Question

Hi I am creating an application that will draw lines over the top of a MapView using Overlays. For the moment I have set GPS points that I want to use the projection.toPixel() method on. The method does not throw up any errors but the points do not stay fixed on the map when the map is moved as they should. Is there something I am missing?

//Zoom into certain area
public boolean onKeyDown(int keyCode, KeyEvent event)
{ 
    MapController mc = mapView.getController();

    switch(keyCode)
    {
        case KeyEvent.KEYCODE_1:
            mc.zoomIn();
            break;

        case KeyEvent.KEYCODE_2:
            mc.zoomOut();
            break;

        case KeyEvent.KEYCODE_4:
            //Create new path
            path = new Path();

            //Get Location Data from GPS

            p = new GeoPoint(
                    (int) (54.9886454 * 1E6),
                    (int) (-7.522208 * 1E6));

            //Convert GPS location to Screen Pixels
            Point screenPts1 = new Point();

            mapView.getProjection().toPixels(p,  screenPts1);

            //Start Path and pass locations
            path.moveTo(screenPts1.x, screenPts1.y);
            path.lineTo(screenPts1.x, screenPts1.y);

            System.out.println("New Path ");
            break;  

        case KeyEvent.KEYCODE_5:                
            //Get Location Data from GPS

            p = new GeoPoint(
                    (int) (54.9984931 * 1E6),
                    (int) (-7.522208 * 1E6));

            //Convert GPS location to Screen Pixels
            Point screenPts2 = new Point();
            mapView.getProjection().toPixels(p,  screenPts2);

            //Pass locations to the path
            path.lineTo(screenPts2.x, screenPts2.y);

            System.out.println("Continue Path ");
            break;  

        case KeyEvent.KEYCODE_6:                
            //Get Location Data from GPS

            p = new GeoPoint(
                    (int) (54.9994777 * 1E6),
                    (int) (-7.5005787 * 1E6));

            //Convert GPS location to Screen Pixels
            Point screenPts3 = new Point();
            mapView.getProjection().toPixels(p, screenPts3);

            //Pass locations to the path
            path.lineTo(screenPts3.x, screenPts3.y);

            //Close the path and add it to the _graphics array for it to be drawn
            _graphics.add(path);

            System.out.println("End Path ");

            break;  
    }
    return super.onKeyDown(keyCode, event);
}

Overlay Class draw Method:

public boolean draw(Canvas canvas, MapView mapView, boolean shadow,long when)
    {       
        super.draw(canvas,mapView,shadow);

        //-- Create new paint object --
        Paint mPaint = new Paint();
        mPaint.setDither(true);
        mPaint.setColor(Color.RED);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(2);

        //-- Take all the paths from the _graphics array and draw them to the Screen --
        for (Path path : _graphics) 
        {
            canvas.drawPath(path, mPaint);  
        }

        return true;
    }

Answer 1

From documentation of MapView.getProjection() method: “The Projection of the map in its current state. You should not hold on to this object for more than one draw, since the projection of the map could change.”

So when you move the map, you have to recalculate the pixel locations, because the projection with which you calculated the previous pixel locations is no longer valid and thus the pixel locations are not valid anymore either.