view - Android ImageView Scaling and translating issue

Question

I’m developing an android application (API 19 4.4) and I encounter some issue with ImageViews. I have a SurfaceView, in which I dynamically add ImageViews which I want to react to touch events. On so far, I have managed to make the ImageView move and scale smoothly but I have an annoying behavior.

When I scale down the image to a certain limit (I would say half the original size) and I try to move it, the image flicker. After a short analysis, it seems that it’s switching its position symmetrically around the finger point on the screen, cumulating distance, and finally gets out of sight (all that happens very fast ( < 1s). I think I am missing something with the relative value of the touch event to the ImageView/SurfaceView, but I’m a quite a noob and I’m stucked…

Here is my code

public class MyImageView extends ImageView {
private ScaleGestureDetector mScaleDetector ;
private static final int MAX_SIZE = 1024;

private static final String TAG = "MyImageView";
PointF DownPT = new PointF(); // Record Mouse Position When Pressed Down
PointF StartPT = new PointF(); // Record Start Position of 'img'

public MyImageView(Context context) {
    super(context);
    mScaleDetector = new ScaleGestureDetector(context,new MySimpleOnScaleGestureListener());
    setBackgroundColor(Color.RED);
    setScaleType(ScaleType.MATRIX);
    setAdjustViewBounds(true);
    RelativeLayout.LayoutParams lp = new RelativeLayout.LayoutParams(RelativeLayout.LayoutParams.WRAP_CONTENT, RelativeLayout.LayoutParams.WRAP_CONTENT);

    lp.setMargins(-MAX_SIZE, -MAX_SIZE, -MAX_SIZE, -MAX_SIZE);
    this.setLayoutParams(lp);
    this.setX(MAX_SIZE);
    this.setY(MAX_SIZE);

}

int firstPointerID;
boolean inScaling=false;
@Override
public boolean onTouchEvent(MotionEvent event) {
    // get pointer index from the event object
    int pointerIndex = event.getActionIndex();
    // get pointer ID
    int pointerId = event.getPointerId(pointerIndex);
    //First send event to scale detector to find out, if it's a scale
    boolean res = mScaleDetector.onTouchEvent(event);

    if (!mScaleDetector.isInProgress()) {
        int eid = event.getAction();
        switch (eid & MotionEvent.ACTION_MASK)
        {
        case MotionEvent.ACTION_MOVE :
            if(pointerId == firstPointerID) {

                PointF mv = new PointF( (int)(event.getX() - DownPT.x), (int)( event.getY() - DownPT.y));

                this.setX((int)(StartPT.x+mv.x));
                this.setY((int)(StartPT.y+mv.y));
                StartPT = new PointF( this.getX(), this.getY() );

            }
            break;
        case MotionEvent.ACTION_DOWN : {
            firstPointerID = pointerId;
            DownPT.x = (int) event.getX();
            DownPT.y = (int) event.getY();
            StartPT = new PointF( this.getX(), this.getY() );
            break;
        }
        case MotionEvent.ACTION_POINTER_DOWN: {
            break;
        }
        case MotionEvent.ACTION_UP:
        case MotionEvent.ACTION_POINTER_UP:
        case MotionEvent.ACTION_CANCEL: {
            firstPointerID = -1;
            break;
        }
        default :
            break;
        }
        return true;
    }
    return true;

}

public boolean onScaling(ScaleGestureDetector detector) {

    this.setScaleX(this.getScaleX()*detector.getScaleFactor());
    this.setScaleY(this.getScaleY()*detector.getScaleFactor());
    invalidate();
    return true;
}

private class MySimpleOnScaleGestureListener extends SimpleOnScaleGestureListener {


    @Override
    public boolean onScale(ScaleGestureDetector detector) {
        return onScaling(detector);
    }

    @Override
    public boolean onScaleBegin(ScaleGestureDetector detector) {
        Log.d(TAG, "onScaleBegin");
        return true;
    }

    @Override
    public void onScaleEnd(ScaleGestureDetector arg0) {
        Log.d(TAG, "onScaleEnd");
    }
}

}

I have also another questions about rotations. How should I implement this? Could I use the ScalegestureDetector in some way or have I to make this works in the view touch event? I would like to be able to scale and rotate in the same gesture (and move in another).

Thank for helping me, I would really appreciate!

Sorry for my english

Answer 1

this is a working example of two fingers move/scale/rotate (note: the code is quite short due to smart detector used - see MatrixGestureDetector):

class ViewPort extends View {
    List<Layer> layers = new LinkedList<Layer>();
    int[] ids = {R.drawable.layer0, R.drawable.layer1, R.drawable.layer2};

    public ViewPort(Context context) {
        super(context);
        Resources res = getResources();
        for (int i = 0; i < ids.length; i++) {
            Layer l = new Layer(context, this, BitmapFactory.decodeResource(res, ids[i]));
            layers.add(l);
        }
    }

    @Override
    protected void onDraw(Canvas canvas) {
        for (Layer l : layers) {
            l.draw(canvas);
        }
    }

    private Layer target;

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        if (event.getAction() == MotionEvent.ACTION_DOWN) {
            target = null;
            for (int i = layers.size() - 1; i >= 0; i--) {
                Layer l = layers.get(i);
                if (l.contains(event)) {
                    target = l;
                    layers.remove(l);
                    layers.add(l);
                    invalidate();
                    break;
                }
            }
        }
        if (target == null) {
            return false;
        }
        return target.onTouchEvent(event);
    }
}

class Layer implements MatrixGestureDetector.OnMatrixChangeListener {
    Matrix matrix = new Matrix();
    Matrix inverse = new Matrix();
    RectF bounds;
    View parent;
    Bitmap bitmap;
    MatrixGestureDetector mgd = new MatrixGestureDetector(matrix, this);

    public Layer(Context ctx, View p, Bitmap b) {
        parent = p;
        bitmap = b;
        bounds = new RectF(0, 0, b.getWidth(), b.getHeight());
        matrix.postTranslate(50 + (float) Math.random() * 50, 50 + (float) Math.random() * 50);
    }

    public boolean contains(MotionEvent event) {
        matrix.invert(inverse);
        float[] pts = {event.getX(), event.getY()};
        inverse.mapPoints(pts);
        if (!bounds.contains(pts[0], pts[1])) {
            return false;
        }
        return Color.alpha(bitmap.getPixel((int) pts[0], (int) pts[1])) != 0;
    }

    public boolean onTouchEvent(MotionEvent event) {
        mgd.onTouchEvent(event);
        return true;
    }

    @Override
    public void onChange(Matrix matrix) {
        parent.invalidate();
    }

    public void draw(Canvas canvas) {
        canvas.drawBitmap(bitmap, matrix, null);
    }
}

class MatrixGestureDetector {
    private static final String TAG = "MatrixGestureDetector";

    private int ptpIdx = 0;
    private Matrix mTempMatrix = new Matrix();
    private Matrix mMatrix;
    private OnMatrixChangeListener mListener;
    private float[] mSrc = new float[4];
    private float[] mDst = new float[4];
    private int mCount;

    interface OnMatrixChangeListener {
        void onChange(Matrix matrix);
    }

    public MatrixGestureDetector(Matrix matrix, MatrixGestureDetector.OnMatrixChangeListener listener) {
        this.mMatrix = matrix;
        this.mListener = listener;
    }

    public void onTouchEvent(MotionEvent event) {
        if (event.getPointerCount() > 2) {
            return;
        }

        int action = event.getActionMasked();
        int index = event.getActionIndex();

        switch (action) {
            case MotionEvent.ACTION_DOWN:
            case MotionEvent.ACTION_POINTER_DOWN:
                int idx = index * 2;
                mSrc[idx] = event.getX(index);
                mSrc[idx + 1] = event.getY(index);
                mCount++;
                ptpIdx = 0;
                break;

            case MotionEvent.ACTION_MOVE:
                for (int i = 0; i < mCount; i++) {
                    idx = ptpIdx + i * 2;
                    mDst[idx] = event.getX(i);
                    mDst[idx + 1] = event.getY(i);
                }
                mTempMatrix.setPolyToPoly(mSrc, ptpIdx, mDst, ptpIdx, mCount);
                mMatrix.postConcat(mTempMatrix);
                if(mListener != null) {
                    mListener.onChange(mMatrix);
                }
                System.arraycopy(mDst, 0, mSrc, 0, mDst.length);
                break;

            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_POINTER_UP:
                if (event.getPointerId(index) == 0) ptpIdx = 2;
                mCount--;
                break;
        }
    }
}