Server Side SVG
If you've been using SVG or reading XML.com, you probably know about the Adobe SVG Viewer, and you may have heard of the Apache Batik project. Although Batik is most widely known for its SVG viewer component, it's more than that. Batik, according to its web site, is a “Java technology-based toolkit for applications that want to use images in the Scalable Vector Graphics (SVG) format for various purposes, such as viewing, generation or manipulation.”
The Batik viewer application uses the JSVGCanvas
component, which is a Java class that accepts SVG as input and
displays it on screen. In this article, we'll use two of the other
Batik components, SVGGraphics2D, and the Batik
transcoders. The SVGGraphics2D class is the inverse of
JSVGCanvas; you draw into an SVGGraphics2D
environment using the standard Java two-dimensional graphics methods,
and the result is an SVG document. The transcoders take an SVG
document as input and produce either JPG or PNG as output.
The context in which we'll use these tools is a servlet that generates geometric art in the style of Piet Mondrian. If the client supports SVG, the servlet will return an SVG document. Otherwise, it will return a JPEG or PNG image, depending upon client support for those image formats.
The web page that we show users will let them choose the orientation and color scheme of their painting. See the screenshot below for an example of how this might look.
Here's the HTML:
<html>
<head>
<title>Art-O-Matic</title>
</head>
<body>
<h2>Art-O-Matic</h2>
<p>
Yes, you too can become a famous artist! With a few simple clicks
of the mouse, you can generate geometric art in the style of
Piet Mondrian.
</p>
<form id="artForm"
action="http://jde:8080/artmaker/servlet/ArtMaker">
<p>
What kind of picture would you like?
<br />
<input type="radio" name="picType" value="landscape"
checked="checked" /> Landscape
<input type="radio" name="picType" value="portrait" /> Portrait
<input type="radio" name="picType" value="square" /> Square
</p>
<p>
Which color scheme would you like?
<br />
<input type="radio" name="scheme" value="bright"
checked="checked" /> Vivid colors
<input type="radio" name="scheme" value="pastel" /> Soft pastel
</p>
</body>
</html>
Determining whether the client has SVG support or not must be handled on the client side. We'll add this script, taken from a Sun Microsystems technical note, to do this detection.
<script type="text/javascript">
<!-- <![CDATA[
var hasSVGSupport = false; // does client have SVG support?
var useVBMethod = false; // use VBScript or JavaScript?
/*
Internet Explorer returns 0 as the number of MIME types,
so this code will not be executed by it. This is our indication
to use VBScript to detect SVG support.
*/
if (navigator.mimeTypes != null
&& navigator.mimeTypes.length > 0)
{
if (navigator.mimeTypes["image/svg+xml"] != null)
{
hasSVGSupport = true;
}
}
else
{
useVBMethod = true;
}
// ]]> -->
</script>
<!--
Visual Basic Script to detect support of Adobe SVG plugin.
This code is not run on browsers which report they have MIME types,
and it is also not run by browsers which do not have VBScript support.
-->
<script type="text/vbscript">
On Error Resume Next
If useVBMethod = true Then
hasSVGSupport = IsObject(CreateObject("Adobe.SVGCtl"))
End If
</script>
In order to have the web page send this information back to the
server, we'll have to change the <form> tag and add
a hidden field to our form:
<form id="artForm"
action="http://jde:8080/artmaker/servlet/ArtMaker"
onsubmit="return setSVGStatus();">
<input type="hidden" name="imgType" value="jpg" />
Which requires an additional script to set the hidden field before we send the data to the server:
<script type="text/javascript">
<!-- <![CDATA[
function setSVGStatus()
{
if (hasSVGSupport)
{
var theForm = document.getElementById("artForm");
theForm.imgType.value = "svg";
}
return true;
}
// ]]> -->
</script>
You may see the source for the entire HTML file.
|
Questions about this article? Share them with the author and other readers in our forum. |
| Post your comments |
First, we need Java code to draw the actual painting into a
Graphics2D context. You may see the
Artist.java source. There's nothing particularly
special about it; it's just a few draw and
fill calls in a recursive function. The only thing to
note is the constructor, which requires a width, height, and palette
(which will be BRIGHT or PASTEL):
public Artist( int width, int height, int colorType );
Now, on to the servlet. We'll do all the work in the
doPost method. We start by sending out header information
to keep the results from being cached:
public class ArtMaker extends HttpServlet {
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
Artist mondrian = null;
try
{
response.setHeader("Cache-Control",
"no-cache, no-store, must-revalidate");
response.setHeader("Cache-Control",
"post-check=0, pre-check=0");
String agent =
request.getHeader("User-Agent").toLowerCase();
/* netscape chokes on Pragma no-cache so only
send it to explorer */
if (agent.indexOf("explorer") > -1){
response.setHeader("Pragma", "no-cache");
}
response.setHeader("Expires",
"Thu, 01 Dec 1994 16:00:00 GMT");
We then grab the parameters, use them to set some local variables,
and create an appropriate Artist.
String picType = request.getParameter( "picType" );
String colorScheme = request.getParameter( "scheme" );
String imgType = request.getParameter( "imgType" );
Artist mondrian = null;
int width;
int height;
int palette;
if (picType.equals("landscape"))
{
width = 400;
height = 300;
}
else if (picType.equals("portrait"))
{
width = 200;
height = 300;
}
else
{
width = 250;
height = 250;
}
palette = (colorScheme.equals("bright")) ?
Artist.BRIGHT : Artist.PASTEL;
mondrian = new Artist( width, height, palette );
Once this is set, we need to create an SVG document and an SVG
graphics environment. Since every parser implementation has its own
way of storing the objects, you must ask the
SVGDOMImplementation class to give you the details. Once
you know the details, you can ask the implementation to create a
document with the appropriate namespace, which is in
svgNS. The third argument to createDocument
is a reference to a list of entities defined for the document. There
aren't any in this case, so we set the third argument to
null.
DOMImplementation domImpl =
SVGDOMImplementation.getDOMImplementation();
String svgNS = SVGDOMImplementation.SVG_NAMESPACE_URI;
Document document =
domImpl.createDocument(svgNS, "svg", null);
We then construct the SVGGraphics2D object and
associate it with the document that it will create. We set the graphic
environment's dimensions to the desired height and width with ten
extra pixels for padding around the edges.
SVGGraphics2D svgGraphicsEnvironment = new SVGGraphics2D(document);
svgGraphicsEnvironment.setSVGCanvasSize(
new Dimension( width + 10, height + 10 )
);
|
Related Reading
|
Everything is now set up. We just tell our artist to paint into that graphic environment, and the corresponding SVG document will be built up automatically.
mondrian.paint( svgGraphicsEnvironment );
Now the document is in memory, but we will need it in text form to
send back to the client. We accomplish this by streaming the document
to a ByteArrayOutputStream, whose initial size is 8192
bytes, which should be enough for most normal paintings that this code
will generate. The second parameter to stream is set to
true, indicating that the output should use CSS styles
instead of style attributes.
ByteArrayOutputStream baos = new ByteArrayOutputStream( 8192 ); Writer svgOutput = new OutputStreamWriter( baos, "UTF-8" ); svgGraphicsEnvironment.stream( svgOutput, true );
We now determine whether we need to send back an SVG, JPEG, or PNG
image. The client has told us whether we can use SVG or not. The
information about PNG support is in the HTTP request's
Accept header. In each case, we take the SVG output byte
array, convert it to a string, and pass it on to the appropriate
emit function.
if (imgType.equals("svg"))
{
emitSVG( request, response, baos.toString() );
}
else
{
if (request.getHeader("Accept").indexOf("png") >= 0)
{
emitPNG( request, response, baos.toString() );
}
else
{
emitJPG( request, response, baos.toString() );
}
}
The remainder of the doPost method is the error
trapping; in case anything fails, we send back an HTML page with an
error message.
catch (Exception e)
{
PrintWriter out = response.getWriter();
response.setContentType("text/html");
out.println("<?xml version="1.0" encoding="utf-8"?>");
out.println("<!DOCTYPE");
out.println("html PUBLIC \"-//W3C//DTD XHTML 1.1//EN\"");
out.println("\"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd\">");
out.println("<html><head><title>Error</title></head>");
out.println("<body>");
out.println("<p>Unable to create painting.</p>");
out.println("<pre>");
e.printStackTrace(out);
out.println("</pre>");
out.println("</body></html>");
}
This leaves the three emit...
methods. emitSVG is trivial; we just send the string back
to the client with an appropriate MIME type:
public void emitSVG( HttpServletRequest request,
HttpServletResponse response,
String svgString )
{
response.setContentType( "image/svg+xml" );
try {
response.getWriter().write( svgString );
response.getWriter().flush();
}
catch (Exception e)
{
e.printStackTrace();
}
}
To send back a JPEG or PNG file, we have to use Batik's
transcoder. Input to the transcoder can be a document, a stream, or a
Reader; output can be a URI, stream, or
Writer. For the JPEG transcoder, you may set the output
quality by calling the addTranscodingHint method. Since
we're sending a series of bytes back to the client rather than a
string, we're sending output to
response.getOutputStream() rather than
response.getWriter().
public void emitJPG( HttpServletRequest request,
HttpServletResponse response, String svgString )
{
response.setContentType("image/jpeg");
JPEGTranscoder t = new JPEGTranscoder();
t.addTranscodingHint(JPEGTranscoder.KEY_QUALITY,
new Float(.8));
TranscoderInput input =
new TranscoderInput( new StringReader(svgString) );
try {
TranscoderOutput output =
new TranscoderOutput(response.getOutputStream());
t.transcode(input, output);
response.getOutputStream().close();
}
catch (Exception e)
{
e.printStackTrace();
}
}
Similar code sends PNG output; you may give it an additional hint
with KEY_FORCE_TRANSPARENT_WHITE if you want fully
transparent pixels to appear as white. This is good for older browsers
that don't support PNG transparency completely.
public void emitPNG ( HttpServletRequest request,
HttpServletResponse response, String svgString )
{
response.setContentType("image/png");
PNGTranscoder t = new PNGTranscoder();
TranscoderInput input =
new TranscoderInput( new StringReader(svgString) );
try {
TranscoderOutput output =
new TranscoderOutput(response.getOutputStream());
t.transcode(input, output);
response.getOutputStream().close();
}
catch (Exception e)
{
e.printStackTrace();
}
}
You may examine the
entire servlet as one file. If you install Artist and
Artmaker in a suitable servlet container such as Jakarta Tomcat, you too can
produce art like this, which I've shrunk to half size to take up less
screen space.
Other Batik tools which we haven't covered here let you convert TrueType fonts to SVG and extend SVG to include custom tags. If you want a powerful, cross-platform toolkit for building applications that let you view, construct, and convert SVG documents, Batik is the answer.
XML.com Copyright © 1998-2006 O'Reilly Media, Inc.