iTextSharp 提取签名图像

原文
本文使用 iTextSharp 5.5.13.2，记录使用 iTextSharp 提取图片时，获得的知识点。

pdf 中的签名并不是单纯的一张图片，它是由一张基础的底色图和一张蒙版图片组成。需要提取底图和Mask。

下面是提取签名的底图和Mask的方法

public class ExtImageRenderListener : IRenderListener
{
    public List<System.Drawing.Image> Images = new List<System.Drawing.Image>();
    public List<System.Drawing.Image> Masks = new List<System.Drawing.Image>();

    public void BeginTextBlock()
    { }

    public void EndTextBlock()
    { }

    public void RenderText(TextRenderInfo renderInfo)
    { }

    public void RenderImage(ImageRenderInfo renderInfo)
    {
        PdfImageObject imageObject = renderInfo.GetImage();
        if (imageObject == null)
        {
            Console.WriteLine("Image {0} could not be read.", renderInfo.GetRef().Number);
        }
        else
        {
            Images.Add(imageObject.GetDrawingImage());
            PRStream maskStream = (PRStream)imageObject.GetDictionary().GetAsStream(PdfName.SMASK);
            if (maskStream != null)
            {
                PdfImageObject maskImageObject = new PdfImageObject(maskStream);
                Masks.Add(maskImageObject.GetDrawingImage());
            }
            else
            {
                Masks.Add(null);
            }
        }
    }
}

String source = @"E:\1.pdf";
int count = 0;

using (PdfReader sourceReader = new PdfReader(source))
{

    AcroFields acroFields = sourceReader.AcroFields;
    foreach (string name in acroFields.GetSignatureNames())
    {
        PdfStream xObject = (PdfStream)PdfReader.GetPdfObjectRelease(sourceReader.AcroFields.GetNormalAppearance(name));
        PdfDictionary resources = xObject.GetAsDict(PdfName.RESOURCES);
        ExtImageRenderListener strategy = new ExtImageRenderListener();
        PdfContentStreamProcessor processor = new PdfContentStreamProcessor(strategy);
        processor.ProcessContent(ContentByteUtils.GetContentBytesFromContentObject(xObject), resources);
        System.Drawing.Image drawingImage = strategy.Images[0];
        System.Drawing.Image drawingMask = strategy.Masks[0];
        if (drawingImage != null)
        {
            drawingImage.Save(count++ + @".png");
        }
        if (drawingMask != null)
        {
            drawingMask.Save(count++ + @"Mask.png");
        }
    }
}