public class MainActivity extends AppCompatActivity implements OnMapReadyCallback {

    private static final String TAG = MainActivity.class.getSimpleName();
    private static final int DIAMETER_IN_METERS = 300;

    private final LatLng MARKER_LOCATION = new LatLng(37.762555, -122.244221);

    private GoogleMap mGoogleMap;
    private MapFragment mMapFragment;

    private Marker mMarker = null;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mMapFragment = (MapFragment) getFragmentManager()
                .findFragmentById(R.id.map_fragment);
        mMapFragment.getMapAsync(this);
    }

    @Override
    public void onMapReady(GoogleMap googleMap) {
        mGoogleMap = googleMap;

        mGoogleMap.setOnCameraIdleListener(new GoogleMap.OnCameraIdleListener() {
            @Override
            public void onCameraIdle() {

                if (mMarker != null) {
                    mMarker.remove();
                }
                mMarker = createResizedMarker(MARKER_LOCATION);
            }
        });
        mGoogleMap.animateCamera(CameraUpdateFactory.newLatLngZoom(MARKER_LOCATION, 16));
    }

    private Marker createResizedMarker(LatLng marker_location) {

        final Drawable iconDrawable = getResources().getDrawable(R.drawable.ic_letter_m);
        final double meters_per_pixel = (Math.cos(mGoogleMap.getCameraPosition().target.latitude * Math.PI / 180) * 2 * Math.PI * 6378137) / (256 * Math.pow(2, mGoogleMap.getCameraPosition().zoom));
        final int diameter = (int)(DIAMETER_IN_METERS / meters_per_pixel);
        final Bitmap bitmap = Bitmap.createBitmap(diameter, diameter, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);
        iconDrawable.setBounds(0, 0, canvas.getWidth(), canvas.getHeight());
        iconDrawable.draw(canvas);

        Marker marker = mGoogleMap.addMarker(new MarkerOptions()
                .position(MARKER_LOCATION)
                .anchor(0.5f, 0.5f)
                .icon(BitmapDescriptorFactory.fromBitmap(bitmap)));

        return marker;
    }
}

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