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IntersectingLineDetectionHere's some functions that I wrote to help with collision detection in a retro arcade remake of Asteroids using Python and Pygame. Some simple assertion tests are included at the bottom. Features: The entry point for an intersect test is the getIntersectPoint function. # geometry.py # # Geometry functions to find intersecting lines. # Thes calc's use this formula for a straight line: # y = mx + b where m is the gradient and b is the y value when x=0 # # See here for background http://www.mathopenref.com/coordintersection.html # # Throughout the code the variable p is a point tuple representing (x,y) # # Copyright (C) 2008 Nick Redshaw # # This program is free software: you can redistribute it and/or modify # it under the terms of the GNU General Public License as published by # the Free Software Foundation, either version 3 of the License, or # (at your option) any later version. # # This program is distributed in the hope that it will be useful, # but WITHOUT ANY WARRANTY; without even the implied warranty of # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the # GNU General Public License for more details. # # You should have received a copy of the GNU General Public License # along with this program. If not, see . from __future__ import division from pygame import Rect # Calc the gradient 'm' of a line between p1 and p2 def calculateGradient(p1, p2): # Ensure that the line is not vertical if (p1[0] != p2[0]): m = (p1[1]  p2[1]) / (p1[0]  p2[0]) return m else: return None # Calc the point 'b' where line crosses the Y axis def calculateYAxisIntersect(p, m): return p[1]  (m * p[0]) # Calc the point where two infinitely long lines (p1 to p2 and p3 to p4) intersect. # Handle parallel lines and vertical lines (the later has infinate 'm'). # Returns a point tuple of points like this ((x,y),...) or None # In non parallel cases the tuple will contain just one point. # For parallel lines that lay on top of one another the tuple will contain # all four points of the two lines def getIntersectPoint(p1, p2, p3, p4): m1 = calculateGradient(p1, p2) m2 = calculateGradient(p3, p4) # See if the the lines are parallel if (m1 != m2): # Not parallel # See if either line is vertical if (m1 is not None and m2 is not None): # Neither line vertical b1 = calculateYAxisIntersect(p1, m1) b2 = calculateYAxisIntersect(p3, m2) x = (b2  b1) / (m1  m2) y = (m1 * x) + b1 else: # Line 1 is vertical so use line 2's values if (m1 is None): b2 = calculateYAxisIntersect(p3, m2) x = p1[0] y = (m2 * x) + b2 # Line 2 is vertical so use line 1's values elif (m2 is None): b1 = calculateYAxisIntersect(p1, m1) x = p3[0] y = (m1 * x) + b1 else: assert false return ((x,y),) else: # Parallel lines with same 'b' value must be the same line so they intersect # everywhere in this case we return the start and end points of both lines # the calculateIntersectPoint method will sort out which of these points # lays on both line segments b1, b2 = None, None # vertical lines have no b value if m1 is not None: b1 = calculateYAxisIntersect(p1, m1) if m2 is not None: b2 = calculateYAxisIntersect(p3, m2) # If these parallel lines lay on one another if b1 == b2: return p1,p2,p3,p4 else: return None # For line segments (ie not infinitely long lines) the intersect point # may not lay on both lines. # # If the point where two lines intersect is inside both line's bounding # rectangles then the lines intersect. Returns intersect point if the line # intesect o None if not def calculateIntersectPoint(p1, p2, p3, p4): p = getIntersectPoint(p1, p2, p3, p4) if p is not None: width = p2[0]  p1[0] height = p2[1]  p1[1] r1 = Rect(p1, (width , height)) r1.normalize() width = p4[0]  p3[0] height = p4[1]  p3[1] r2 = Rect(p3, (width, height)) r2.normalize() # Ensure both rects have a width and height of at least 'tolerance' else the # collidepoint check of the Rect class will fail as it doesn't include the bottom # and right hand side 'pixels' of the rectangle tolerance = 1 if r1.width < tolerance: r1.width = tolerance if r1.height < tolerance: r1.height = tolerance if r2.width < tolerance: r2.width = tolerance if r2.height < tolerance: r2.height = tolerance for point in p: try: res1 = r1.collidepoint(point) res2 = r2.collidepoint(point) if res1 and res2: point = [int(pp) for pp in point] return point except: # sometimes the value in a point are too large for PyGame's Rect class str = "point was invalid ", point print str # This is the case where the infinately long lines crossed but # the line segments didn't return None else: return None # Test script below... if __name__ == "__main__": # line 1 and 2 cross, 1 and 3 don't but would if extended, 2 and 3 are parallel # line 5 is horizontal, line 4 is vertical p1 = (1,5) p2 = (4,7) p3 = (4,5) p4 = (3,7) p5 = (4,1) p6 = (3,3) p7 = (3,1) p8 = (3,10) p9 = (0,6) p10 = (5,6) p11 = (472.0, 116.0) p12 = (542.0, 116.0) assert None != calculateIntersectPoint(p1, p2, p3, p4), "line 1 line 2 should intersect" assert None != calculateIntersectPoint(p3, p4, p1, p2), "line 2 line 1 should intersect" assert None == calculateIntersectPoint(p1, p2, p5, p6), "line 1 line 3 shouldn't intersect" assert None == calculateIntersectPoint(p3, p4, p5, p6), "line 2 line 3 shouldn't intersect" assert None != calculateIntersectPoint(p1, p2, p7, p8), "line 1 line 4 should intersect" assert None != calculateIntersectPoint(p7, p8, p1, p2), "line 4 line 1 should intersect" assert None != calculateIntersectPoint(p1, p2, p9, p10), "line 1 line 5 should intersect" assert None != calculateIntersectPoint(p9, p10, p1, p2), "line 5 line 1 should intersect" assert None != calculateIntersectPoint(p7, p8, p9, p10), "line 4 line 5 should intersect" assert None != calculateIntersectPoint(p9, p10, p7, p8), "line 5 line 4 should intersect" print "\nSUCCESS! All asserts passed for doLinesIntersect" [edit] 
