Question

I am currently creating an android game and implemented collision detection a while back. I am simply drawing a Rect around sprites using their position, width and height and seeing if they intersect other Rects. However, my sprites now rotate depending on their trajectory, but I cannot find how to rotate the Rect so the bound is correct. Any suggestions?

Thanks
Andy

Answer 1

Rect objects are usually axis-aligned, and so they only need 4 values: top, left, bottom, right.

If you want to rotate your rectangle, you’ll need to convert it to eight values representing the co-ordinate of each vertex.

You can easily calculate the centre value by averaging all the x- and y-values.

Then it’s just basic maths. Here’s something from StackOverflow:

Rotating a point about another point (2D)

Your eight values, or four corners are (assuming counter-clockwise from the top right):

v0 : (right, top)
v1 : (left, top)
v2 : (left, bottom)
v3 : (right, bottom)

Create your own rectangle object to cope with this, and compute intersections etc.

Note that I’ve talked about how to rotate the rectangle’s vertices. If you still want a bounding box, this is normally still considered to be axis-aligned, so you could take the max and min of the rotated vertices and construct a new (larger) rectangle. That might not be what you want though.