Question

if i make an ellipse bounded by a rectangle. how can i rotate it ? as in if i have rx,ry,cx,cy,topleft x,toplefy, bottomrightx,bottomrighty

iused the formula

angle=taninverse(ry/rx)
and i keep adding the angle that is angle=angle + taninverse(ry/rx)

the angle is too small to see the difference.
please provide another formula to calctulate teh angle with the given parmeteres

to have a look at the ellipse with the bounded rectangle frame
http://svg-edit.googlecode.com/svn/branches/2.5.1/editor/svg-editor.html

Answer 1

Have you tried rotating by a larger angle to see what happens? Do the values of rx and ry change?

If not, you probably should compute ‘atan(ry/rx)’ and store the result in a variable like so:

double delta = Math.atan(ry / rx);

Then when you rotate

angle += delta; // or angle = angle + delta --- these are equivalent

Alternatively, rather than calculating the angle from ry and rx, you may wish to simply keep a value such as

double delta = Math.PI / 6;

This simply gives the angle to rotate through without a calculation. The advantage is that you can simply set this delta variable to any value you wish, large or small. As above, you can increment angle by this delta value.