public override void draw(Graphics graphics, GISView view) { Point screenpoint = view.ToScreenPoint(centroid); graphics.FillEllipse(new SolidBrush(Color.Red), new Rectangle(screenpoint.X - 3, screenpoint.Y - 3, 6, 6)); }

这是一个 C# 语言中的方法，它覆盖了父类的 draw 方法。该方法接受两个参数：一个 Graphics 对象和一个 GISView 对象。在方法中，首先通过 GISView 对象的 ToScreenPoint 方法将当前对象的质心坐标转为屏幕坐标，然后使用 Graphics 对象在屏幕上绘制一个填充颜色为红色的椭圆形，其坐标为以质心为中心，边长为 6 的正方形。该方法的作用是在屏幕上绘制当前对象的符号化表示。

相关问题

package demo; import java.awt.Color; import java.awt.Dimension; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.Point; import java.awt.geom.Path2D; import javax.swing.JFrame; import javax.swing.JPanel; public class PointTest extends JFrame{ int[] xPoints = { 71, 163, 163, 256}; int[] yPoints = { 89, 89, 254, 254}; // 创建多段线路径 Path2D path = new Path2D.Double(); Point point2 = new Point(117, 131); public PointTest() { path.moveTo(xPoints[0], yPoints[0]); for (int i = 1; i < xPoints.length; i++) { path.lineTo(xPoints[i], yPoints[i]); } System.out.println(path.contains(point2)); } public static void main(String[] args) { PointTest frame = new PointTest(); Canvas panel = new Canvas(); panel.path2d = frame.path; panel.point = frame.point; panel.point2 = frame.point2; frame.setContentPane(panel); frame.pack(); frame.setDefaultCloseOperation(3); frame.setLocationRelativeTo(null); frame.setVisible(true); } } class Canvas extends JPanel { Path2D path2d; Point point, point2; @Override public void paintComponent(Graphics g) { Graphics2D g2d = (Graphics2D) g; g2d.setColor(Color.BLUE); g2d.draw(path2d); g2d.setColor(Color.RED); g2d.fillOval(point.x, point.y, 4, 4); g2d.fillOval(point2.x, point2.y, 4, 4); } @Override public Dimension getPreferredSize() { return new Dimension(300, 200); } } 如何让point2只在线path上时候才为true

要确定`point2`只在线`path`上时才为`true`，可以使用`Path2D`的`contains()`方法来判断。但是在当前的代码中，`path`只是一条简单的多段线路径，并不包含闭合的区域。因此，我们可以使用`Path2D`的`contains()`方法来判断点是否在路径上。

以下是修改后的代码，其中添加了判断点是否在线上的逻辑：

package demo;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.geom.Path2D;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class PointTest extends JFrame{
    int[] xPoints = { 71, 163, 163, 256};
    int[] yPoints = { 89, 89, 254, 254};

    // 创建多段线路径
    Path2D path = new Path2D.Double();
    Point point2 = new Point(117, 131);

    public PointTest() {
        path.moveTo(xPoints[0], yPoints[0]);
        for (int i = 1; i < xPoints.length; i++) {
            path.lineTo(xPoints[i], yPoints[i]);
        }
        path.closePath();

        System.out.println(isPointOnPath(point2));
    }

    public boolean isPointOnPath(Point point) {
        return path.contains(point);
    }

    public static void main(String[] args) {
        PointTest frame = new PointTest();
        Canvas panel = new Canvas();
        panel.path2d = frame.path;
        panel.point = frame.point;
        panel.point2 = frame.point2;
        frame.setContentPane(panel);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}

class Canvas extends JPanel {
    Path2D path2d;
    Point point, point2;

    @Override
    public void paintComponent(Graphics g) {
        Graphics2D g2d = (Graphics2D) g;
        g2d.setColor(Color.BLUE);
        g2d.draw(path2d);
        g2d.setColor(Color.RED);
        g2d.fillOval(point.x, point.y, 4, 4);
        g2d.fillOval(point2.x, point2.y, 4, 4);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(300, 200);
    }
}

在`PointTest`类中添加了一个`isPointOnPath()`方法，用于判断点是否在线上。在`PointTest`构造函数中，调用`isPointOnPath()`方法判断`point2`是否在线上，并输出结果。

这样修改后，当`point2`在线上时，输出为`true`，不在线上时，输出为`false`。

package work; import java.applet.Applet; import java.awt.Color; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.geom.Line2D; import java.awt.geom.Point2D; public class CyrusBeckAlgorithmApplet extends Applet { private static final long serialVersionUID = 1L; private Point2D.Double[] clipWindow; private Point2D.Double[][] lines; private double[][] vectors; private double[] p1, p2, D; @Override public void init() { clipWindow = new Point2D.Double[3]; clipWindow[0] = new Point2D.Double(200, 275); clipWindow[1] = new Point2D.Double(250.0 / 3, 100); clipWindow[2] = new Point2D.Double(950.0 / 3, 100); lines = new Point2D.Double[2][2]; lines[0][0] = new Point2D.Double(0, 120); lines[0][1] = new Point2D.Double(400, 120); lines[1][0] = new Point2D.Double(0, 180); lines[1][1] = new Point2D.Double(400, 180); vectors = new double[2][2]; D = new double[2]; } @Override public void paint(Graphics g) { super.paint(g); Graphics2D g2d = (Graphics2D) g; // draw clip window g2d.setColor(Color.BLACK); g2d.draw(new Line2D.Double(clipWindow[0], clipWindow[1])); g2d.draw(new Line2D.Double(clipWindow[1], clipWindow[2])); g2d.draw(new Line2D.Double(clipWindow[2], clipWindow[0])); // draw lines for (int i = 0; i < lines.length; i++) { Point2D.Double p1 = lines[i][0]; Point2D.Double p2 = lines[i][1]; cyrusBeckClip(g2d, p1, p2); } } private void cyrusBeckClip(Graphics2D g2d, Point2D.Double p1, Point2D.Double p2) { double tE = 0, tL = 1; double dx = p2.x - p1.x; double dy = p2.y - p1.y; for (int i = 0; i < clipWindow.length; i++) { Point2D.Double P1 = clipWindow[i]; Point2D.Double P2 = clipWindow[(i + 1) % clipWindow.length]; double nx = -(P2.y - P1.y); double ny = P2.x - P1.x; double D = -nx * P1.x - ny * P1.y; double numerator = nx * p1.x + ny * p1.y + D; double denominator = -(nx * dx + ny * dy); if (denominator == 0) { if (numerator < 0) { return; } } else { double t = numerator / denominator; if (denominator < 0) { tE = Math.max(tE, t); } else { tL = Math.min(tL, t); } } } if (tE <= tL) { double x1 = p1.x + tE * dx; double y1 = p1.y + tE * dy; double x2 = p1.x + tL * dx; double y2 = p1.y + tL * dy; g2d.setColor(Color.BLUE); g2d.draw(new Line2D.Double(p1, new Point2D.Double(x1, y1))); g2d.setColor(Color.RED); g2d.draw(new Line2D.Double(new Point2D.Double(x1, y1), new Point2D.Double(x2, y2))); g2d.setColor(Color.BLUE); g2d.draw(new Line2D.Double(new Point2D.Double(x2, y2), p2)); } } } 将此代码改为 Java 应用程序运行

将代码中的 `extends Applet` 和 `public void init()` 方法删除，并将 `public void paint(Graphics g)` 方法改为 `public static void main(String[] args)` 方法，代码如下：

package work;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Line2D;
import java.awt.geom.Point2D;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class CyrusBeckAlgorithmApp {

    private static final Point2D.Double[] clipWindow = { new Point2D.Double(200, 275),
            new Point2D.Double(250.0 / 3, 100), new Point2D.Double(950.0 / 3, 100) };

    private static final Point2D.Double[][] lines = { { new Point2D.Double(0, 120), new Point2D.Double(400, 120) },
            { new Point2D.Double(0, 180), new Point2D.Double(400, 180) } };

    public static void main(String[] args) {
        JFrame frame = new JFrame("Cyrus Beck Algorithm");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel panel = new JPanel() {
            private static final long serialVersionUID = 1L;

            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);
                Graphics2D g2d = (Graphics2D) g;
                // draw clip window
                g2d.setColor(Color.BLACK);
                g2d.draw(new Line2D.Double(clipWindow[0], clipWindow[1]));
                g2d.draw(new Line2D.Double(clipWindow[1], clipWindow[2]));
                g2d.draw(new Line2D.Double(clipWindow[2], clipWindow[0]));
                // draw lines
                for (int i = 0; i < lines.length; i++) {
                    Point2D.Double p1 = lines[i][0];
                    Point2D.Double p2 = lines[i][1];
                    cyrusBeckClip(g2d, p1, p2);
                }
            }
        };
        frame.add(panel);
        frame.setSize(400, 300);
        frame.setVisible(true);
    }

    private static void cyrusBeckClip(Graphics2D g2d, Point2D.Double p1, Point2D.Double p2) {
        double tE = 0, tL = 1;
        double dx = p2.x - p1.x;
        double dy = p2.y - p1.y;
        for (int i = 0; i < clipWindow.length; i++) {
            Point2D.Double P1 = clipWindow[i];
            Point2D.Double P2 = clipWindow[(i + 1) % clipWindow.length];
            double nx = -(P2.y - P1.y);
            double ny = P2.x - P1.x;
            double D = -nx * P1.x - ny * P1.y;
            double numerator = nx * p1.x + ny * p1.y + D;
            double denominator = -(nx * dx + ny * dy);
            if (denominator == 0) {
                if (numerator < 0) {
                    return;
                }
            } else {
                double t = numerator / denominator;
                if (denominator < 0) {
                    tE = Math.max(tE, t);
                } else {
                    tL = Math.min(tL, t);
                }
            }
        }
        if (tE <= tL) {
            double x1 = p1.x + tE * dx;
            double y1 = p1.y + tE * dy;
            double x2 = p1.x + tL * dx;
            double y2 = p1.y + tL * dy;
            g2d.setColor(Color.BLUE);
            g2d.draw(new Line2D.Double(p1, new Point2D.Double(x1, y1)));
            g2d.setColor(Color.RED);
            g2d.draw(new Line2D.Double(new Point2D.Double(x1, y1), new Point2D.Double(x2, y2)));
            g2d.setColor(Color.BLUE);
            g2d.draw(new Line2D.Double(new Point2D.Double(x2, y2), p2));
        }
    }

}