java - 获取 BufferedImage 上像素的颜色

Question

我想获取 BufferedImage 上像素的颜色。我将 BufferedImage 的背景设置为白色，并在 BufferedImage 上画一条从 (100, 100) 到 (100, 200) 的线。然后，我将 BufferedImage 绘制到 JPanel 上。有这条线，但背景不是白色的。为什么？

此外，getRGB 方法对 R、G 和 B 返回 0，即使它不是 getRGB(100, 100)。怎么了？

代码:

public class PixelColour extends JPanel{

    public void paintComponent(Graphics g){
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        BufferedImage bi = new BufferedImage(500, 500, BufferedImage.TYPE_INT_ARGB);
        Graphics2D gbi = bi.createGraphics();
        gbi.setColor(Color.black);
        gbi.setBackground(Color.white);
        gbi.drawLine(100, 100, 100, 200);
        g2.drawImage(bi, null, 0, 0);
        int rgb = bi.getRGB(100, 100);
        int red = (rgb >> 16) & 0xFF;
        int green = (rgb >> 8) & 0xFF;
        int blue = (rgb & 0xFF);
        System.out.println(red + " " + green + " " + blue);
    }

    public static void main(String[] args) throws IOException{
        PixelColour pc = new PixelColour();
        JFrame frame = new JFrame("Pixel colour");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(pc);
        frame.setSize(500,500);
        frame.setVisible(true);
    }
}

Answer 1

在gbi.setBackground(Color.white)之后添加gbi.clearRect(0,0,bi.getWidth(), bi.getHeight());

clearRect() 将背景颜色绘制到图像上。如果您只是设置新的背景颜色，则不会更改图像。

public void paintComponent(Graphics g){
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D) g;
    BufferedImage bi = new BufferedImage(500, 500, BufferedImage.TYPE_INT_ARGB);
    Graphics2D gbi = bi.createGraphics();
    gbi.setColor(Color.black);
    gbi.setBackground(Color.white);

    // here
    gbi.clearRect(0, 0, bi.getWidth(), bi.getHeight());

    gbi.drawLine(100, 100, 100, 200);
    g2.drawImage(bi, null, 0, 0);
    int rgb = bi.getRGB(50, 50);    // off the black line
    int red = (rgb >> 16) & 0xFF;
    int green = (rgb >> 8) & 0xFF;
    int blue = (rgb & 0xFF);
    System.out.println(red + " " + green + " " + blue);
}

这会打印

255 255 255
255 255 255