I am trying to implement two finger rotation in android however, it is not quite working as expected. The goal is to implement rotation like Google Earth does (two-finger rotating the image around the focal point). Currently my rotation listener looks like this:
private class RotationGestureListener {
private static final int INVALID_POINTER_ID = -1;
private float fX, fY, sX, sY, focalX, focalY;
private int ptrID1, ptrID2;
public RotationGestureListener(){
ptrID1 = INVALID_POINTER_ID;
ptrID2 = INVALID_POINTER_ID;
}
public boolean onTouchEvent(MotionEvent event){
switch (event.getActionMasked()) {
case MotionEvent.ACTION_DOWN:
sX = event.getX();
sY = event.getY();
ptrID1 = event.getPointerId(0);
break;
case MotionEvent.ACTION_POINTER_DOWN:
fX = event.getX();
fY = event.getY();
focalX = getMidpoint(fX, sX);
focalY = getMidpoint(fY, sY);
ptrID2 = event.getPointerId(event.getActionIndex());
break;
case MotionEvent.ACTION_MOVE:
if(ptrID1 != INVALID_POINTER_ID && ptrID2 != INVALID_POINTER_ID){
float nfX, nfY, nsX, nsY;
nfX = event.getX(event.findPointerIndex(ptrID1));
nfY = event.getY(event.findPointerIndex(ptrID1));
nsX = event.getX(event.findPointerIndex(ptrID2));
nsY = event.getY(event.findPointerIndex(ptrID2));
float angle = angleBtwLines(fX, fY, nfX, nfY, sX, sY, nsX, nsY);
rotateImage(angle, focalX, focalY);
fX = nfX;
fY = nfY;
sX = nfX;
sY = nfY;
}
break;
case MotionEvent.ACTION_UP:
ptrID1 = INVALID_POINTER_ID;
break;
case MotionEvent.ACTION_POINTER_UP:
ptrID2 = INVALID_POINTER_ID;
break;
}
return false;
}
private float getMidpoint(float a, float b){
return (a + b) / 2;
}
private float angleBtwLines (float fx1, float fy1, float fx2, float fy2, float sx1, float sy1, float sx2, float sy2){
float angle1 = (float) Math.atan2(fy1 - fy2, fx1 - fx2);
float angle2 = (float) Math.atan2(sy1 - sy2, sx1 - sx2);
return (float) Math.toDegrees((angle1-angle2));
}
}
However whenever I rotate the angle of rotation is much larger and it sometimes it rotates to the wrong side. Any ideas on how to fix this?
By the way I am testing it on a Motorola Atrix, so it does not have the touchscreen bug.
Thanks
public class RotationGestureDetector {
private static final int INVALID_POINTER_ID = -1;
private float fX, fY, sX, sY;
private int ptrID1, ptrID2;
private float mAngle;
private OnRotationGestureListener mListener;
public float getAngle() {
return mAngle;
}
public RotationGestureDetector(OnRotationGestureListener listener){
mListener = listener;
ptrID1 = INVALID_POINTER_ID;
ptrID2 = INVALID_POINTER_ID;
}
public boolean onTouchEvent(MotionEvent event){
switch (event.getActionMasked()) {
case MotionEvent.ACTION_DOWN:
ptrID1 = event.getPointerId(event.getActionIndex());
break;
case MotionEvent.ACTION_POINTER_DOWN:
ptrID2 = event.getPointerId(event.getActionIndex());
sX = event.getX(event.findPointerIndex(ptrID1));
sY = event.getY(event.findPointerIndex(ptrID1));
fX = event.getX(event.findPointerIndex(ptrID2));
fY = event.getY(event.findPointerIndex(ptrID2));
break;
case MotionEvent.ACTION_MOVE:
if(ptrID1 != INVALID_POINTER_ID && ptrID2 != INVALID_POINTER_ID){
float nfX, nfY, nsX, nsY;
nsX = event.getX(event.findPointerIndex(ptrID1));
nsY = event.getY(event.findPointerIndex(ptrID1));
nfX = event.getX(event.findPointerIndex(ptrID2));
nfY = event.getY(event.findPointerIndex(ptrID2));
mAngle = angleBetweenLines(fX, fY, sX, sY, nfX, nfY, nsX, nsY);
if (mListener != null) {
mListener.OnRotation(this);
}
}
break;
case MotionEvent.ACTION_UP:
ptrID1 = INVALID_POINTER_ID;
break;
case MotionEvent.ACTION_POINTER_UP:
ptrID2 = INVALID_POINTER_ID;
break;
case MotionEvent.ACTION_CANCEL:
ptrID1 = INVALID_POINTER_ID;
ptrID2 = INVALID_POINTER_ID;
break;
}
return true;
}
private float angleBetweenLines (float fX, float fY, float sX, float sY, float nfX, float nfY, float nsX, float nsY)
{
float angle1 = (float) Math.atan2( (fY - sY), (fX - sX) );
float angle2 = (float) Math.atan2( (nfY - nsY), (nfX - nsX) );
float angle = ((float)Math.toDegrees(angle1 - angle2)) % 360;
if (angle < -180.f) angle += 360.0f;
if (angle > 180.f) angle -= 360.0f;
return angle;
}
public static interface OnRotationGestureListener {
public void OnRotation(RotationGestureDetector rotationDetector);
}
}
RotationGestureDetector.javamRotationDetector of type RotationGestureDetector in your activity class and create a new instance of the detector during the initialization (onCreate method for example) and give as parameter a class implementing the onRotation method (here the activity = this). onTouchEvent, send the touch events received to the gesture detector with 'mRotationDetector.onTouchEvent(event);' RotationGestureDetector.OnRotationGestureListener in your activity and add the method 'public void OnRotation(RotationGestureDetector rotationDetector)' in the activity. In this method, get the angle with rotationDetector.getAngle() public class MyActivity extends Activity implements RotationGestureDetector.OnRotationGestureListener {
private RotationGestureDetector mRotationDetector;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mRotationDetector = new RotationGestureDetector(this);
}
@Override
public boolean onTouchEvent(MotionEvent event){
mRotationDetector.onTouchEvent(event);
return super.onTouchEvent(event);
}
@Override
public void OnRotation(RotationGestureDetector rotationDetector) {
float angle = rotationDetector.getAngle();
Log.d("RotationGestureDetector", "Rotation: " + Float.toString(angle));
}
}
You can also use the RotationGestureDetector class in a View instead of an Activity.