Android Two finger rotation

I tried a combination of answers that are here but it still didn't work perfectly so I had to modify it a little bit.

This code gives you the delta angle on each rotation, it works perfectly to me, I'm using it to rotate an object in OpenGL.

public class RotationGestureDetector {
private static final int INVALID_POINTER_ID = -1;
private float fX, fY, sX, sY, focalX, focalY;
private int ptrID1, ptrID2;
private float mAngle;
private boolean firstTouch;

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:
            sX = event.getX();
            sY = event.getY();
            ptrID1 = event.getPointerId(0);
            mAngle = 0;
            firstTouch = true;
            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());
            mAngle = 0;
            firstTouch = true;
            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));
                if (firstTouch) {
                    mAngle = 0;
                    firstTouch = false;
                } else {
                    mAngle = angleBetweenLines(fX, fY, sX, sY, nfX, nfY, nsX, nsY);
                }

                if (mListener != null) {
                    mListener.OnRotation(this);
                }
                fX = nfX;
                fY = nfY;
                sX = nsX;
                sY = nsY;
            }
            break;
        case MotionEvent.ACTION_UP:
            ptrID1 = INVALID_POINTER_ID;
            break;
        case MotionEvent.ACTION_POINTER_UP:
            ptrID2 = INVALID_POINTER_ID;
            break;
    }
    return true;
}

private float getMidpoint(float a, float b){
    return (a + b) / 2;
}

float findAngleDelta( float angle1, float angle2 )
{
    float From = ClipAngleTo0_360( angle2 );
    float To   = ClipAngleTo0_360( angle1 );

    float Dist  = To - From;

    if ( Dist < -180.0f )
    {
        Dist += 360.0f;
    }
    else if ( Dist > 180.0f )
    {
        Dist -= 360.0f;
    }

    return Dist;
}

float ClipAngleTo0_360( float Angle ) { 
    return Angle % 360.0f; 
}

private float angleBetweenLines (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 findAngleDelta((float)Math.toDegrees(angle1),(float)Math.toDegrees(angle2));
}

public static interface OnRotationGestureListener {
    public boolean OnRotation(RotationGestureDetector rotationDetector);
}
}

Here's my improvement on Leszek's answer. I found that his didn't work for small views as when a touch went outside the view the angle calculation was wrong. The solution is to get the raw location instead of just getX/Y.

Credit to this thread for getting the raw points on a rotatable view.

public class RotationGestureDetector {

    private static final int INVALID_POINTER_ID = -1;
    private PointF mFPoint = new PointF();
    private PointF mSPoint = new PointF();
    private int mPtrID1, mPtrID2;
    private float mAngle;
    private View mView;

    private OnRotationGestureListener mListener;

    public float getAngle() {
        return mAngle;
    }

    public RotationGestureDetector(OnRotationGestureListener listener, View v) {
        mListener = listener;
        mView = v;
        mPtrID1 = INVALID_POINTER_ID;
        mPtrID2 = INVALID_POINTER_ID;
    }

    public boolean onTouchEvent(MotionEvent event) {


        switch (event.getActionMasked()) {
            case MotionEvent.ACTION_OUTSIDE:
                Log.d(this, "ACTION_OUTSIDE");
                break;
            case MotionEvent.ACTION_DOWN:
                Log.v(this, "ACTION_DOWN");
                mPtrID1 = event.getPointerId(event.getActionIndex());
                break;
            case MotionEvent.ACTION_POINTER_DOWN:
                Log.v(this, "ACTION_POINTER_DOWN");
                mPtrID2 = event.getPointerId(event.getActionIndex());

                getRawPoint(event, mPtrID1, mSPoint);
                getRawPoint(event, mPtrID2, mFPoint);

                break;
            case MotionEvent.ACTION_MOVE:
                if (mPtrID1 != INVALID_POINTER_ID && mPtrID2 != INVALID_POINTER_ID) {
                    PointF nfPoint = new PointF();
                    PointF nsPoint = new PointF();

                    getRawPoint(event, mPtrID1, nsPoint);
                    getRawPoint(event, mPtrID2, nfPoint);

                    mAngle = angleBetweenLines(mFPoint, mSPoint, nfPoint, nsPoint);

                    if (mListener != null) {
                        mListener.onRotation(this);
                    }
                }
                break;
            case MotionEvent.ACTION_UP:
                mPtrID1 = INVALID_POINTER_ID;
                break;
            case MotionEvent.ACTION_POINTER_UP:
                mPtrID2 = INVALID_POINTER_ID;
                break;
            case MotionEvent.ACTION_CANCEL:
                mPtrID1 = INVALID_POINTER_ID;
                mPtrID2 = INVALID_POINTER_ID;
                break;
            default:
                break;
        }
        return true;
    }

    void getRawPoint(MotionEvent ev, int index, PointF point) {
        final int[] location = { 0, 0 };
        mView.getLocationOnScreen(location);

        float x = ev.getX(index);
        float y = ev.getY(index);

        double angle = Math.toDegrees(Math.atan2(y, x));
        angle += mView.getRotation();

        final float length = PointF.length(x, y);

        x = (float) (length * Math.cos(Math.toRadians(angle))) + location[0];
        y = (float) (length * Math.sin(Math.toRadians(angle))) + location[1];

        point.set(x, y);
    }

    private float angleBetweenLines(PointF fPoint, PointF sPoint, PointF nFpoint, PointF nSpoint) {
        float angle1 = (float) Math.atan2((fPoint.y - sPoint.y), (fPoint.x - sPoint.x));
        float angle2 = (float) Math.atan2((nFpoint.y - nSpoint.y), (nFpoint.x - nSpoint.x));

        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 interface OnRotationGestureListener {
        void onRotation(RotationGestureDetector rotationDetector);
    }
}

Improvements of the class:

  • angle returned is total since rotation has begun
  • removing unnecessary functions
  • simplification
  • get position of first pointer only after second pointer is down
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);
    }
}

How to use it:

  1. Put the above class in a separate file RotationGestureDetector.java
  2. create a private field mRotationDetector 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).
  3. In the method onTouchEvent, send the touch events received to the gesture detector with 'mRotationDetector.onTouchEvent(event);'
  4. Implements 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()

Example:

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));
    }

}

Note:

You can also use the RotationGestureDetector class in a View instead of an Activity.


There are still some mistakes, here is the solution that worked perfect for me...

instead of 

float angle = angleBtwLines(fX, fY, nfX, nfY, sX, sY, nsX, nsY);

you need to write

float angle = angleBtwLines(fX, fY, sX, sY, nfX, nfY, nsX, nsY);

And angleBetweenLines should be

private float angleBetweenLines (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 findAngleDelta((float)Math.toDegrees(angle1),(float)Math.toDegrees(angle2));
}

Then the angle you get is the angle you should rotate the image by...

ImageAngle += angle...

Tags:

Android

Rotation

Gesture Recognition

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