Collision detection with rotating crosses (or similar)

Windows
1

Hi,

everywhere i read, “Don’t do pixel perfect collision, because it’s ineffective / time consuming” … but, when I have an image, let’s say a rotating cross, where collision should only count when the player hits one of the four rotating ‘blades’ of the cross and not the empty space in-between, how can this be done otherwise?

When I put bounding boxes around the four ends of the cross, how can I take rotation into account then?

Any tips, example code, or whatever?

2

let’s say the middle point of your rotating fan/cross is at _origin and the fan’s blades have a length of 10.

Let’s you have 4 blades ( a cross) and right now it is unrotated. One blade is top, one is right bottom left etc.

So the blade to the right should have the position 10, 0 at the start right?

But for rotation we need to calculate the sins and cosines, I tried to make it into a simple function:

_origin = new Vector2(0,0);
_bladeLength = 10;
_cross.angle = 0;

(…)

//rotate!
_cross.angle += 0.1f;

// Calculate the end points of the blades!

//Calculate the position from the angle
float sin0 = (float) Math.sin(_cross.angle);
float cos0 = (float) Math.cos(_cross.angle);

_blade1.position.x = sin0 * bladeLength + _origin.x;
_blade1.position.y = cos0*bladeLength + _origin.y;

//The opposite blade simply has the negative sin/cos values

_blade2.position.x = -sin0 * bladeLength + _origin.x;
_blade2.position.y = -cos0*bladeLength + _origin.y;

//Now for the other 2 blades we need to take the angle and add 90°, aka half pi

sin0 = Math.sin(_cross.angle + Math.PI/2);
cos0 = Math.cos(_cross.angle + Math.PI/2);

_blade3.position.x = sin0 * bladeLength + _origin.x;
_blade3.position.y = cos0bladeLength + _origin.y;
_blade4.position.x = -sin0 * bladeLength + _origin.x;
_blade4.position.y = -cos0bladeLength + _origin.y;

Hope this helps

1 Like
3

Thanks for the detailled answer. :wink:

Alas, it doesn’t help me much… your example shows how to calculate the end points of a rotating cross, but not how to do proper collision detection with a player sprite/texture. What if the player doesn’t touch the ‘end point’ of one of the blades, but somewhere between the center/origin of the rotating cross and the end point? (assuming it’s a bigger rotating cross / a smaller player sprite)

4

Two rectangles that go along the blade?

5

Yeah, use rectangles that rotate around the center. You can apply the transformation kosmonaut explained to the rectangles and use them for collision detection.

6

Umm… what’s a transformation kosmonaut?

Alas, I don’t really know any geometry math… or it has been a too long time since I last used it.

7

So as a result you only have points to go off?

Do you then need to create virtual rectangles and check if objects are in between all the points you have?