绕轴旋转椭圆

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.

Answer 2

There are some animation classes in Android. The package android.animation is available since API 11 and provides the ability animate object properties. android.view.animation is available from API 1 and provides animations for resizing, moving, and rotating. Both also offer XML attributes so you can also set you animation in XML. The main classes to check out are android.view.animation.Animation and android.animation.Animator.