Multi Touch in Android – Translate, Scale, and Rotate

Here’s a quick and easy implementation of Android’s multi touch feature – one finger to move, two to zoom, and three to rotate the image.

Assuming you have a basic understanding of 2D matrix transformations, the Matrix class in Android uses a 3×3 matrix to achieve all of the 2D transformations.

The source code and pre -built APK is available. See the end of this post.

Main activity –

package com.multitouch.example;

import android.os.Bundle;
import android.util.FloatMath;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.widget.ImageView;

public class MultiTouch extends Activity implements OnTouchListener {

    // these matrices will be used to move and zoom image
    private Matrix matrix = new Matrix();
    private Matrix savedMatrix = new Matrix();
    // we can be in one of these 3 states
    private static final int NONE = 0;
    private static final int DRAG = 1;
    private static final int ZOOM = 2;
    private int mode = NONE;
    // remember some things for zooming
    private PointF start = new PointF();
    private PointF mid = new PointF();
    private float oldDist = 1f;
    private float d = 0f;
    private float newRot = 0f;
    private float[] lastEvent = null;

    public void onCreate(Bundle savedInstanceState) {
        ImageView view = (ImageView) findViewById(;

    public boolean onTouch(View v, MotionEvent event) {
        // handle touch events here
        ImageView view = (ImageView) v;
        switch (event.getAction() & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN:
                start.set(event.getX(), event.getY());
                mode = DRAG;
                lastEvent = null;
            case MotionEvent.ACTION_POINTER_DOWN:
                oldDist = spacing(event);
                if (oldDist > 10f) {
                    midPoint(mid, event);
                    mode = ZOOM;
                lastEvent = new float[4];
                lastEvent[0] = event.getX(0);
                lastEvent[1] = event.getX(1);
                lastEvent[2] = event.getY(0);
                lastEvent[3] = event.getY(1);
                d = rotation(event);
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_POINTER_UP:
                mode = NONE;
                lastEvent = null;
            case MotionEvent.ACTION_MOVE:
                if (mode == DRAG) {
                    float dx = event.getX() - start.x;
                    float dy = event.getY() - start.y;
                    matrix.postTranslate(dx, dy);
                } else if (mode == ZOOM) {
                    float newDist = spacing(event);
                    if (newDist > 10f) {
                        float scale = (newDist / oldDist);
                        matrix.postScale(scale, scale, mid.x, mid.y);
                    if (lastEvent != null && event.getPointerCount() == 3) {
                        newRot = rotation(event);
                        float r = newRot - d;
                        float[] values = new float[9];
                        float tx = values[2];
                        float ty = values[5];
                        float sx = values[0];
                        float xc = (view.getWidth() / 2) * sx;
                        float yc = (view.getHeight() / 2) * sx;
                        matrix.postRotate(r, tx + xc, ty + yc);

        return true;

     * 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);

     * Calculate the degree to be rotated by.
     * @param event
     * @return Degrees
    private float rotation(MotionEvent event) {
        double delta_x = (event.getX(0) - event.getX(1));
        double delta_y = (event.getY(0) - event.getY(1));
        double radians = Math.atan2(delta_y, delta_x);
        return (float) Math.toDegrees(radians);

An important function that is always used is postXx(). This function concats a new matrix of the type Xx to the existing matrix object. Using setXx() will reset the matrix’s Xx property.
Main layout – main.xml

<?xml version="1.0" encoding="utf-8"?>


<ImageView android:id="@+id/imageView"
android:scaleType="matrix" />


Browse and download the source code on GitHub.

[HOWTO] Create a hidden loopback device in a .jpg file

Well, I’m sure that you are definitely curious about how do you do this. Let me tell you this, it’s really simple.

  • Image File(any format will do or even any file will do)
  • Linux(I’ve only tested it on this platform but I guess Mac would also do)

Let’s get started, shall we?
First create a standard loopback device.

dd if=/dev/zero of=hiddenimage bs=1M count=10

Let’s see what does this do:
It creates a file called “hiddenimage”, with it’s size as 10MB.
Then, create a filesystem on it.

mkfs.ext4 hiddenimage

mke2fs 1.41.8 (11-July-2009) hiddenimage is not a block special device. Proceed anyway? (y,n) y Filesystem label= OS type: Linux Block size=1024 (log=0) Fragment size=1024 (log=0) 2560 inodes, 10240 blocks 512 blocks (5.00%) reserved for the super user First data block=1 Maximum filesystem blocks=10485760 2 block groups 8192 blocks per group, 8192 fragments per group 1280 inodes per group Superblock backups stored on blocks: 8193 Writing inode tables: done Creating journal (1024 blocks): done Writing superblocks and filesystem accounting information: done This filesystem will be automatically checked every 39 mounts or 180 days, whichever comes first. Use tune2fs -c or -i to override.

Continue reading “[HOWTO] Create a hidden loopback device in a .jpg file”