Basic static/moving circle collisions. (Fixes issue #5)

[python-bulletml.git] / bulletml / collision.py

diff --git a/bulletml/collision.py b/bulletml/collision.py
new file mode 100644 (file)
index 0000000..f185745
--- /dev/null
+++ b/bulletml/collision.py
@@ -0,0 +1,81 @@
+"""Simple collision check.
+
+This module provides simple collision checking appropriate for
+shmups. It provides a routine to check whether two moving circles
+collided during the past frame.
+
+Basic Usage:
+
+    from bulletml.collision import collides
+
+    for bullet in bullets:
+        if collides(player, bullet): ... # Kill the player.
+"""
+
+from __future__ import division
+
+def overlaps(a, b):
+    """Return true if two circles are overlapping.
+
+    Usually, you'll want to use the 'collides' method instead, but
+    this one can be useful for just checking to see if the player has
+    entered an area or hit a stationary oject.
+    """
+
+    dx = a.x - b.x
+    dy = a.y - b.y
+    radius = getattr(a, 'radius', 0.5) + getattr(b, 'radius', 0.5)
+
+    return dx * dx + dy * dy <= radius * radius
+
+def collides(a, b):
+    """Return true if the two moving circles collide.
+
+    a and b should have the following attributes:
+
+    x, y - required, current position
+    px, py - not required, defaults to x, y, previous frame position
+    radius - not required, defaults to 0.5
+
+    """
+    # Current locations.
+    xa = a.x
+    xb = b.x
+    ya = a.y
+    yb = b.y
+
+    # Treat b as a point, we only need one radius.
+    radius = getattr(a, 'radius', 0.5) + getattr(b, 'radius', 0.5)
+
+    # Previous frame locations.
+    pxa = getattr(a, 'px', xa)
+    pya = getattr(a, 'py', ya)
+    pxb = getattr(b, 'px', xb)
+    pyb = getattr(b, 'py', yb)
+
+    # Translate b's final position to be relative to a's start.
+    # And now, circle/line collision.
+    dir_x = pxa + (xb - xa) - pxb
+    dir_y = pya + (yb - ya) - pyb
+
+    if abs(dir_x) < 0.0001 and abs(dir_y) < 0.0001:
+        # b did not move relative to a, so do point/circle.
+        dx = pxb - pxa
+        dy = pyb - pya
+        return dx * dx + dy * dy < radius * radius
+
+    diff_x = pxa - pxb
+    diff_y = pya - pyb
+
+    # dot(diff, dir) / dot(dir, dir)
+    t = (diff_x * dir_x + diff_y * dir_y) / (dir_x * dir_x + dir_y * dir_y)
+    if t < 0:
+        t = 0
+    elif t > 1:
+        t = 1
+
+    dist_x = pxa - (pxb + dir_x * t)
+    dist_y = pya - (pyb + dir_y * t)
+
+    # dist_sq < radius_sq
+    return dist_x * dist_x + dist_y * dist_y <= radius * radius