Using a back buffer

[Nate Lockwood]

I have a class that extends Canvass that I use to display a histogram. The user can "drag" lines that are drawn over the histogram which is repainted when the mouse is moved. I noticed a flicker and wanted to draw the histogram only once in a back buffer and use it as a background for the lines drawn from the mouse move. I attempted to adapt from an example but now all I get is a light grey background. Here's some abbreviated code.

		BufferedImage backImage = null;

		@Override
		public void paint(Graphics g) {
			Graphics g2 = (Graphics)g;
			if (backImage == null) {
				backImage = (BufferedImage) this.createImage(256, 256);
				g2 = backImage.createGraphics();


				// draw the histogram
				g2.setColor(Color.GREEN);
				for (int i = 0; i < histogram.length; ++i)
					g2.drawLine(i, 255, i, 255 - histogram[i]);
			}

			g2.drawImage(backImage, 0, 0, null);
			// draw the lines
			g2.setColor(Color.BLACK);
			g2.drawLine(loX, loY, midX, midY);
			g2.drawLine(midX, midY, hiX, hiY);
		
			g2.dispose();
		}

This is my first shot at using a back buffer in Java - can anyone suggest what's wrong?

[Darryl Burke]

Nate Lockwood wrote:I have a class that extends Canvass

Since that's not a class in the standard JDK, you need to provide more information about it.

Why don't you use Swing (extend JComponent or JPanel), which provides built-in double buffering?

[Rob Spoor]

Darryl Burke wrote:

Nate Lockwood wrote:I have a class that extends Canvass

Since that's not a class in the standard JDK

Not?

[Nate Lockwood]

I figured it out and the flicker is gone as I wished!
The back buffer and the lines (there's more to this than my abbreviated code of course) need to be drawn to the component Graphics object.

        BufferedImage backImage = null;  
  
        @Override  
        public void paint(Graphics g) {  
            Graphics g2 = (Graphics)g;  
            if (backImage == null) {  
                backImage = (BufferedImage) this.createImage(256, 256);  
                g2 = backImage.createGraphics();  
  
  
                // draw the histogram  
                g2.setColor(Color.GREEN);  
                for (int i = 0; i < histogram.length; ++i)  
                    g2.drawLine(i, 255, i, 255 - histogram[i]);  
               g2.dispose();
            }  
  
            // the background is in the back buffer
            g.drawImage(backImage, 0, 0, null);  
            // draw the lines  
            g.setColor(Color.BLACK);  
            g.drawLine(loX, loY, midX, midY);  
            g.drawLine(midX, midY, hiX, hiY);  
          
            g.dispose();  
        }  

I had experimented with both an extended JPanel and an extended Canvas before I decided on a Canvas but can't remember exactly why - but Canvas does what I want.
BTW I did all of this in the '90s in Borland C working in Windows 98 and later Windows 95. I got it working but never did understand the graphics well. Java is better IMHO as I can understand what I'm doing.

[Darryl Burke]

Rob Spoor wrote:

Darryl Burke wrote:

Nate Lockwood wrote:I have a class that extends Canvass

Since that's not a class in the standard JDK

Not?

That's not Canvass

[Nate Lockwood]

That's not Canvass

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