# Collision for mobile game

I'm writing a little game in as3 using Starling, and I need to check collision between 2 boats that can rotate. I don't need the pixel perfect collision, but bounds collision is not enough too. The boat look more or less like this:

I was thinking about create one square on the back of the boat and a triangle on the front, than for each boat, check if the square collide with the other boat square or triangle, and the same for the triangle.

I just don't know how to do that, I don't know if it's possible with the Shape.hitTest, or if it's the best way to do that.

What can I do?

• I suggest you to use Box2D (using physics editor). It will be more easy to handle collision. Hope to help you! Also you can see the Citrus Engine.
– user32901
Jul 9 '13 at 7:16
• I think your idea of using a square and triangle is good. Even if Shape.hitTest doesn't help, it won't be difficult to write a hitDetection function yourself. Unless you are using physics based collisions/simulations, Don't use Box2d, will be overkill.
– Ani
Aug 8 '13 at 10:05

You can treat each boat as a rectangle, and then check for collisions as they rotate using projected axis.

There is an excellent class for doing just that, by Jozef Chút'ka, available at GitHub.

You can also check out his blog post about it: http://blog.yoz.sk/2010/11/4-000-000-rectangle-collisions-per-second/

package sk.yoz.math
{
public class FastCollisions
{
public static function rectangles(
r1p1x:Number, r1p1y:Number, r1p2x:Number, r1p2y:Number,
r1p3x:Number, r1p3y:Number, r1p4x:Number, r1p4y:Number,
r2p1x:Number, r2p1y:Number, r2p2x:Number, r2p2y:Number,
r2p3x:Number, r2p3y:Number, r2p4x:Number, r2p4y:Number):Boolean
{
if(!isProjectedAxisCollision(r1p1x, r1p1y, r1p2x, r1p2y,
r2p1x, r2p1y, r2p2x, r2p2y, r2p3x, r2p3y, r2p4x, r2p4y))
return false;

if(!isProjectedAxisCollision(r1p2x, r1p2y, r1p3x, r1p3y,
r2p1x, r2p1y, r2p2x, r2p2y, r2p3x, r2p3y, r2p4x, r2p4y))
return false;

if(!isProjectedAxisCollision(r2p1x, r2p1y, r2p2x, r2p2y,
r1p1x, r1p1y, r1p2x, r1p2y, r1p3x, r1p3y, r1p4x, r1p4y))
return false;

if(!isProjectedAxisCollision(r2p2x, r2p2y, r2p3x, r2p3y,
r1p1x, r1p1y, r1p2x, r1p2y, r1p3x, r1p3y, r1p4x, r1p4y))
return false;

return true;
}

public static function isProjectedAxisCollision(
b1x:Number, b1y:Number, b2x:Number, b2y:Number,
p1x:Number, p1y:Number, p2x:Number, p2y:Number,
p3x:Number, p3y:Number, p4x:Number, p4y:Number):Boolean
{
var x1:Number, x2:Number, x3:Number, x4:Number;
var y1:Number, y2:Number, y3:Number, y4:Number;
if(b1x == b2x)
{
x1 = x2 = x3 = x4 = b1x;
y1 = p1y;
y2 = p2y;
y3 = p3y;
y4 = p4y;

if(b1y > b2y)
{
if((y1 > b1y && y2 > b1y && y3 > b1y && y4 > b1y) ||
(y1 < b2y && y2 < b2y && y3 < b2y && y4 < b2y))
return false;
}
else
{
if((y1 > b2y && y2 > b2y && y3 > b2y && y4 > b2y) ||
(y1 < b1y && y2 < b1y && y3 < b1y && y4 < b1y))
return false;
}
return true;
}
else if(b1y == b2y)
{
x1 = p1x;
x2 = p2x;
x3 = p3x;
x4 = p4x;
y1 = y2 = y3 = y4 = b1y;
}
else
{
var a:Number = (b1y - b2y) / (b1x - b2x);
var ia:Number = 1 / a;
var t1:Number = b2x * a - b2y;
var t2:Number = 1 / (a + ia);

x1 = (p1y + t1 + p1x * ia) * t2;
x2 = (p2y + t1 + p2x * ia) * t2;
x3 = (p3y + t1 + p3x * ia) * t2;
x4 = (p4y + t1 + p4x * ia) * t2;

y1 = p1y + (p1x - x1) * ia;
y2 = p2y + (p2x - x2) * ia;
y3 = p3y + (p3x - x3) * ia;
y4 = p4y + (p4x - x4) * ia;
}

if(b1x > b2x)
{
if((x1 > b1x && x2 > b1x && x3 > b1x && x4 > b1x) ||
(x1 < b2x && x2 < b2x && x3 < b2x && x4 < b2x))
return false;
}
else
{
if((x1 > b2x && x2 > b2x && x3 > b2x && x4 > b2x) ||
(x1 < b1x && x2 < b1x && x3 < b1x && x4 < b1x))
return false;
}
return true;
}

public static function pointInRectangle(px:Number, py:Number,
r1x:Number, r1y:Number, r2x:Number, r2y:Number,
r3x:Number, r3y:Number, r4x:Number, r4y:Number):Boolean
{
var a:Number, x:Number, y:Number;
if(r1x == r2x)
{
x = r1x, y = py;
if(y > (r1y > r2y ? r1y : r2y)) return false;
if(y < (r1y < r2y ? r1y : r2y)) return false;
}
else if(r1y == r2y)
{
x = px, y = r1y;
}
else
{
a = (r1y - r2y) / (r1x - r2x);
x = (py - r2y + r2x * a + px / a) / (a + 1 / a);
y = py + (px - x) / a;
}

if(x > (r1x > r2x ? r1x : r2x)) return false;
if(x < (r1x < r2x ? r1x : r2x)) return false;

if(r2x == r3x)
{
x = r2x, y = py;
if(y > (r2y > r3y ? r2y : r3y)) return false;
if(y < (r2y < r3y ? r2y : r3y)) return false;
}
else if(r2y == r3y)
{
x = px, y = r2y;
}
else
{
a = (r2y - r3y) / (r2x - r3x);
x = (py - r3y + r3x * a + px / a) / (a + 1 / a);
y = py + (px - x) / a;
}

if(x > (r2x > r3x ? r2x : r3x)) return false;
if(x < (r2x < r3x ? r2x : r3x)) return false;
return true;
}
}
}


I hope that helps. Cheers!