背景
有许多与阅读和解析SVG路径相关的未解答的问题：
> Easiest way to read in svg path data with Java?
> Re: parsing a svg file using batik svg parser
> How to convert SVG Path (SVGOMPathElement) to array of points?
这个问题和答案旨在解决所有这些问题.
问题
SVG路径元素包含data attribute(d).有时需要从SVG文件中加载,解析和提取路径信息.
题
如何使用Java从SVG文件加载,解析和提取SVG路径信息？
解决方法:
概观
使用Apache Batik加载和解析SVG文件.该解决方案在将SVG文件转换为MetaPost的初步阶段显示Java代码.这应该提供有关如何使用Java从SVG文件加载,解析和提取内容的一般概念.
图书馆
您将需要以下库：
batik-anim.jar
batik-awt-util.jar
batik-bridge.jar
batik-css.jar
batik-dom.jar
batik-ext.jar
batik-gvt.jar
batik-parser.jar
batik-script.jar
batik-svg-dom.jar
batik-svggen.jar
batik-util.jar
batik-xml.jar
xml-apis-ext.jar
加载SVG文件
主应用程序将SVG文件加载到DOM中,然后将DOM转换为SVG DOM. initSVGDOM()方法调用非常重要.在不调用initSVGDOM()的情况下,从DOM中提取SVG DOM元素的方法将不可用.
import java.io.File;
import java.io.IOException;
import java.net.URI;
import org.apache.batik.bridge.BridgeContext;
import org.apache.batik.bridge.DocumentLoader;
import org.apache.batik.bridge.GVTBuilder;
import org.apache.batik.bridge.UserAgent;
import org.apache.batik.bridge.UserAgentAdapter;
import org.apache.batik.dom.svg.SAXSVGDocumentFactory;
import org.apache.batik.dom.svg.SVGOMSVGElement;
import org.apache.batik.util.XMLResourceDescriptor;
import org.w3c.dom.Document;
import org.w3c.dom.NodeList;
/**
* Responsible for converting all SVG path elements into MetaPost curves.
*/
public class SVGMetaPost {
private static final String PATH_ELEMENT_NAME = "path";
private Document svgDocument;
/**
* Creates an SVG Document given a URI.
*
* @param uri Path to the file.
* @throws Exception Something went wrong parsing the SVG file.
*/
public SVGMetaPost( String uri ) throws IOException {
setSVGDocument( createSVGDocument( uri ) );
}
/**
* Finds all the path nodes and converts them to MetaPost code.
*/
public void run() {
NodeList pathNodes = getPathElements();
int pathNodeCount = pathNodes.getLength();
for( int iPathNode = 0; iPathNode < pathNodeCount; iPathNode++ ) {
MetaPostPath mpp = new MetaPostPath( pathNodes.item( iPathNode ) );
System.out.println( mpp.toCode() );
}
}
/**
* Returns a list of elements in the SVG document with names that
* match PATH_ELEMENT_NAME.
*
* @return The list of "path" elements in the SVG document.
*/
private NodeList getPathElements() {
return getSVGDocumentRoot().getElementsByTagName( PATH_ELEMENT_NAME );
}
/**
* Returns an SVGOMSVGElement that is the document's root element.
*
* @return The SVG document typecast into an SVGOMSVGElement.
*/
private SVGOMSVGElement getSVGDocumentRoot() {
return (SVGOMSVGElement)getSVGDocument().getDocumentElement();
}
/**
* This will set the document to parse. This method also initializes
* the SVG DOM enhancements, which are necessary to perform SVG and CSS
* manipulations. The initialization is also required to extract information
* from the SVG path elements.
*
* @param document The document that contains SVG content.
*/
public void setSVGDocument( Document document ) {
initSVGDOM( document );
this.svgDocument = document;
}
/**
* Returns the SVG document parsed upon instantiating this class.
*
* @return A valid, parsed, non-null SVG document instance.
*/
public Document getSVGDocument() {
return this.svgDocument;
}
/**
* Enhance the SVG DOM for the given document to provide CSS- and SVG-specific
* DOM interfaces.
*
* @param document The document to enhance.
* @link http://wiki.apache.org/xmlgraphics-batik/BootSvgAndCssDom
*/
private void initSVGDOM( Document document ) {
UserAgent userAgent = new UserAgentAdapter();
DocumentLoader loader = new DocumentLoader( userAgent );
BridgeContext bridgeContext = new BridgeContext( userAgent, loader );
bridgeContext.setDynamicState( BridgeContext.DYNAMIC );
// Enable CSS- and SVG-specific enhancements.
(new GVTBuilder()).build( bridgeContext, document );
}
/**
* Use the SAXSVGDocumentFactory to parse the given URI into a DOM.
*
* @param uri The path to the SVG file to read.
* @return A Document instance that represents the SVG file.
* @throws Exception The file could not be read.
*/
private Document createSVGDocument( String uri ) throws IOException {
String parser = XMLResourceDescriptor.getXMLParserClassName();
SAXSVGDocumentFactory factory = new SAXSVGDocumentFactory( parser );
return factory.createDocument( uri );
}
/**
* Reads a file and parses the path elements.
*
* @param args args[0] - Filename to parse.
* @throws IOException Error reading the SVG file.
*/
public static void main( String args[] ) throws IOException {
URI uri = new File( args[0] ).toURI();
SVGMetaPost converter = new SVGMetaPost( uri.toString() );
converter.run();
}
}
注意：除非另有说明,否则调用initSVGDOM()应该是Batik的默认行为.唉,它不是,发现这个宝石意味着在他们的网站上阅读documentation buried.
解析SVG DOM
解析SVG DOM是相对微不足道的. toCode()方法是该类的主力：
import org.apache.batik.dom.svg.SVGItem;
import org.apache.batik.dom.svg.SVGOMPathElement;
import org.w3c.dom.Node;
import org.w3c.dom.svg.SVGPathSegList;
/**
* Responsible for converting an SVG path element to MetaPost. This
* will convert just the bezier curve portion of the path element, not
* its style. Typically the SVG path data is provided from the "d" attribute
* of an SVG path node.
*/
public class MetaPostPath extends MetaPost {
private SVGOMPathElement pathElement;
/**
* Use to create an instance of a class that can parse an SVG path
* element to produce MetaPost code.
*
* @param pathNode The path node containing a "d" attribute (output as MetaPost code).
*/
public MetaPostPath( Node pathNode ) {
setPathNode( pathNode );
}
/**
* Converts this object's SVG path to a MetaPost draw statement.
*
* @return A string that represents the MetaPost code for a path element.
*/
public String toCode() {
StringBuilder sb = new StringBuilder( 16384 );
SVGOMPathElement pathElement = getPathElement();
SVGPathSegList pathList = pathElement.getNormalizedPathSegList();
int pathObjects = pathList.getNumberOfItems();
sb.append( ( new MetaPostComment( getId() ) ).toString() );
for( int i = 0; i < pathObjects; i++ ) {
SVGItem item = (SVGItem)pathList.getItem( i );
sb.append( String.format( "%s%n", item.getValueAsString() ) );
}
return sb.toString();
}
/**
* Returns the value for the id attribute of the path element. If the
* id isn't present, this will probably throw a NullPointerException.
*
* @return A non-null, but possibly empty String.
*/
private String getId() {
return getPathElement().getAttributes().getNamedItem( "id" ).getNodeValue();
}
/**
* Typecasts the given pathNode to an SVGOMPathElement for later analysis.
*
* @param pathNode The path element that contains curves, lines, and other
* SVG instructions.
*/
private void setPathNode( Node pathNode ) {
this.pathElement = (SVGOMPathElement)pathNode;
}
/**
* Returns an SVG document element that contains path instructions (usually
* for drawing on a canvas).
*
* @return An object that contains a list of items representing pen
* movements.
*/
private SVGOMPathElement getPathElement() {
return this.pathElement;
}
}
建立
编译因环境而异.类似于以下的脚本应该有所帮助：
#!/bin/bash
mkdir -p ./build
javac -cp ./lib/* -d ./build ./source/*.java
确保将所有.jar文件放入./lib目录中.将源文件放入./source目录.
跑
创建一个脚本(或批处理文件)来执行该程序：
#!/bin/bash
java -cp ./lib/*:./build SVGMetaPost $1
产量
对包含有效SVG路径的文件运行时,会产生：
$./run.sh stripe/trigon.svg
% path8078-6
M 864.1712 779.3069
C 864.1712 779.3069 868.04065 815.6211 871.4032 833.4621
C 873.4048 844.08203 874.91724 855.0544 879.0846 864.82227
C 884.24023 876.9065 895.2377 887.9899 900.0184 897.3661
C 904.7991 906.7422 907.3466 918.3257 907.3466 918.3257
C 907.3466 918.3257 892.80817 887.6536 864.1712 887.3086
C 835.53424 886.9637 820.9958 918.3257 820.9958 918.3257
C 820.9958 918.3257 823.6176 906.59644 828.32404 897.3661
C 833.0304 888.1356 844.10223 876.9065 849.2578 864.82227
C 853.4252 855.05444 854.9376 844.08203 856.93915 833.4621
C 860.3017 815.6211 864.17114 779.3069 864.17114 779.3069
z
从这里开始,应该清楚如何使用Java将SVG路径数据读入相应的SVG对象.
附录
请注意,从SVG转换为MetaPost的最简单方法是：
>将SVG转换为PDF(例如,使用Inkscape或rsvg-convert).
>使用pstoedit将PDF转换为MetaPost.