Hi. I want to get a 2D object to rotate around another one. They are both in world coordinates. Here's what I have so far: Point t = center of object that I want to rotate around. Point p = position of the object that I want to rotate. I've concatenated 3 matrices (each 3x3): Matrix concat = TransInv*Rotation*Translate.

where Translate is:
1 0 -t.x
0 1 -t.y
0 0   1

Rotation is:
cos(a) -sin(a) 0
sin(a)  cos(a) 0
  0       0    1

TransInv is:
1 0 t.x
0 1 t.y
0 0  1

and I get:
| cos(a) -sin(a)  -t.x*cos(a) + t.y*sin(a) + t.x |
| sin(a)  cos(a)  -t.x*sin(a) - t.y*cos(a) + t.y |
|   0       0                1                   |

So then I use that matrix and transform 'p':

Point temp;
temp.x = cos(a)*p.x + -sin(a)*p.y + -t.x*cos(a) + t.y*sin(a) + t.x;
temp.y = sin(a)*p.x +  cos(a)*p.y + -t.x*sin(a) - t.y*cos(a) + t.y;
p = temp;

[Every time before I apply that transform on p, I reposition it to it's original location (because angle 'a' accumulates each frame)] And so, what I get is that as the object rotates below position 't', it strays really far away from its circular path but as the object rotates above position 't', it's fine. What the heck am I doing wrong here? Thank you. [Edited by - 16bit_port on February 9, 2008 9:05:37 PM]

It looks like you may have reversed the order of your transformations, depending on whether your system uses premultiplication or postmultiplication of vectors.

I tried reversing the orders of the matrices and even tried

temp.x = cos(a)*p.x + -sin(a)*p.y + t.x;temp.y = sin(a)*p.x +  cos(a)*p.y + t.y;

but neither works.

With the first one, it's all over the place on the screen. With the second one, it's no where to be seen.

So, I'm pretty sure mine is correct for the most part and I'm just forgetting a small minor detail. Anybody?

Try:

temp.x = cos(a)*(p.x - t.x) - sin(a)*(p.y - t.y) + t.x;temp.y = sin(a)*(p.x - t.x) + cos(a)*(p.y - t.y) + t.y;

If this doesn't work, you probably have an error somewhere else.