为什么Graphics的draw方法不尊重笔画属性？

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 -

正如你所看到的,Graphics.drawRect完全忽略BasicStroke.CAP_ROUND和BasicStroke.JOIN_ROUND属性.为什么？

Answer 1

正如Learning Java 2D,第1部分所述:

java.awt.BasicStroke.CAP_ROUND:这使得圆形帽以端点为中心,笔直径为直径.

关键词是"居中".我相信,总是这样的情况,当用粗笔划绘制时,Java2D将沿着起始坐标和结束坐标处的像素中心之间的假想的,无穷小的细线中心线的粗细.例如,当绘制7像素厚的垂直蓝线时,Java2D在正在绘制的假想线段的每一侧绘制3个像素.

在您的示例中,厚度为5像素.您需要偏移坐标以在图形剪辑中完全绘制笔划.通过移动2个像素(或THICKNESS/2),圆角变为可见:

//...
        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:你有效地切断了中间的边框,所以角落似乎没有圆角.考虑到这一点(这里仅针对偏移量,也需要调整宽度)

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

编辑

固定的草率像素计数:-)