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Home » Newsletter » March 2010

By V. Subhash

In a previous article, I showed you how to rasterize a PDF document by first exporting all page content as an image file and then rendering the images in a different PDF document. In this article, you will see how to export PDF page content as vector images.

Windows Metafile and PDFDocument.GetPageMetafile()

Windows metafile is Microsoft file format for storing GDI instructions. GDI instructions from a metafile can be "played back" to re-create the same appearance. This also means that the appearance is scalable. (Raster images are not scalable.)

In the above-mentioned article, the method PDFDocument.GetPageMetafile() was used for exporting vector images of PDF pages. However, the System.Drawing.Metafile class of the .NET framework does not support saving to EMF/WMF formats. If you attempt to save to one of those formats, the resultant file will be a PNG file.

Enter Unmanaged Code

In the code example shown below, two native GDI methods are used to ensure that the metafile is saved as a EMF file. A Metafile instance can have a huge memory footprint. So, the code is meant for small documents. The garbage collections instructions can help but will not save your program in all cases.

using System;
using System.Collections.Generic;
using System.Text;
using System.Drawing;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;
using Gnostice.PDFOne;

namespace com.gnostice.net.pdfone.examples {
  class PDF_to_EMF {

    [DllImport("gdi32.dll")]
    static extern IntPtr CopyEnhMetaFile(  // Copy EMF to file
        IntPtr hemfSrc,   // Handle to EMF
        String lpszFile   // File
    );

    [DllImport("gdi32.dll")]
    static extern bool DeleteEnhMetaFile(  
        IntPtr hemf
    );

    static void Main(string[] args) {
      // Create a metafile
      Metafile metafile;

      // Create a handle to a metafile
      IntPtr iptrMetafileHandle, iptrMetafileHandleCopy;

      // Load a PDF document
      PDFDocument doc =
         new PDFDocument("your-license-key");
      // Load the input document and get its page count
      doc.Load("input_doc.pdf");
      Console.WriteLine("Document loaded.");


      // Iterate through all pages in the document
      for (int I = 1; I <= doc.GetPageCount(); i++) {
        // Export current page as a metafile
        metafile = doc.GetPageMetafile(i);

        // Get a handle to the metafile
        iptrMetafileHandle = metafile.GetHenhmetafile();

        // Export metafile to an image file
        iptrMetafileHandleCopy = CopyEnhMetaFile(
            iptrMetafileHandle,
            "image_of_page_#" + i.ToString() + ".emf");

        // Delete source and copied metafiles from memory
        DeleteEnhMetaFile(iptrMetafileHandle);
        DeleteEnhMetaFile(iptrMetafileHandleCopy);

        /*
        // To save metafile as a raster image,
        // comment four previous statements.
        // Metafile.Save does not save to EMF/WMF.
        Metafile.Save(
            "image_of_page_#" + i.ToString() + ".png",
            ImageFormat.Png);
        */

        Console.Write("\rPage #1-" + I + " exported.");

        // Release any resources used by the metafile
        metafile.Dispose();
      }

      // Clean up
      doc.Close();
      Console.WriteLine("\nDocument closed.");
      doc.Dispose();

      Console.WriteLine("Press Enter to exit.");
      Console.In.ReadLine();
    }
  }
}

Exported Images - PNG vs EMF

The image below shows the difference between the EMF and PNG exports of the metafile, contents of both of which have been magnified. The PNG image has lost some detail. The EMF has retained fidelity, as the GDI instructions were rescaled and played back each time the image was magnified.

Update (April 17, 2013)
How to convert to higher resolutions?

In Version 5 of PDFOne .NET, we introduced new overloads of the PDFDocument.GetPageMetafile method which allow you to specify a DPI value at which the contents of the PDF pages need to be exported. Using this option, you will be able to export PDF page content at high resolutions.

Update (November 14, 2014)
Can this code be used in an ASP.NET Web application?

This article uses a console application. While you could adapt it for an ASP.NET application to do that, we wouldn't recommend that.

• Conversion of big PDFs to EMF would take longer than an HTTP request life cycle. Your web application would timeout on long-running tasks.
• System.Drawing.Metafile class of the .NET framework does not provide a method to save a metafile to WMF or EMF. It can save to PNG or JPEG. Conversion of a metafile instance to EMF/WMF requires unmanaged or Win32 API calls, which is not possible for an ASP.NET application, even in high-trust environments.

For these reasons, we suggest that you use an ASP.NET application only to receive the requests for conversion and perform the actual conversion asynchronously using a "Windows Service" application, which runs as a background process on the same server or another computer. A Windows Service application can call Win32 API without restriction and does not timeout like a web application. We have published another article where this ASP.NET front end with a Windows service file conversion backend is demonstrated. Unfortunately, that article is for a Delphi product but you will get the idea of how to do it. You just need to write the Windows Service application in C#.