# Incorrectly says that 2 rectangles collide: whats going wrong?

I am using and expanding a function that detects whether two 2d rotated rectangles collide. The function is from here.

My Problem: The function incorrectly detects a collision between the following rectangles.

Rect1 = centreX=0, centreY=0, width=8, height=4 angle=0
Rect1 = centreX=16, centreY=0, width=8, height=4 angle=0

Am I wrong or should those rectangles not collide? How should I edit the function so that it detects pixel perfect collisions between rotated rectangles? I have a feeling I need to edit the last 2 lines but I'm not sure.

``````int RotRectsCollision(_RotRect * rr1, _RotRect * rr2)
{
_Vector2D A, B,   // vertices of the rotated rr2
C,      // center of rr2
BL, TR; // vertices of rr2 (bottom-left, top-right)

float ang = rr1->ang - rr2->ang, // orientation of rotated rr1
cosa = cos(ang),           // precalculated trigonometic -
sina = sin(ang);           // - values for repeated use

float t, x, a;      // temporary variables for various uses
float dx;           // deltaX for linear equations
float ext1, ext2;   // min/max vertical values

// move rr2 to make rr1 cannonic
C = rr2->C;
SubVectors2D(&C, &rr1->C);

// rotate rr2 clockwise by rr2->ang to make rr2 axis-aligned
RotateVector2DClockwise(&C, rr2->ang);

// calculate vertices of (moved and axis-aligned := 'ma') rr2
BL = TR = C;
SubVectors2D(&BL, &rr2->S);

// calculate vertices of (rotated := 'r') rr1
A.x = -rr1->S.y*sina; B.x = A.x; t = rr1->S.x*cosa; A.x += t; B.x -= t;
A.y =  rr1->S.y*cosa; B.y = A.y; t = rr1->S.x*sina; A.y += t; B.y -= t;

t = sina*cosa;

// verify that A is vertical min/max, B is horizontal min/max
if (t < 0)
{
t = A.x; A.x = B.x; B.x = t;
t = A.y; A.y = B.y; B.y = t;
}

// verify that B is horizontal minimum (leftest-vertex)
if (sina < 0) { B.x = -B.x; B.y = -B.y; }

// if rr2(ma) isn't in the horizontal range of
// colliding with rr1(r), collision is impossible
if (B.x > TR.x || B.x > -BL.x) return 0;

// if rr1(r) is axis-aligned, vertical min/max are easy to get
if (t == 0) {ext1 = A.y; ext2 = -ext1; }
// else, find vertical min/max in the range [BL.x, TR.x]
else
{
x = BL.x-A.x; a = TR.x-A.x;
ext1 = A.y;
// if the first vertical min/max isn't in (BL.x, TR.x), then
// find the vertical min/max on BL.x or on TR.x
if (a*x > 0)
{
dx = A.x;
if (x < 0) { dx -= B.x; ext1 -= B.y; x = a; }
else       { dx += B.x; ext1 += B.y; }
ext1 *= x; ext1 /= dx; ext1 += A.y;
}

x = BL.x+A.x; a = TR.x+A.x;
ext2 = -A.y;
// if the second vertical min/max isn't in (BL.x, TR.x), then
// find the local vertical min/max on BL.x or on TR.x
if (a*x > 0)
{
dx = -A.x;
if (x < 0) { dx -= B.x; ext2 -= B.y; x = a; }
else       { dx += B.x; ext2 += B.y; }
ext2 *= x; ext2 /= dx; ext2 -= A.y;
}
}

// check whether rr2(ma) is in the vertical range of colliding with rr1(r)
// (for the horizontal range of rr2)
return !((ext1 < BL.y && ext2 < BL.y) ||
(ext1 > TR.y && ext2 > TR.y));
}
``````
-
In your first snippet, you initialize Rect1 twice. Did you mean Rect1 and Rect2? –  Marton Aug 20 '12 at 6:29
Try stepping through your function with the debugger and do the math on paper. You'll easily see where you're doing something wrong, or if you initialized your variables incorrectly. –  Darcara Aug 20 '12 at 19:27