Image Transcoder Tutorial


	The Transcoder API

The org.apache.batik.transcoder package defines 5 major classes:

Transcoder -
Defines the interface that all transcoders implement. You can transcode a specific input using a specific output by invoking the transcode method. Although there is no assumption on the input and output format, a specific transcoder may or may not support a particular type of input or output. For example, the image transcoders accept an SVG org.w3c.dom.Document, a Reader, an InputStream, or a URI as an input but only supports a byte stream for the output.

TranscoderInput -
Defines the input of a transcoder. There are various ways to create an input and the most commons are already part of the API. The default implementation lets you create an input using a org.w3c.dom.Document, a Reader, an InputStream, a org.xml.sax.XMLReader, or a URI.

TranscoderOutput -
Defines the output of a transcoder. There are various ways to create an output and the most commons are already part of the API. The default implementation lets you create an output using a org.w3c.dom.Document, a Writer, an OutputStream, a org.xml.sax.XMLFilter, or a URI.

TranscodingHints -
The TranscodingHints class contains different hints which can be used to control the various options or parameters of a transcoder. Each transcoder provides its own set of hints. A hint is specified by (key, value) pair. For example, the JPEGTranscoder provides a hint to control the encoding quality.

ErrorHandler -
This class provides a way to get the errors and/or warnings that might occur while transcoding. A default implementation is provided but you can, for example, implement your own handler that display a dialog instead of stack trace.


	How to Use the Image Transcoder API

The org.apache.batik.transcoder.image package provides an easy way to transcode an SVG document to a raster image such as JPEG, PNG or Tiff. Additional raster image formats can be added by subclassing the ImageTranscoder and implementing the writeImage method. Although, in next sections, the examples will use the JPEG transcoder, the PNG transcoder works the same way.


	Creating an Image

The following example, using the JPEGTranscoder shows how to trasnform an SVG document to a JPEG image.

SaveAsJpeg.java
===============

import java.io.*;
import org.apache.batik.transcoder.image.JPEGTranscoder;
import org.apache.batik.transcoder.TranscoderInput;
import org.apache.batik.transcoder.TranscoderOutput;

public class SaveAsJPEG {

    public static void main(String [] args) throws Exception {

        // create a JPEG transcoder
        JPEGTranscoder t = new JPEGTranscoder();
        // set the transcoding hints
        t.addTranscodingHint(JPEGTranscoder.KEY_QUALITY,
                             new Float(.8));
        // create the transcoder input
        String svgURI = new File(args[0]).toURL().toString();
        TranscoderInput input = new TranscoderInput(svgURI);
        // create the transcoder output
        OutputStream ostream = new FileOutputStream("out.jpg");
        TranscoderOutput output = new TranscoderOutput(ostream);
        // save the image
        t.transcode(input, output);
        // flush and close the stream then exit
        ostream.flush();
        ostream.close();
        System.exit(0);
    }
}

The code creates a JPEGTranscoder and sets the transcoding hints. The first hint indicates the XML parser to use. The second hint sets the encoding quality. Then, an input and an output are created. The input is created using the first command line argument which should represent a URI. The output is a byte stream representing a file called "out.jpg". Finally, the transcode method is invoked and the byte stream is closed.

Although not shown above, the program might have specified additional hints to indicate a user style sheet, the preferred language of the document or the background color.

Try this:

Compile and run the program: SaveAsJPEG.java. You will need an SVG document.
% java SaveAsJPEG <filename>.svg

Take a look at: out.jpg


	Defining the Size of the Image

By adding the following line of code to the previous example, you will specify the raster image size (in pixels). The new transcoding hint KEY_WIDTH lets you specify the raster image width. As the raster image height is not provided (using the KEY_HEIGHT), the transcoder will compute the raster image dimension by keeping the aspect ratio of the SVG document.


	t.addTranscodingHint(JPEGTranscoder.KEY_WIDTH, new Float(100));

The transcoder will have the same behavior if you specify the KEY_HEIGHT without initializing the KEY_WIDTH. In all cases (though both keys are provided), the transcoder will preserve the apsect ratio of the SVG document.


	Selecting an Area of Interest

The image transcoder lets you specify an area of interest (ie. a part of the SVG document). The key KEY_AOI enables to select the region of the SVG document to render. The value of this key must be a java.awt.Rectangle specified in pixels and described in the SVG document's space. The following example shows how you can split an SVG document into 4 tiles.

SaveAsJPEGTiles.java
====================

import java.io.*;
import java.awt.*;
import org.apache.batik.transcoder.image.JPEGTranscoder;
import org.apache.batik.transcoder.TranscoderInput;
import org.apache.batik.transcoder.TranscoderOutput;

public class SaveAsJPEGTiles {

    JPEGTranscoder trans = new JPEGTranscoder();

    public SaveAsJPEGTiles() {
        trans.addTranscodingHint(JPEGTranscoder.KEY_QUALITY,
                                 new Float(.8));
    }

    public void tile(String inputFilename,
                     String outputFilename,
                     Rectangle aoi) throws Exception {
        trans.addTranscodingHint(JPEGTranscoder.KEY_WIDTH,
                                 new Float(aoi.width));
        trans.addTranscodingHint(JPEGTranscoder.KEY_HEIGHT,
                                 new Float(aoi.height));
        trans.addTranscodingHint(JPEGTranscoder.KEY_AOI, aoi);
        String svgURI = new File(inputFilename).toURL().toString();
        TranscoderInput input = new TranscoderInput(svgURI);
        OutputStream ostream = new FileOutputStream(outputFilename);
        TranscoderOutput output = new TranscoderOutput(ostream);
        trans.transcode(input, output);
        ostream.flush();
        ostream.close();
    }

    public static void main(String [] args) throws Exception {
        SaveAsJPEGTiles p = new SaveAsJPEGTiles();
        String in = "samples/anne.svg";
        int documentWidth = 450;
        int documentHeight = 500;
        int dw2 = documentWidth / 2;
        int dh2 = documentHeight / 2;
        p.tile(in, "tileTopLeft.jpg", new Rectangle(0, 0, dw2, dh2));
        p.tile(in, "tileTopRight.jpg", new Rectangle(dw2, 0, dw2, dh2));
        p.tile(in, "tileBottomLeft.jpg", new Rectangle(0, dh2, dw2, dh2));
        p.tile(in, "tileBottomRight.jpg", new Rectangle(dw2, dh2, dw2, dh2));
        System.exit(0);
    }
}

This code splits the same document "anne.svg" into four tiles of the same size. Considering the document and its original size, we can determine four regions. Then we rasterize each region using the KEY_AOI key. Note that we also specify the image width and height to be the same as the area of interest width and height (so we keep a 1:1 zoom factor). You can of course combine the KEY_WIDTH, KEY_HEIGHT keys with the KEY_AOI. In that case, first the area of interest will determine which part of the SVG document has to be rendered - then that part could be zoom in or out depending on the specified raster image size.

Try this:

Compile and run the program: SaveAsJPEGTiles.java. You will need the "anne.svg" document.
% java SaveAsJPEGTiles

Take a look at: tileTopRight.jpg, tileTopRight.jpg, tileBottomRight.jpg and tileBottomLeft.jpg


	Other Transcoding Hints

The ImageTranscoder provides additional TranscodingHints that lets you customize the generated images.

ImageTranscoder.KEY_MEDIA -
This hint lets you choose the CSS medium to use. The author of the SVG document to transcode can control CSS media using the CSS media rule. Example:

trans.addTranscodingHint(ImageTranscoder.KEY_MEDIA, "print");

ImageTranscoder.KEY_ALTERNATE_STYLESHEET -
This hint lets you choose an alternate stylesheet the author of the SVG document to transcode might have provided using the xml-stylesheet processing instruction. Example:

trans.addTranscodingHint(ImageTranscoder.KEY_ALTERNATE_STYLESHEET, alternateStylesheetName);

ImageTranscoder.KEY_USER_STYLESHEET_URI -
This hint lets you use a user stylesheet. User stylesheet can override some styles of the SVG document to transcode. Example:

trans.addTranscodingHint(ImageTranscoder.KEY_USER_STYLESHEET_URI, "http://...");

ImageTranscoder.KEY_PIXEL_TO_MM -
This hint lets you use the pixel to millimeter conversion factor. This factor is used to determine how units are converted into pixels. Example:

// 96dpi
trans.addTranscodingHint(ImageTranscoder.KEY_PIXEL_TO_MM, new Float(0.2645833f));
or
// 72dpi
trans.addTranscodingHint(ImageTranscoder.KEY_PIXEL_TO_MM, new Float(0.3528f));

ImageTranscoder.KEY_BACKGROUND_COLOR -
This hint lets you choose a background color. Example:

trans.addTranscodingHint(ImageTranscoder.KEY_BACKGROUND_COLOR, Color.white);


	Generating an Image from an SVG DOM Tree

The following code creates and saves an SVG DOM tree.

DOMRasterizer.java
==================

import java.io.*;
import org.apache.batik.transcoder.image.JPEGTranscoder;
import org.apache.batik.transcoder.TranscoderInput;
import org.apache.batik.transcoder.TranscoderOutput;
import org.apache.batik.dom.svg.SVGDOMImplementation;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.DOMImplementation;

public class DOMRasterizer {

    public Document createDocument() {
        DOMImplementation impl = SVGDOMImplementation.getDOMImplementation();
        String svgNS = SVGDOMImplementation.SVG_NAMESPACE_URI;
        Document document =
            impl.createDocument(svgNS, "svg", null);
        Element root = document.getDocumentElement();
        root.setAttributeNS(null, "width", "450");
        root.setAttributeNS(null, "height", "500");

        Element e;
        e = document.createElementNS(svgNS, "rect");
        e.setAttributeNS(null, "x", "10");
        e.setAttributeNS(null, "y", "10");
        e.setAttributeNS(null, "width", "200");
        e.setAttributeNS(null, "height", "300");
        e.setAttributeNS(null, "style", "fill:red;stroke:black;stroke-width:4");
        root.appendChild(e);

        e = document.createElementNS(svgNS, "circle");
        e.setAttributeNS(null, "cx", "225");
        e.setAttributeNS(null, "cy", "250");
        e.setAttributeNS(null, "r", "100");
        e.setAttributeNS(null, "style", "fill:green;fill-opacity:.5");
        root.appendChild(e);

        return document;
    }

    public void save(Document document) throws Exception {
        JPEGTranscoder t = new JPEGTranscoder();
        t.addTranscodingHint(JPEGTranscoder.KEY_QUALITY,
                             new Float(.8));
        TranscoderInput input = new TranscoderInput(document);
        OutputStream ostream = new FileOutputStream("out.jpg");
        TranscoderOutput output = new TranscoderOutput(ostream);
        t.transcode(input, output);
        ostream.flush();
        ostream.close();
    }

    public static void main(String [] args) throws Exception {
        DOMRasterizer rasterizer = new DOMRasterizer();
        Document document = rasterizer.createDocument();
        rasterizer.save(document);
        System.exit(0);
    }
}

This code is divided into two distinct parts.

Creating an SVG DOM tree -

See the createDocument method
Three steps are required at this time. The first one consists on getting the Batik SVG DOM implementation (via the SVGDOMImplementation class). Then, you can create a org.w3c.dom.Document (which is an SVG Document by the way) by invoking the createDocument method with the svg namespace URI and the "svg" document element. At last, you can get the document element and start building your DOM tree.

Rasterizing your DOM -

See the save method
Similar to the previous examples, you can transcode an SVG document to a raster image by creating a TranscoderInput with this time, the SVG Document.

Try this:

Compile and run the program: DOMRasterizer.java.
% java DOMRasterizer

Take a look at: out.jpg