为什么图形不显示？绘制方法尊重笔划属性吗？

Question

我想创建一个带圆角的自定义边框。

代码 -

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Component;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Insets;
import java.awt.RenderingHints;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.border.AbstractBorder;

class JRoundedCornerBorder extends AbstractBorder 
{   
    private static final long serialVersionUID = 7644739936531926341L;
    private static final int THICKNESS = 5;

    JRoundedCornerBorder()
    {
        super();
    }

    @Override
    public void paintBorder(Component c, Graphics g, int x, int y, int width, int height) 
    {
        Graphics2D g2 = (Graphics2D)g.create();

        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        if(c.hasFocus())
        {
            g2.setColor(Color.BLUE);
        }
        else
        {
            g2.setColor(Color.BLACK);
        }
        g2.setStroke(new BasicStroke(THICKNESS, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND));
        g2.drawRect(x, y, width - 1, height - 1);

        g2.dispose();
    }

    @Override
    public Insets getBorderInsets(Component c) 
    {
        return new Insets(THICKNESS, THICKNESS, THICKNESS, THICKNESS);
    }

    @Override
    public Insets getBorderInsets(Component c, Insets insets) 
    {
        insets.left = insets.top = insets.right = insets.bottom = THICKNESS;
        return insets;
    }

    public static void main(String[] args) 
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            @Override
            public void run() 
            {
                final JFrame frame = new JFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new FlowLayout());

                // Add button with custom border
                final JButton button = new JButton("Hello");
                button.setBorder(new JRoundedCornerBorder());
                frame.add(button);

                // Add button without custom border
                frame.add(new JButton("Goodbye"));

                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
}

结果 -

如您所见、Graphics.drawRect 完全忽略BasicStroke.CAP_ROUND 和BasicStroke.JOIN_ROUND 属性。为什么？

Answer 1

正如学习 Java 2D，第 1 部分中所述：

java.awt.BasicStroke.CAP_ROUND：这将创建一个以端点为中心的圆形帽，其直径为笔宽度。

关键词是“居中”。我相信，当使用粗笔画进行绘制时，Java2D 总是会沿着起始坐标和结束坐标的像素中心之间的假设的无限细线将线条的粗细居中。例如，当绘制一条 7 像素粗的垂直蓝线时，Java2D 在所绘制的假设线段的每一侧绘制 3 个像素。

在您的示例中，厚度为 5 像素。您需要偏移坐标才能将笔划完全绘制在图形剪辑内。通过移动 2 个像素（或 THICKNESS/2），圆角变得可见：

//...
        g2.setStroke(new BasicStroke(THICKNESS, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND));
        g2.drawRect(x + THICKNESS/2, y + THICKNESS/2, width - 2*(THICKNESS/2) - 1, height - 2*(THICKNESS/2) - 1);

        g2.dispose();
    }

    @Override
    public Insets getBorderInsets(Component c) {
        return new Insets(THICKNESS + THICKNESS/2, THICKNESS + THICKNESS/2, THICKNESS + THICKNESS/2, THICKNESS + THICKNESS/2);
    }
//...

Answer 2

问题是偏移：您实际上切断了中间的边框，因此角落看起来不是圆角的。考虑到这一点（这里仅用于偏移，还需要调整宽度）

   g2.drawRect(x + thickness/2, y + thickness/2, 
       width - 1 - thickness, height - 1 - thickness);

编辑

修复了马虎的像素计数:-)