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2.5 Example: Off-Screen Image Creation

Since JCCKit 1.1 convenient methods allow easy off-screen image creation. This is especially useful if you have to create images on the server of a Web application. Note, that off-screen image creation needs J2SE 1.4 or higher.

The code below is a small example which takes a properties file (like this one) and creates a PNG image of specified size. The important lines of code are highlighted:

import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;

import jcckit.Graphics2DPlotCanvas;
import jcckit.data.DataPlot;
import jcckit.util.ConfigParameters;
import jcckit.util.PropertiesBasedConfigData;

public class ImagePlotter
  public static void main(String[] args) throws IOException
    System.getProperties().setProperty("java.awt.headless", "true");

    // Handles the commandline argument
    if (args.length < 3)
      System.out.println("Usage: java -cp classycle.jar ImagePlotter " 
                         + "  ");
    String propertiesFile = args[0];
    int width = Integer.parseInt(args[1]);
    int height = Integer.parseInt(args[2]);

    // Sets up a Graphics2DPlotCanvas
    ConfigParameters config
        = new ConfigParameters(new PropertiesBasedConfigData(propertiesFile));
    Graphics2DPlotCanvas plotCanvas = new Graphics2DPlotCanvas(config);

    // Draws the chart into a off-screen image
    BufferedImage image 
        = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

    // Writes the off-screen image into a PNG file
    ImageIO.write(image, "png", new File(propertiesFile + ".png"));
Explanations of the highlighted statements:
  1. Setting java.awt.headless = true allows to draw off-screen images on a server without any graphical window system running. For example, in a Unix environment no X-Server is needed (but some X libraries might be still necessary).
  2. Double-buffering has to be switched off. This is possible since JCCKit 1.1.
  3. A BufferedImage is an off-screen image.
  4. The method draw2DInto() of Graphics2DPlotCanvas draws the chart into the buffered image as it would be drawn onto the real screen if one runs Graphics2DPlotCanvas as a Java application with the specified properties file.

    Instead of drawing into a Graphics2D context one can also draw into a Graphics context by using drawInto() of GraphicsPlotCanvas. Note: This only works if there is no rotated text because rotated texts are created by double-buffering. In a chart with a CartesianCoordinateSystem this implies that the default label of the y-axis has to be removed because it is a rotated piece of text.

  5. Since J2SE 1.4 the method ImageIO.write() is a convenient way to encode images and write them into files.