Android: Find absolute click position on the ImageView after zooming (using pinch to zoom using Matrix Layout)

Question

I am using code found in Hello Android, 3rd Edition for Pinch to Zoom functionality. After using this the pinch to zoom works fine, but after zooming i want to get the absolute click position on the image view.

Here is my code

package org.example.touch;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.Matrix;
import android.graphics.PointF;
import android.os.Bundle;
import android.util.FloatMath;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.widget.GridView;
import android.widget.ImageView;

public class Touch extends Activity implements OnTouchListener {
private static final String TAG = "Touch";

private static final float MIN_ZOOM = 1.0f;
private static final float MAX_ZOOM = 5.0f;

// These matrices will be used to move and zoom image
Matrix matrix = new Matrix();
Matrix savedMatrix = new Matrix();

// We can be in one of these 3 states
static final int NONE = 0;
static final int DRAG = 1;
static final int ZOOM = 2;
int mode = NONE;

// Remember some things for zooming
PointF start = new PointF();
PointF mid = new PointF();
float oldDist = 1f;

ImageView image2;
image2 = (ImageView)this.findViewById(R.id.imageView2);
image2.setOnTouchListener(new OnTouchListener() {

                public boolean onTouch(View v, MotionEvent event) {

                     ImageView view = (ImageView) image2;
                      // Handle touch events here...
                        switch (event.getAction() & MotionEvent.ACTION_MASK) {
                        case MotionEvent.ACTION_DOWN:
                            savedMatrix.set(matrix);
                            start.set(event.getX(), event.getY());
                            mode = DRAG;
                            break;
                        case MotionEvent.ACTION_POINTER_DOWN:
                            oldDist = spacing(event);
                            if (oldDist > 10f) {
                                savedMatrix.set(matrix);
                                midPoint(mid, event);
                                mode = ZOOM;
                            }
                            break;
                        case MotionEvent.ACTION_UP:
                        case MotionEvent.ACTION_POINTER_UP:
                            mode = NONE;
                            break;
                        case MotionEvent.ACTION_MOVE:
                            if (mode == DRAG) {
                                matrix.set(savedMatrix);
                                matrix.postTranslate(event.getX() - start.x, event.getY() - start.y);
                            }
                            else if (mode == ZOOM) {
                                float newDist = spacing(event);
                                if (newDist > 10f) {
                                    matrix.set(savedMatrix);
                                    float scale = newDist / oldDist;
                                    matrix.postScale(scale, scale, mid.x, mid.y);
                                }
                            }
                            break;
                        }
                        view.setImageMatrix(matrix);
}

        /** Determine the space between the first two fingers */
        private float spacing(MotionEvent event) {
            float x = event.getX(0) - event.getX(1);
            float y = event.getY(0) - event.getY(1);
            return FloatMath.sqrt(x * x + y * y);
        }

        /** Calculate the mid point of the first two fingers */
        private void midPoint(PointF point, MotionEvent event) {
            float x = event.getX(0) + event.getX(1);
            float y = event.getY(0) + event.getY(1);
            point.set(x / 2, y / 2);
        }

Answer 1

You can use the matrix that is currently applied to the imageview to map points to/from the original default points.

//points from a touch/click event on the zoomed/translated imageview
float X;
float Y;

float initialPoints = new float[]{X, Y};

//invert the matrix and map points back
    if(transformMatrix.invert(inverseTransformMatrix))
            inverseTransformMatrix.mapPoints(initialFocalPoints);

initialPoints now contains the original points, i.e where the click point would be if no matrix was applied to the imageview

EDIT: should have added this in there somewhere so you know what they are.

Matrix inverseTransformMatrix = new Matrix();
Matrix transformMatrix = new Matrix();

Answer 2

Apply the below logic

float[] values = new float[9];
matrix.getValues(values);

calculate scalefactor before pinchzoom;

scaleX=values[0];
scaleY=vales[4];

Now calculate the relative touch coordinates according to iamge after pinch zoom

float relativeX = ((event.getX() - values[2])*sacleX) / values[0];
float relativeY = ((event.getY() - values[5])*scaleY) / values[4];