Calculating side of intersecting rectangle in collision

Question

Hi all,
I'm trying to determine the side of a rectangle in which a colliding object has intersected with it (i'm working on some bouncing bullet code)
anyway, this is what I have so far..
I know, the bounding rectangle of the object being hit (tile)
(lets say 0,0,64,64 (top, left, width, height))
I know the projectile is moving 5 pixels per update and has a bounding box of pos.X, pos.Y, 16, 16
So when the bullet and tile collide, the rectangles intersect and so I know a collision has happened
but I want to now know which side of the tile object was intersected so i can do
If top or bottom is the intersecting side then reflect the Y velocity of the bullet (but leave the X projectile)
if left or right is intersected then reflect X and not Y
This should then give me this effect
TTTTTT
or
\ .T
.\ T
./ T
/. T
but to get that effect I need to know which side of the tile was intersected.
I also know the position of the bullet at collision and i know the last 12 positions leading up to collision (as i store them in an array for a bouncing action upon collision of other objects such as the player)
so from that I could work out the trajectory of the bullet through the bounding rectangle of the tile (even though I dont know how but I'm sure mathematics can be my friend on that one :p)
with all this knowledge, is there a way to achieve what I'm trying to do?
thanks
Edited by: harveyball on 05-Jun-2009 14:43

Answer 1

Hi again,
so i've found an algorithm to do my intersection and I've implemented a quick php version of the intersection code
which is
function intersectLine($line1A, $line1B, $line2A, $line2B)
     $insersectionPoint=array();
     $A1 = $line1B['y']-$line1A['y'];
     $B1 = $line1A['x']-$line1B['x'];
     $C1 = $A1 * $line1A['x'] + $B1 * $line1A['y'];
     $A2 = $line2B['y']-$line2A['y'];
     $B2 = $line2A['x']-$line2B['x'];
     $C2 = $A2 * $line2A['x'] + $B2 * $line2A['y'];
     $det = $A1 * $B2 - $A2 * $B1;
    if($det == 0)
          $insersectionPoint=array("x"=>-1, "y"=>-1);
     else
        $insersectionPoint['x'] = ($B2 * $C1 - $B1 * $C2) / $det;
        $insersectionPoint['y'] = ($A1 * $C2 - $A2 * $C1) / $det;     
     return $insersectionPoint;
     }however, i only want to know if the lines between the given points have actually touched (not the potential of the line if infinity extended)
My code will return an intersection even if the point hasnt reached that part of the line.
Can someone help me with the mathematical bit of doing the min and max of the points to work out the actual collision of lines.
I found this snippet of data from google
This gives you the location of the intersection of two lines, but what if you have line segments, not lines. In this case, you need to make sure that the point you found is on both of the line segments. If your line segment goes from (x1,y1) to (x2,y2), then to check if (x,y) is on that segment, you just need to check that min(x1,x2) ≤ x ≤ max(x1,x2), and do the same thing for y. You must be careful about double precision issues though. If your point is right on the edge of the segment, or if the segment is horizontal or vertical, a simple comparison might be problematic. In these cases, you can either do your comparisons with some tolerance, or else use a fraction class.
which is obviously what i want to do but I dont understand what this means
min(x1,x2) ≤ x ≤ max(x1,x2)
what exactly does ≤ x ≤ mean?
Can someone aid in writing the simplified version of the algorithm?
(java or pseduo is fine :) i just wrote my test function in php because it was quicker)