如何将圆圈放在线的中心？

Question

我有一条对角线，还有一个距离为 100 米的圆。问题是圆圈并不是真正位于线的中心。我知道这很简单，但我只是对如何做感到困惑。有人可以帮助我如何将圆圈放在线的中心吗？

这是我到目前为止所尝试过的：

public void paint(Graphics g)
{
    super.paint(g);

    Graphics2D g2d = (Graphics2D) g;
    g2d.setBackground(Color.white);

    int x0_pixel = 0;
    int y0_pixel = 0;

    int x1_pixel = getWidth();
    int y1_pixel = getHeight();

    int x0_world = 0;
    int y0_world = 0;
    double x1_world = 2000; // meters
    double y1_world = 1125; // meters

    double x_ratio = (double) x1_pixel / x1_world;
    double y_ratio = (double) y1_pixel / y1_world;

    int xFrom = 0;
    int yFrom = 0;

    double xTo = x1_world;
    double yTo = y1_world;

    int FromX_pixel = convertToPixelX(xFrom, x_ratio);
    int FromY_pixel = convertToPixelY(y1_pixel, yFrom, y_ratio);

    int ToX_pixel = convertToPixelX((int) xTo, x_ratio);
    int ToY_pixel = convertToPixelY(y1_pixel, (int) yTo, y_ratio);

    g2d.setColor(Color.RED);
    g2d.drawLine(FromX_pixel, FromY_pixel, ToX_pixel, ToY_pixel);

    double theta = Math.atan(yTo / xTo);

    int len = (int) Math.sqrt(xTo * xTo + yTo * yTo);

    int interval = 100;

    final double cosTheta = Math.cos(theta);
    final double sinTheta = Math.sin(theta);

    for (int distance = xFrom; distance <= len; distance += interval)
    {

        double distance_x = distance * cosTheta;
        double distance_y = distance * sinTheta;

        int x_circle_pixel = convertToPixelX(distance_x, x_ratio);
        int y_circle_pixel = convertToPixelY(y1_pixel, distance_y, y_ratio);

        g2d.drawOval(x_circle_pixel, y_circle_pixel, 50, 50);

        g2d.setColor(Color.BLUE);

    }

    Toolkit.getDefaultToolkit().

            sync();

    g2d.dispose();

}

private static int convertToPixelY(int y_offset, double y_world, double y_ratio)
{
    return (int) (y_offset - (y_world * y_ratio));
}

private static int convertToPixelX(double x_world, double x_ratio)
{
    return (int) (x_world * x_ratio);
}

Answer 1

当您绘制椭圆形时，前两个参数是包含椭圆形的矩形的左上角。接下来的两个参数是同一边界矩形的宽度和高度。您当前的代码将左上角放置在线条本身上，但您实际想要的是边界矩形的中心放置在线条上。解决问题的方法是简单地将左上角移动直径的 1/2。你的代码应该是这样的：

public class GraphicsFoo extends JPanel {
   // avoid using magic numbers:
   private static final int CIRCLE_DIAMETER = 50; 

   //....

   // override a JComponent's paintComponent method, not its paint method
   @Override
   protected void paintComponent(Graphics g) {
       super.paintComponent(g);

       Graphics2D g2d = (Graphics2D) g;
       g2d.setBackground(Color.white);

       // make your graphics smooth
       g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, 
             RenderingHints.VALUE_ANTIALIAS_ON);

       // ...

       final double cosTheta = Math.cos(theta);
       final double sinTheta = Math.sin(theta);

       for (int distance = xFrom; distance <= len; distance += interval)
       {

           //....

           // *** here's the key: ***    
           g2d.drawOval(
              x_circle_pixel - CIRCLE_DIAMETER / 2, 
              y_circle_pixel - CIRCLE_DIAMETER / 2, 
              CIRCLE_DIAMETER, CIRCLE_DIAMETER);

           g2d.setColor(Color.BLUE);

       }