Android Maps API v2 绘制圆圈 [英] Android Maps API v2 draw circle

问题描述

使用以下代码绘制圆圈(取自 Google Play 服务地图"示例):

Having the following code to draw circle (taken from Google Play Services "maps" sample):

    PolylineOptions options = new PolylineOptions();
    int radius = 5; //What is that?
    int numPoints = 100;
    double phase = 2 * Math.PI / numPoints;
    for (int i = 0; i <= numPoints; i++) {
        options.add(new LatLng(SYDNEY.latitude + radius * Math.sin(i * phase),
                SYDNEY.longitude + radius * Math.cos(i * phase)));
    }
    int color = Color.RED;
    mMap.addPolyline(options
            .color(color)
            .width(2));

这是在世界不同地区绘制的内容:

This is what gets drawn on different part of the world:

正如你所见，圆并不是真正的圆，甚至第二个基本上也是椭圆.

As you see circles are not really circles and even second one is ellipse basically.

我猜圆的抗锯齿"取决于 int numPoints 变量中的点数.

I guess that "anti-aliasing" of circle depending on number of points in int numPoints variable.

1. 示例代码中的 int radius = 5 变量是什么?我的意思是它是什么衡量标准?
2. 主要问题是，以米为单位绘制具有给定半径的漂亮圆的正确方法是什么?类似于我们在 api v1 中使用 canvas.drawCircle()
的东西

1. What is int radius = 5 variable in example code? I mean what measure it is?
2. And main question what would be correct way of drawing nice circle with given radius in meters? Something smiliar to what we had in api v1 with canvas.drawCircle()

更新--------------------

UPDATE --------------------

好的，在提高数学后，我能够画正确"的圆圈:

OK after improving math I was able to draw "right" circle:

private void addCircle(LatLng latLng, double radius)
    {
        double R = 6371d; // earth's mean radius in km
        double d = radius/R; //radius given in km
        double lat1 = Math.toRadians(latLng.latitude);
        double lon1 = Math.toRadians(latLng.longitude);         
        PolylineOptions options = new PolylineOptions();
        for (int x = 0; x <= 360; x++)
        {                      
            double brng = Math.toRadians(x);
            double latitudeRad = Math.asin(Math.sin(lat1)*Math.cos(d) + Math.cos(lat1)*Math.sin(d)*Math.cos(brng));
            double longitudeRad = (lon1 + Math.atan2(Math.sin(brng)*Math.sin(d)*Math.cos(lat1), Math.cos(d)-Math.sin(lat1)*Math.sin(latitudeRad)));             
            options.add(new LatLng(Math.toDegrees(latitudeRad), Math.toDegrees(longitudeRad)));
        }           
        mMap.addPolyline(options.color(Color.BLACK).width(2));          
    }

然而，我认为圆的抗锯齿有点无法控制，并且在某些缩放级别上圆可能会变得丑陋:

However anti-aliasing of circle I guess is somewhat beyond control, and on some zoom levels circle might get ugly:

推荐答案

如何在 Google Maps v2 (bitmap) 中绘制圆圈

How to draw circle in Google Maps v2 (bitmap)

// 1. some variables:

    private static final double EARTH_RADIUS = 6378100.0;
    private int offset;

// 2. convert meters to pixels between 2 points in current zoom:

    private int convertMetersToPixels(double lat, double lng, double radiusInMeters) {

         double lat1 = radiusInMeters / EARTH_RADIUS;
         double lng1 = radiusInMeters / (EARTH_RADIUS * Math.cos((Math.PI * lat / 180)));

         double lat2 = lat + lat1 * 180 / Math.PI;
         double lng2 = lng + lng1 * 180 / Math.PI; 

         Point p1 = YourActivity.getMap().getProjection().toScreenLocation(new LatLng(lat, lng));
         Point p2 = YourActivity.getMap().getProjection().toScreenLocation(new LatLng(lat2, lng2));

         return Math.abs(p1.x - p2.x);
    }

// 3. bitmap creation:

    private Bitmap getBitmap() {

        // fill color
        Paint paint1 = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint1.setColor(0x110000FF);
        paint1.setStyle(Style.FILL);

        // stroke color
        Paint paint2 = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint2.setColor(0xFF0000FF);
        paint2.setStyle(Style.STROKE);

        // icon
        Bitmap icon = BitmapFactory.decodeResource(YourActivity.getResources(), R.drawable.blue);

        // circle radius - 200 meters
        int radius = offset = convertMetersToPixels(lat, lng, 200);

        // if zoom too small
        if (radius < icon.getWidth() / 2) {

            radius = icon.getWidth() / 2;
        }

        // create empty bitmap 
        Bitmap b = Bitmap.createBitmap(radius * 2, radius * 2, Config.ARGB_8888);
        Canvas c = new Canvas(b);

        // draw blue area if area > icon size
        if (radius != icon.getWidth() / 2) {

            c.drawCircle(radius, radius, radius, paint1);
            c.drawCircle(radius, radius, radius, paint2);
        }

        // draw icon
        c.drawBitmap(icon, radius - icon.getWidth() / 2, radius - icon.getHeight() / 2, new Paint());

        return b;
    }

// 4. calculate image offset:

    private LatLng getCoords(double lat, double lng) {

        LatLng latLng = new LatLng(lat, lng);

        Projection proj = YourActivity.getMap().getProjection();
        Point p = proj.toScreenLocation(latLng);
        p.set(p.x, p.y + offset);

        return proj.fromScreenLocation(p);
    }

// 5. draw:

        MarkerOptions options = new MarkerOptions();
            options.position(getCoords(lat, lng));
            options.icon(BitmapDescriptorFactory.fromBitmap(getBitmap()));

            marker = YourActivity.getMap().addMarker(options);

和结果:

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