Get new position of coordinate after rotation with Matrix

Question

I'm wondering how to use a Matrix to get the new position of a coordinate within a rectangle after rotation. What I would like to do is:

1. Define a rectangle
2. Define a coordinate within that rectangle
3. Rotate the rectangle
4. Get the new position of the coordinate after the rotation

The parts I can't figure out are 2 & 4. Any ideas?

Answer 1

I have created a simple Demo for this. It has a little extra, so you can also see how to use this in Drawing.

main.xml

<?xml version="1.0" encoding="utf-8"?> <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"     android:id="@+id/container"     android:layout_width="fill_parent"     android:layout_height="fill_parent">     <SeekBar         android:id="@+id/seekBar1"         android:layout_width="fill_parent"         android:layout_height="wrap_content"         android:layout_alignParentBottom="true"         android:layout_centerHorizontal="true" /> </RelativeLayout> 

And the Activity:

package nl.entreco.android.testrotation;  import android.app.Activity; import android.content.Context; import android.graphics.Canvas; import android.graphics.Color; import android.graphics.Matrix; import android.graphics.Paint; import android.graphics.Point; import android.graphics.Rect; import android.os.Bundle; import android.util.Log; import android.view.View; import android.widget.RelativeLayout; import android.widget.SeekBar; import android.widget.SeekBar.OnSeekBarChangeListener;  public class RotationActivity extends Activity implements OnSeekBarChangeListener {       private MyDrawing myDrawing;     private SeekBar mSeekbar;      /** Called when the activity is first created. */     @Override     public void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.main);          Rect rect = new Rect(150,150,440,630);          int x = (int) (rect.left + Math.random() * rect.width());         int y = (int) (rect.top + Math.random() * rect.height());         Point coordinate = new Point(x, y);           // To draw the rect we create a CustomView         myDrawing = new MyDrawing(this, rect, coordinate);          RelativeLayout rl = (RelativeLayout)findViewById(R.id.container);         rl.addView(myDrawing);           mSeekbar = (SeekBar)findViewById(R.id.seekBar1);         mSeekbar.setMax(360);         mSeekbar.setOnSeekBarChangeListener(this);     }      private class MyDrawing extends View     {         private Rect myRect;         private Point myPoint;         private Paint rectPaint;         private Paint pointPaint;          private Matrix transform;          public MyDrawing(Context context, Rect rect, Point point)         {             super(context);              // Store the Rect and Point             myRect = rect;             myPoint = point;              // Create Paint so we can see something :)             rectPaint = new Paint();             rectPaint.setColor(Color.GREEN);             pointPaint = new Paint();             pointPaint.setColor(Color.YELLOW);              // Create a matrix to do rotation             transform = new Matrix();          }           /**         * Add the Rotation to our Transform matrix.         *          * A new point, with the rotated coordinates will be returned         * @param degrees         * @return         */         public Point rotate(float degrees)         {             // This is to rotate about the Rectangles center             transform.setRotate(degrees, myRect.exactCenterX(),     myRect.exactCenterY());              // Create new float[] to hold the rotated coordinates             float[] pts = new float[2];              // Initialize the array with our Coordinate             pts[0] = myPoint.x;             pts[1] = myPoint.y;              // Use the Matrix to map the points             transform.mapPoints(pts);              // NOTE: pts will be changed by transform.mapPoints call             // after the call, pts will hold the new cooridnates              // Now, create a new Point from our new coordinates             Point newPoint = new Point((int)pts[0], (int)pts[1]);              // Return the new point             return newPoint;         }          @Override         public void onDraw(Canvas canvas)         {             if(myRect != null && myPoint != null)             {                 // This is an easy way to apply the same transformation (e.g. rotation)                 // To the complete canvas.                 canvas.setMatrix(transform);                  // With the Canvas being rotated, we can simply draw                 // All our elements (Rect and Point)                  canvas.drawRect(myRect, rectPaint);                 canvas.drawCircle(myPoint.x, myPoint.y, 5, pointPaint);             }         }     }      @Override     public void onProgressChanged(SeekBar seekBar, int progress,boolean fromUser) {          Point newCoordinates = myDrawing.rotate(progress);           // Now -> our float[] pts contains the new x,y coordinates         Log.d("test", "Before Rotate myPoint("+newCoordinates.x+","+newCoordinates.y+")");         myDrawing.invalidate();      }      @Override     public void onStartTrackingTouch(SeekBar seekBar) {}      @Override     public void onStopTrackingTouch(SeekBar seekBar) {} } 

Answer 2

Use Matrix.mapPoints to transform 2D points by matrix.