When to create a manual back buffer when using JPanel?

Question

I've read that JPanel has a double buffering system, but I've also seen people manually create their own back buffer using BufferedImage, which allows the user to add to the image before actually rendering.

How does the JPanel double buffering work, and why would someone create their own back buffer if JPanel already has double buffering? What is an example of a time where BufferedImage as your buffer would be needed/recommended?

Answer 1

From what I've seen it's usually because:

• They are already loading an image which serves as a background image and wish to draw on top of that (they probably could just draw the image, leaving it to be hardware accelerated for future renders while drawing everything else using the same Graphics object used to call drawImage)
• Because they wish to have a simple way to scale their graphics as their panel resizes with drawImage(Image, 0, 0, panelWidth, panelHeight, ImageObserver) and don't wish to do the extra work on calculating dynamic drawing coordinates for the graphics based on the panel's current dimensions (although this leads to not so good looking graphics, and if the scaled image is not cached per resize there could be performance issues as well).
• Because it's old code in the days before Swing was double buffered, or people following code from old books.

The graphics of what you draw in the double buffered painting methods (like paintComponent(Graphics)) will be visible all at once when all of the relevant painting methods are complete. That being the case there's no reason to use a BufferedImage as a way to manually ensure all the Graphics get shown at once.