I’ve been working on a tile-engine lately and I’ve hit a bit of a snag when doing object to object collision. My moving objects are defined as cylinders, but I’m having trouble with the two dimensional circle part of it, so consider this a circle to circle problem. I can calculate the exact time and point of the collision, that’s not the problem, but currently the physics in my engine are, that an object can move in a dimension if it isn’t blocked in that dimension. That means that if I would diagonally jump into a horizontal wall, only the y-dimension would be blocked, so I stop moving in the y dimension, but keep moving in the x and z dimensions. If that doesn’t make sense, try the link to the current build at the end of this post.

This is the current code I’m using.

```
private function movingObjektBlock(Red:RigidBody, Blue:RigidBody, LINE:Line):Number {
var RR:Number = Red.radius+Blue.radius;
var dX:Number = Red.x-Blue.x;
var dY:Number = Red.y-Blue.y;
//
var Vx:Number = Red.Vl.x-Blue.Vl.x;
var Vy:Number = Red.Vl.y-Blue.Vl.y;
//
//
var a = Vx*Vx+Vy*Vy;
var b = 2*dX*Vx+2*dY*Vy;
var c = dX*dX+dY*dY-(RR*RR);
//
var Ta = (-b+Math.sqrt(b*b-(4*a*c)))/(2*a);
var Tb = (-b-Math.sqrt(b*b-(4*a*c)))/(2*a);
//
var T1:Number
var T2:Number
if (Ta<Tb) {
T1 = Ta
T2 = Tb
} else {
T1 = Tb
T2 = Ta
}
//
var z1:Number = LINE.getZforT(T1)
var z2:Number = LINE.getZforT(T2)
var T:Number = -1
if ((z1 > Blue.z && z1 < Blue.z + Blue.height) || (z1 + Red.height > Blue.z && z1 + Red.height < Blue.z + Blue.height) || (z1 < Blue.z && z1 + Red.height > Blue.z + Blue.height)) {
//Collision
T = T1
} else if ((z2 > Blue.z && z2 < Blue.z + Blue.height) || (z2 + Red.height > Blue.z && z2 + Red.height < Blue.z + Blue.height) || (z2 < Blue.z && z2 + Red.height > Blue.z + Blue.height)) {
//Collision
T = T2
}
//
//
if (T>0 && T<1) {
return (T);
} else {
return (-1);
}
}
```

The problem with just one time is, that I wouldn’t know the time for the individual dimensions. What I was thinking of, is moving the circles to the state of collision. Draw a line from centre to centre and get the left and right normal of that line. This would be the virtual dimension that the other circle isn’t blocking. If I project the remaining movement vector on that line, would that be the solution to my problem?

I’m a bit fried at the moment so any other suggestions are welcome.

Arrows to move, space to jump

c = fire

x = incendary grenade

z = frag grenade