Question

我有以下代码来绘制RadialGradientPaint圆圈：

public class Character {
    public static void draw(Graphics2D g2d){
        g2d.setColor(Color.RED);
        g2d.fillRect(0, 0, 1000, 750);
        drawVisibilityPolygon(g2d);
    }

    private static void drawVisibilityPolygon(Graphics2D g2d){
        Point center = new Point(1000 / 2, 750 / 2);
        float radius = 200;
        float[] dist = {
            0f,
            1f
        };
        Color[] colors = {
            new Color(0, 0, 0, 0),
            new Color(0, 0, 0, 255)
        };
        drawGradientCircle(g2d, radius, dist, colors, center);
    }

    private static void drawGradientCircle(Graphics2D g2d, float radius, float[] dist, Color[] colors, Point2D center){
        RadialGradientPaint rgp = new RadialGradientPaint(center, radius, dist, colors);
        g2d.setPaint(rgp);
        g2d.fill(new Ellipse2D.Double(center.getX() - radius, center.getY() - radius, radius * 2, radius * 2));
    }
}

draw方法由Display类调用：

public class Display extends JPanel {
    @Override
    public void paintComponent(Graphics g){
        // SETUP
        BufferedImage base = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_ARGB);
        Graphics2D g2d = base.createGraphics();
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2d.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
        g2d.setColor(Color.WHITE);
        g2d.fillRect(0, 0, getWidth(), getHeight());

        // DRAW STUFF
        Character.draw(g2d);

        // FINISH UP
        g.drawImage(base, 0, 0, null);
        g2d.dispose();
        g.dispose();
    }
}

显示类包含在JFrame：

中

public class Window extends JFrame {
    private Display display = new Display();

    public Window(String title, int width, int height){
        super(title);
        add(display);
        setSize(width, height);
        setIgnoreRepaint(false);
        setLocationRelativeTo(null);
        setResizable(false);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        setVisible(true);
    }

    public Display getDisplay() {
        return display;
    }
}

这样可以正常工作，但我想用一种颜色填充圆圈外部（只有那部分）。

我尝试过使用AlphaComposite，但我无法弄清楚如何使用它来做到这一点。我可以制作一个更大的圆圈并减少dist1值但绘制时间太长（在我的机器上大约10ms而不是图像中的2ms）。我想把它放进游戏中。

我该怎么做？如果这有明显的解决方案，请提前致谢并对不起，但我无法指责它。

如果您需要更多信息，请询问。

Answer 1

使用非不透明颜色绘画会导致背景显示，因此您必须设置背景，因此您必须设置适当的背景。
看看这个演示：

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RadialGradientPaint;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Point2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.WindowConstants;
import javax.swing.border.LineBorder;
import javax.swing.border.TitledBorder;

public class APanel extends JPanel{

    APanel(){

        setPreferredSize(new Dimension(600,600));
        setBackground(Color.YELLOW);
        setBorder(new TitledBorder(new LineBorder(new Color(169, 169, 169)),
                "Gradient Circle", TitledBorder.LEADING, TitledBorder.BELOW_TOP, null, null));
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        draw((Graphics2D) g);
    }

    public static void draw(Graphics2D g2d){

        g2d.setColor(Color.RED);
        g2d.fillRect(100,100, 400, 400);
        drawVisibilityCircle(g2d);
    }

    private static void drawVisibilityCircle(Graphics2D g2d){
        Point center = new Point(300, 300);
        float radius = 200;
        float[] dist = { 0f, 1f};
        Color[] colors = { new Color(0, 0, 0, 0), new Color(0, 0, 0, 255)};
        //workaround to prevent background color from showing
        drawBackGroundCircle(g2d, radius, Color.WHITE, center);
        drawGradientCircle(g2d, radius, dist, colors, center);
    }

    private static void drawBackGroundCircle(Graphics2D g2d, float radius, Color color, Point2D center){

        g2d.setColor(color);
        radius -= 1;//make radius a bit smaller to prevent fuzzy edge
        g2d.fill(new Ellipse2D.Double(center.getX() - radius, center.getY()
                                                    - radius, radius * 2, radius * 2));
    }

    private static void drawGradientCircle(Graphics2D g2d, float radius, float[] dist, Color[] colors, Point2D center){
        RadialGradientPaint rgp = new RadialGradientPaint(center, radius, dist, colors);
        g2d.setPaint(rgp);
        g2d.fill(new Ellipse2D.Double(center.getX() - radius, center.getY() - radius, radius * 2, radius * 2));
    }

    public static void main(String[] args){

        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
        JPanel panel = new APanel();
        frame.getContentPane().add(panel);
        frame.pack();
        frame.setVisible(true);
    }
}

Java绘制一个RadialGradientPaint圆切成矩形

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