Java BufferedImage 保存不需要的背景颜色

Question

提前感谢您的帮助

说明： 该程序绘制、显示和保存图像。它的工作原理如下：对象本身扩展了 Frame。在构造函数中，对象创建一个 BufferedImage，并调用一个在该图像上绘制的方法。然后，它将图像显示到框架上。最后，它将图像保存到一个文件中（我不在乎它使用什么格式）。主程序创建对象，其余的由它完成。

问题： 保存的文件总是有彩色背景！这特别奇怪，因为显示的图像很好。如果我在 ImageIO.write() 中使用“jpg”格式，则背景为红色。如果我使用“png”格式，则背景为深灰色。

我在这上面花了一段时间，但我仍然不知道到底发生了什么！

    import java.awt.Frame ;
    import java.awt.image.BufferedImage ;
    import java.io.IOException ;
    import java.awt.event.WindowEvent ;
    import java.awt.event.WindowAdapter ;
    import java.awt.Toolkit ;
    import java.awt.Graphics2D ;
    import java.awt.Graphics ;
    import java.awt.Color ;
    import java.awt.Dimension ;
    import javax.imageio.ImageIO ;
    import java.io.File ;
    import java.awt.geom.Rectangle2D;

    public class HGrapher extends Frame{
       private BufferedImage img ;
       private float colors[][] ; //the colors for every rectangle
       private double availWidth ;
       private double availHeight ;


       public HGrapher(String saveFileName, int numRects) throws IOException {
          //*add window closer
          addWindowListener(new WindowAdapter() {
             public void windowClosing(WindowEvent e) {
                System.exit(0);            }
          });

          //*get colors to use
          setColors( numRects) ;

          //*figure out the size of the image and frame
          this.availHeight = (3.0/4) * Toolkit.getDefaultToolkit().getScreenSize().height ;
          this.availWidth = (3.0/4) * Toolkit.getDefaultToolkit().getScreenSize().width ;

          //*create the image
          this.img = new BufferedImage( (int)availWidth, (int)availHeight, BufferedImage.TYPE_INT_ARGB);
          Graphics2D drawer = img.createGraphics() ;
          drawer.setBackground(Color.WHITE);

          this.makeImg( drawer) ;
          //*display the image
          this.setSize( new Dimension( (int)availWidth, (int)availHeight ) ) ;
          this.setVisible(true);

          //*save the image
          ImageIO.write(img, "jpg",new File( (saveFileName +".jpg") ) );
       }


       //*draws the image by filling rectangles whose color are specified by this.colors
       public void makeImg( Graphics2D drawer) {
          double rectWidth = this.availWidth / (double)colors.length ;
          for(int i = 0 ; i < colors.length ; i ++) {
             drawer.setColor( new Color( this.colors[i][0], this.colors[i][1], this.colors[i][2],
                                         this.colors[i][3] ) ) ;
             drawer.fill( new Rectangle2D.Double( rectWidth*i, 0, rectWidth, this.availHeight ) ) ;
          }
       }


       //*paint method
       public void paint(Graphics g) {
          Graphics2D drawer = (Graphics2D)g ;
          drawer.drawImage(img, 0, 0, null) ;
       }


       //*creates an array of the colors rectangles are filled with
       public void setColors( int numRects) {
          this.colors = new float[ numRects][4] ;
          //*make every 1st rect red
          for(int i = 0 ; i< colors.length ; i+= 3) {
             this.colors[i][0] = (float).8 ; this.colors[i][1] = (float).1 ; this.colors[i][2] = (float).1 ;
             this.colors[i][3] = (float).8 ;      }
          //*make every 2nd rect green
          for(int i = 1 ; i< colors.length ; i+= 3) {
             this.colors[i][0] = (float).1 ; this.colors[i][1] = (float).8 ; this.colors[i][2] = (float).1 ;
             this.colors[i][3] = (float).8 ;      }
          //*make every 3rd rect
          for(int i = 2 ; i< colors.length ; i+= 3) {
             this.colors[i][0] = (float).1 ; this.colors[i][1] = (float).1 ; this.colors[i][2] = (float).8 ;
             this.colors[i][3] = (float).8 ;      }
       }




       public static void main (String[]args) throws IOException {
          HGrapher hg = new HGrapher("saved", 14) ;
       }

    }

Answer 1

setBackground()只设置了用于清除图像的颜色，并没有真正清除图像。在setBackground() 之后致电Graphics.clearRect(int,int,int,int)。像这样：

//*create the image
this.img = new BufferedImage( (int)availWidth, (int)availHeight, BufferedImage.TYPE_INT_ARGB);
Graphics2D drawer = img.createGraphics() ;
drawer.setBackground(Color.WHITE);
drawer.clearRect(0,0,(int)availWidth,(int)availHeight);