Aaron Johnson Now with 50% less caffeine!

Transparent PNG Charts with JFreeChart

If you’re one of the 3 people in the world that need to create a transparent PNG image using JFreeChart, you’ve come to the right place:

JFreeChart chart = ChartFactory.createXYBarChart(...);
chart.setBackgroundPaint(new Color(255,255,255,0));
...
KeypointPNGEncoderAdapter encoder = new KeypointPNGEncoderAdapter();
encoder.setEncodingAlpha(true);
encoder.encode(chart.createBufferedImage(width, height, BufferedImage.BITMASK, null));

The key is that you must use the KeyPointPNGEncoderAdapter, which is slower for big charts, but is the only one of the two PNG encoders that JFree ships with that has the ability to do alpha transparency. On systems running JDK 1.4 and above, the ImageEncoderFactory will return the SunPNGEncoderAdapter, which does not support alpha transparency.

Also, if you’re planning on using alpha transparency in a web application that needs to support IE, you’ll want to check out this JavaScript fix by Bob Osola.

That’s pretty cool. I must be one of those 3 people.

For the second part, you can also use ChartUtilities if you just want to save the image to a file:

ChartUtilities.saveChartAsPNG(file, chart, width, height, null, true, compressionLevel);

And your lucky 3rd person comes from Australia! Thanks for this tip, mate. Just what I was after. Made my day.

So im the third person using it?

That did the trick! Thanks Aaron.

Hi, I am fourth person…

5th person, thanks!