How to rotate a rectangle OnMousemove

Posted on 2003-11-05
Medium Priority
Last Modified: 2013-11-20
I have an object describing a rectangle using 4 CPoints for its corners, when I'm over a corner I want to hold down the mouse and rotate the object around it's center by moving the mouse.

Any help please....

Question by:dan_hed
  • 4
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LVL 23

Expert Comment

by:Roshan Davis
ID: 9692218

you can use the formula for rotation

x' = (x * cos A) - (y * sin A)
y' = (x * sin A) + (y * cos A)

Try these links also

1). http://www.codeproject.com/gdi/setworldtransform().asp - Using SetWorldTransform() to rotate basic shapes by any angle
2). http://www.codeproject.com/gdi/rotated_dc.asp - Rotate your graphics

Good Luck

Author Comment

ID: 9692308
hmm... right
That's if I know the angle, but what if I have only (x0, y0) - center of rotation, and the old and new x,y ???
How do I get the angle between the two lines ?
LVL 23

Expert Comment

by:Roshan Davis
ID: 9692330
You can use the slope equation

Slope = (y'- y)/(x'- x)
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LVL 23

Expert Comment

by:Roshan Davis
ID: 9692343
Try this code snippet

dy = y2 - y1;
dx = x2 - x1;

assert (!(dx == 0.0 && dy == 0.0));

if (dx == 0.0)  /* Special case, vertical line */
  if (dy > 0.0)
    return 0.0;
    return 180.0;
if (dy == 0.0) /* Special case, horizontal line */
  if (dx > 0.0)
    return 90.0;
    return 270.0;
if (dx > 0.0)
  return 90.0 - atan(dy/dx) * (180 / M_PI);
else if (dx < 0.0)
  return 270.0 - atan(dy/dx) * (180 / M_PI);

Good Luck

Author Comment

ID: 9692354
Yo man,

That's all fine but my math is a bit rusty, know, the slope, know the formula for rotation.
But... how do I get the angle (rads / degrees) ???
LVL 23

Accepted Solution

Roshan Davis earned 750 total points
ID: 9692359
What about the above code?
that will return in degree

Expert Comment

ID: 9693133
Try to call the function as following in your code:

CPoint pt[HANDLE_MAX];
CRect rect(10,10,100,100);


#include <math.h>
#define HANDLE_MAX 4
#define HANDLE_TL 0
#define HANDLE_TR 1
#define HANDLE_BL 2
#define HANDLE_BR 3

BOOL Rotate(const RECT rect,const int nAngle,LPPOINT pPoint) const
      int i;
      CPoint pt,ptCenter;
      double dRadian,dRadianCos,dRadianSin;
      const double PI = 3.141592654;

      ptCenter.x = int((rect.left + rect.right) / 2);
      ptCenter.y = int((rect.top + rect.bottom) / 2);
      dRadian = PI * double(nAngle) / double(180);
      dRadianCos = cos(dRadian);
      dRadianSin = sin(dRadian);

      pPoint[HANDLE_TL] = CPoint(rect.left,rect.top);
      pPoint[HANDLE_TR] = CPoint(rect.right,rect.top);
      pPoint[HANDLE_BR] = CPoint(rect.right,rect.bottom);
      pPoint[HANDLE_BL] = CPoint(rect.left,rect.bottom);

            pt.x = pPoint[i].x - ptCenter.x;
            pt.y = pPoint[i].y - ptCenter.y;

            pPoint[i].x = int(double(ptCenter.x) + double(pt.x)*dRadianCos - double(pt.y)*dRadianSin);
            pPoint[i].y = int(double(ptCenter.y) + double(pt.x)*dRadianSin + double(pt.y)*dRadianCos);

      return TRUE;

Author Comment

ID: 9693258
Hi roshmon,
Not quit it but managed to solve the problem, reminded me of the math I forgot long ago :)

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