  30
  31 def collides(a, b):
  32     """Return true if the two moving circles collide.
  33
  34     a and b should have the following attributes:
  35
  36     x, y - required, current position
  37     px, py - not required, defaults to x, y, previous frame position
  38     radius - not required, defaults to 0.5
  39
  40     """
  41     # Current locations.
  42     xa = a.x
  43     xb = b.x
  44     ya = a.y
  45     yb = b.y
  46
  47     # Treat b as a point, we only need one radius.
  48     radius = getattr(a, 'radius', 0.5) + getattr(b, 'radius', 0.5)
  49
  50     # Previous frame locations.
  51     pxa = getattr(a, 'px', xa)
  52     pya = getattr(a, 'py', ya)
  53     pxb = getattr(b, 'px', xb)
  54     pyb = getattr(b, 'py', yb)
  55
  56     # Translate b's final position to be relative to a's start.
  57     # And now, circle/line collision.
  58     dir_x = pxa + (xb - xa) - pxb
  59     dir_y = pya + (yb - ya) - pyb
  60
  61     if abs(dir_x) < 0.0001 and abs(dir_y) < 0.0001:
  62         # b did not move relative to a, so do point/circle.
  63         dx = pxb - pxa
  64         dy = pyb - pya
  65         return dx * dx + dy * dy < radius * radius
  66
  67     diff_x = pxa - pxb
  68     diff_y = pya - pyb
  69
  70     # dot(diff, dir) / dot(dir, dir)
  71     t = (diff_x * dir_x + diff_y * dir_y) / (dir_x * dir_x + dir_y * dir_y)
  72     if t < 0:
  73         t = 0
  74     elif t > 1:
  75         t = 1
  76
  77     dist_x = pxa - (pxb + dir_x * t)
  78     dist_y = pya - (pyb + dir_y * t)
  79
  80     # dist_sq < radius_sq
  81     return dist_x * dist_x + dist_y * dist_y <= radius * radius