如何使用ITextSharp获得PDF中嵌入图像的分辨率?

我能给你一条完全不同的路吗?您正在查看全局文件中的图像，但您没有看到它们如何在页面中使用。

iTextSharp有一个叫做iTextSharp.text.pdf.parser.PdfReaderContentParser的类，它可以行走PdfReader并告诉你关于它的事情。通过iTextSharp.text.pdf.parser.IRenderListener接口实现信息订阅。对于遇到的每个图像，将使用iTextSharp.text.pdf.parser.ImageRenderInfo对象调用类的RenderImage方法。从这个对象中，你可以得到实际的图像以及当前的转换矩阵，它将告诉你如何将图像放置到文档中。

使用这些信息，您可以创建这样的类:

public class MyImageRenderListener : iTextSharp.text.pdf.parser.IRenderListener {
    //For each page keep a list of various image info
    public Dictionary<int, List<ImageScaleInfo>> Pages = new Dictionary<int, List<ImageScaleInfo>>();
    //Need to manually change the page when using this
    public int CurrentPage { get; set; }
    //Pass through the current page units
    public Single CurrentPageUnits { get; set; }
    //Not used, just interface contracts
    public void BeginTextBlock() { }
    public void EndTextBlock() { }
    public void RenderText(iTextSharp.text.pdf.parser.TextRenderInfo renderInfo) { }
    //Called for each image
    public void RenderImage(iTextSharp.text.pdf.parser.ImageRenderInfo renderInfo) {
        //Get the basic image info
        var img = renderInfo.GetImage().GetDrawingImage();
        var imgWidth = img.Width;
        var imgHeight = img.Height;
        img.Dispose();
        //Get the current transformation matrix
        var ctm = renderInfo.GetImageCTM();
        var ctmWidth = ctm[iTextSharp.text.pdf.parser.Matrix.I11];
        var ctmHeight = ctm[iTextSharp.text.pdf.parser.Matrix.I22];
        //Create new key for our page number if it doesn't exist already
        if (!this.Pages.ContainsKey(CurrentPage)) {
            this.Pages.Add(CurrentPage, new List<ImageScaleInfo>());
        }
        //Add our image info to this page
        this.Pages[CurrentPage].Add(new ImageScaleInfo(imgWidth, imgHeight, ctmWidth, ctmHeight, this.CurrentPageUnits));
    }
}

它使用这个辅助类来存储我们的信息:

public class ImageScaleInfo {
    //The page's unit space, almost always 72
    public Single PageUnits { get; set; }
    //The image's actual dimensions
    public System.Drawing.SizeF ImgSize { get; set; }
    //How the image is placed into the page
    public System.Drawing.SizeF CtmSize { get; set; }
    //Automatically calculate how the image is scaled
    public Single ImgWidthScale { get { return ImgSize.Width / CtmSize.Width; } }
    public Single ImgHeightScale { get { return ImgSize.Height / CtmSize.Height; } }
    //Helper constructor
    public ImageScaleInfo(Single imgWidth, Single imgHeight, Single ctmWidth, Single ctmHeight, Single pageUnits) {
        this.ImgSize = new System.Drawing.SizeF(imgWidth, imgHeight);
        this.CtmSize = new System.Drawing.SizeF(ctmWidth, ctmHeight);
        this.PageUnits = pageUnits;
    }
}

使用起来真的很简单:

//Create an instance of our helper class
var imgList = new MyImageRenderListener();
//Parse the PDF and inspect each image
using (var reader = new PdfReader(testFile)) {
    var proc = new iTextSharp.text.pdf.parser.PdfReaderContentParser(reader);
    for (var i = 1; i <= reader.NumberOfPages; i++) {
        //Get the page object itself
        var p = reader.GetPageN(i);
        //Get the page units. Per spec, page units are expressed as multiples of 1/72 of an inch with a default of 72.
        var pageUnits = (p.Contains(PdfName.USERUNIT) ? p.GetAsNumber(PdfName.USERUNIT).FloatValue : 72);
        //Set the page number so we can find it later
        imgList.CurrentPage = i;
        imgList.CurrentPageUnits = pageUnits;
        //Process the page
        proc.ProcessContent(i, imgList);
    }
}
//Dump out some information
foreach (var p in imgList.Pages) {
    foreach (var i in p.Value) {
        Console.WriteLine(String.Format("Image PPI is {0}x{1}", i.ImgWidthScale * i.PageUnits, i.ImgHeightScale * i.PageUnits));
    }
}

编辑

从@BrunoLowagie下面的评论中，我更新了上面的内容，删除了"magic 72"，并实际尝试查询文档，看看这是否已被覆盖。这种情况不太可能发生，但一两年后会有人发现一些晦涩难懂的PDF，然后抱怨这段代码不能工作，所以宁可安全也不要后悔。