Android - getting the current rotation of a matrix

Question

I need to be able to set the rotation of a matrix rather than add to it. I believe the only way to set the rotation is to know the current rotation of the matrix.

Note: matrix.setRotate() will not do, because the matrix needs to retain its information.

Answer 1

float[] v = new float[9];
matrix.getValues(v);
// translation is simple
float tx = v[Matrix.MTRANS_X];
float ty = v[Matrix.MTRANS_Y];

// calculate real scale
float scalex = v[Matrix.MSCALE_X];
float skewy = v[Matrix.MSKEW_Y];
float rScale = (float) Math.sqrt(scalex * scalex + skewy * skewy);

// calculate the degree of rotation
float rAngle = Math.round(Math.atan2(v[Matrix.MSKEW_X], v[Matrix.MSCALE_X]) * (180 / Math.PI));

from here http://judepereira.com/blog/calculate-the-real-scale-factor-and-the-angle-of-rotation-from-an-android-matrix/

Answer 2

What you can do is call getValues and cache the values. Later when you want them back just call setValues on the matrix.

Update

The rotation matrix and transform matrix relation well explained here

Answer 3

I'm afraid I can't help you with the integration (I've done this in Flash only), but it sounds like you should try to do your matrix calculations yourself, which is best done using "quaternions", which makes adding rotations pretty trivial. See http://introcs.cs.princeton.edu/java/32class/Quaternion.java.html for an example Java implementation. You would want to create a second quaternion matrix that describes your change in rotation and add (in this case, plus) it to your current matrix.