How to save an image to file without rendering to screen?

Question

Does anyone know of a way I can save a buffered image to a file (as a jpg) without having to render it to a screen first? I can create the file fine if I place the image on a JPanel first. However when I try to save the image directly to a file, the program hangs when I try to render the image to a Graphics object. Is there some sort of initialisation carried out on the image when it is displayed on screen which I need to carry out to save the image offscreen?
Just as a background, what I am trying to achieve is to create a chart (using Monarch Charts library) based on a supplied set of data, and then to save this chart as a jpg. It works fine if I first display the chart in a JPanel but not when I try to save it offscreen.
Can anyone help me out here?
Thanks,
Ciaran

Answer 1

Here's the code I use in JFreeChart to create a buffered image - the third line draws the chart using the Graphics2D obtained from the BufferedImage:
    public BufferedImage createBufferedImage(int width, int height) {
        BufferedImage image = new BufferedImage(width , height, BufferedImage.TYPE_INT_RGB);
        Graphics2D g2 = image.createGraphics();
        this.draw(g2, new Rectangle2D.Double(0, 0, width, height), null);
        g2.dispose();
        return image;
    }Writing to a JPEG can be done like this (method taken from the ChartUtilities class in JFreeChart):
    public static void writeChartAsJPEG(OutputStream out, JFreeChart chart, int width, int height)
        throws IOException {
        BufferedImage image = chart.createBufferedImage(width, height);
        JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
        JPEGEncodeParam param = encoder.getDefaultJPEGEncodeParam(image);
        param.setQuality(1.0f, true);
        encoder.encode(image, param);
    }If you want to try some running code, download JFreeChart from:
http://www.object-refinery.com/jfreechart/index.html
...then run this application:
package com.jrefinery.chart.demo;
import java.io.File;
import com.jrefinery.chart.JFreeChart;
import com.jrefinery.chart.ChartFactory;
import com.jrefinery.chart.ChartUtilities;
import com.jrefinery.chart.demo.DemoDatasetFactory;
import com.jrefinery.data.XYDataset;
public class ChartToFile {
    public static void main(String[] args) {
        XYDataset data = DemoDatasetFactory.createTimeSeriesCollection2();
        JFreeChart chart = ChartFactory.createTimeSeriesChart("Sample Chart", "Time", "Rate", data, true);
        File file = new File("MyChart.jpg");
        try {
            ChartUtilities.saveChartAsJPEG(file, chart, 400, 300);
        catch (java.io.IOException e) {
            System.out.println(e.getMessage());
}Regards,
Dave Gilbert
www.object-refinery.com