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 If Pyrex was available when installing, this will used optimized
   8 versions of the functions.
   9
  10 Basic Usage:
  11
  12     from bulletml.collision import collides
  13
  14     for bullet in bullets:
  15         if collides(player, bullet): ... # Kill the player.
  16 """
  17
  18 from __future__ import division
  19
  20 def overlaps(a, b):
  21     """Return true if two circles are overlapping.
  22
  23     Usually, you'll want to use the 'collides' method instead, but
  24     this one can be useful for just checking to see if the player has
  25     entered an area or hit a stationary oject.
  26
  27     (This function is unoptimized.)
  28     """
  29
  30     dx = a.x - b.x
  31     dy = a.y - b.y
  32     radius = getattr(a, 'radius', 0.5) + getattr(b, 'radius', 0.5)
  33
  34     return dx * dx + dy * dy <= radius * radius
  35
  36 def collides(a, b):
  37     """Return true if the two moving circles collide.
  38
  39     a and b should have the following attributes:
  40
  41     x, y - required, current position
  42     px, py - not required, defaults to x, y, previous frame position
  43     radius - not required, defaults to 0.5
  44
  45     (This function is unoptimized.)
  46
  47     """
  48     # Current locations.
  49     xa = a.x
  50     xb = b.x
  51     ya = a.y
  52     yb = b.y
  53
  54     # Treat b as a point, we only need one radius.
  55     radius = getattr(a, 'radius', 0.5) + getattr(b, 'radius', 0.5)
  56
  57     # Previous frame locations.
  58     pxa = getattr(a, 'px', xa)
  59     pya = getattr(a, 'py', ya)
  60     pxb = getattr(b, 'px', xb)
  61     pyb = getattr(b, 'py', yb)
  62
  63     # Translate b's final position to be relative to a's start.
  64     # And now, circle/line collision.
  65     dir_x = pxa + (xb - xa) - pxb
  66     dir_y = pya + (yb - ya) - pyb
  67
  68     if abs(dir_x) < 0.0001 and abs(dir_y) < 0.0001:
  69         # b did not move relative to a, so do point/circle.
  70         dx = pxb - pxa
  71         dy = pyb - pya
  72         return dx * dx + dy * dy < radius * radius
  73
  74     diff_x = pxa - pxb
  75     diff_y = pya - pyb
  76
  77     # dot(diff, dir) / dot(dir, dir)
  78     t = (diff_x * dir_x + diff_y * dir_y) / (dir_x * dir_x + dir_y * dir_y)
  79     if t < 0:
  80         t = 0
  81     elif t > 1:
  82         t = 1
  83
  84     dist_x = pxa - (pxb + dir_x * t)
  85     dist_y = pya - (pyb + dir_y * t)
  86
  87     # dist_sq < radius_sq
  88     return dist_x * dist_x + dist_y * dist_y <= radius * radius
  89
  90 def collides_all(a, others):
  91     """Filter the second argument to those that collide with the first.
  92
  93     This is equivalent to filter(lambda o: collides(a, o), others),
  94     but is much faster when the compiled extension is available (which
  95     it is not currently).
  96
  97     """
  98     return filter(lambda o: collides(a, o), others)
  99
 100 try:
 101     from bulletml._collision import collides, overlaps, collides_all
 102 except ImportError:
 103     pass