如何加载和解析 SVG 文档

Question

背景

有许多与读取和解析 SVG 路径相关的未解决问题：

• 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?

问题

SVG path 元素包含data attribute (d)。有时需要仅从 SVG 文件中加载、解析和提取路径信息。

问题

如何从 SVG 文件中加载、解析和提取 SVG 路径信息？

Answer 1

概述

使用Apache Batik（或EchoSVG）加载和解析SVG文件。该解决方案显示了将 SVG 文件转换为 MetaPost 的初步阶段的 Java 代码。这应该为如何使用 Java 从 SVG 文件中加载、解析和提取内容提供一个总体思路。

库

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。如果不调用initSVGDOM()，则无法使用从 DOM 中提取 SVG DOM 元素的方法。

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，事实并非如此，发现这颗宝石意味着在他们的网站上阅读 documentation buried。

解析 SVG DOM

解析 SVG DOM 就相对简单了。 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;
  }
}

构建

编译会因环境而异。类似于以下的脚本应该会有所帮助：

#!/bin/bash
mkdir -p ./build
javac -cp ./lib/* -d ./build ./source/*.java

请务必将所有.jar 文件放入./lib 目录。把源文件放到./source目录下。

运行

创建一个脚本（或批处理文件）来执行程序：

#!/bin/bash
java -cp ./lib/*:./build SVGMetaPost $1

输出

当对包含有效 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

从这里应该清楚如何将 SVG 路径数据读入其对应的 SVG 对象。

附录

请注意，从 SVG 转换为 MetaPost 的最简单方法是：

1. 将 SVG 转换为 PDF（例如，使用 Inkscape 或 rsvg-convert）。
2. 使用 pstoedit 将 PDF 转换为 MetaPost。