[gameprogrammer] Re: Rotation matrix for 2D point

The general affine transformation:

   x2         x1
      = A  x      + B
   y2         y1

Pure rotation uses the A matrix and is defined as (for
positive angles being clockwise rotations): 

       cosa  -sina
  A = 
       sina  cosa

       0
  B =
       0

Here, we are working in image coordinates, so the y
axis goes downward.

Or:
  x2 = cosa * (x1 - x0) - sina * (y1 - y0) + x0
  y2 = sina * (x1 - x0) + cosa * (y1 - y0) + y0

(where (x0,y0) are the coordinates of the center of
rotation (in the input image) and "a" is the angle of
rotation with clockwise rotations having positive
angles.)


--- Mario de Sousa <mcd@xxxxxxxxxxxxx> wrote:

> Hey, guys.
> 
> Would anyone be so kind as to give me the rotation
> matrix for a 2d point. I can't remeber which trig
> function goes where and my internet proxy is down at
> the moment. 
> 
> Thanks in advance.
> 
> Mario de Sousa
> 
> 
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