我正在开发一个显示 PDF 并允许用户订购文档副本的 Web 应用程序。我们希望在显示 PDF 时即时添加文本，例如“未付费”或“样品”。我已经使用 itextsharp 完成了这项工作。然而，页面图像很容易从水印文本中分离出来，并使用各种免费软件程序提取。
如何将水印添加到PDF中的页面，但将页面图像和水印拼合在一起，使水印成为pdf页面图像的一部分，从而防止水印被去除（除非该人想使用Photoshop）？
【问题讨论】：
标签： asp.net pdf itextsharp
如果我是你，我会走另一条路。使用 iTextSharp（或其他库）将给定文档的每一页提取到一个文件夹中。然后使用一些程序（Ghostscript、Photoshop，也许是 GIMP），您可以将每个页面批量转换为图像。然后将覆盖文本写入图像。最后使用 iTextSharp 将每个文件夹中的所有图像组合回一个 PDF。
我知道这听起来很痛苦，但我假设每个文档你应该只需要这样做一次。
如果您不想走这条路，让我告诉您提取图像所需的操作。下面的大部分代码来自this post。在代码的末尾，我将图像保存到桌面。由于您有原始字节，因此您也可以轻松地将它们泵入System.Drawing.Image 对象并将它们写回听起来像您熟悉的新PdfWriter 对象。以下是针对 iTextSharp 5.1.1.0 的完整工作 WinForms 应用程序
Option Explicit On
Option Strict On
Imports iTextSharp.text
Imports iTextSharp.text.pdf
Imports System.IO
Imports System.Runtime.InteropServices
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
''//File to process
Dim InputFile = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Desktop), "SampleImage.pdf")
''//Bind a reader to our PDF
Dim R As New PdfReader(InputFile)
''//Setup some variable to use below
Dim bytes() As Byte
Dim obj As PdfObject
Dim pd As PdfDictionary
Dim filter, width, height, bpp As String
Dim pixelFormat As System.Drawing.Imaging.PixelFormat
Dim bmp As System.Drawing.Bitmap
Dim bmd As System.Drawing.Imaging.BitmapData
''//Loop through all of the references in the file
Dim xo = R.XrefSize
For I = 0 To xo - 1
''//Get the object
obj = R.GetPdfObject(I)
''//Make sure we have something and that it is a stream
If (obj IsNot Nothing) AndAlso obj.IsStream() Then
''//Case it to a dictionary object
pd = DirectCast(obj, PdfDictionary)
''//See if it has a subtype property that is set to /IMAGE
If pd.Contains(PdfName.SUBTYPE) AndAlso pd.Get(PdfName.SUBTYPE).ToString() = PdfName.IMAGE.ToString() Then
''//Grab various properties of the image
filter = pd.Get(PdfName.FILTER).ToString()
width = pd.Get(PdfName.WIDTH).ToString()
height = pd.Get(PdfName.HEIGHT).ToString()
bpp = pd.Get(PdfName.BITSPERCOMPONENT).ToString()
''//Grab the raw bytes of the image
bytes = PdfReader.GetStreamBytesRaw(DirectCast(obj, PRStream))
''//Images can be encoded in various ways. /DCTDECODE is the simplest because its essentially JPEG and can be treated as such.
''//If your PDFs contain the other types you will need to figure out how to handle those on your own
Select Case filter
Case PdfName.ASCII85DECODE.ToString()
Throw New NotImplementedException("Decoding this filter has not been implemented")
Case PdfName.ASCIIHEXDECODE.ToString()
Throw New NotImplementedException("Decoding this filter has not been implemented")
Case PdfName.FLATEDECODE.ToString()
''//This code from https://stackoverflow.com/questions/802269/itextsharp-extract-images/1220959#1220959
bytes = pdf.PdfReader.FlateDecode(bytes, True)
Select Case Integer.Parse(bpp)
Case 1
pixelFormat = Drawing.Imaging.PixelFormat.Format1bppIndexed
Case 24
pixelFormat = Drawing.Imaging.PixelFormat.Format24bppRgb
Case Else
Throw New Exception("Unknown pixel format " + bpp)
End Select
bmp = New System.Drawing.Bitmap(Int32.Parse(width), Int32.Parse(height), pixelFormat)
bmd = bmp.LockBits(New System.Drawing.Rectangle(0, 0, Int32.Parse(width), Int32.Parse(height)), System.Drawing.Imaging.ImageLockMode.WriteOnly, pixelFormat)
Marshal.Copy(bytes, 0, bmd.Scan0, bytes.Length)
bmp.UnlockBits(bmd)
Using ms As New MemoryStream
bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg)
bytes = ms.GetBuffer()
End Using
Case PdfName.LZWDECODE.ToString()
Throw New NotImplementedException("Decoding this filter has not been implemented")
Case PdfName.RUNLENGTHDECODE.ToString()
Throw New NotImplementedException("Decoding this filter has not been implemented")
Case PdfName.DCTDECODE.ToString()
''//Bytes should be raw JPEG so they should not need to be decoded, hopefully
Case PdfName.CCITTFAXDECODE.ToString()
Throw New NotImplementedException("Decoding this filter has not been implemented")
Case PdfName.JBIG2DECODE.ToString()
Throw New NotImplementedException("Decoding this filter has not been implemented")
Case PdfName.JPXDECODE.ToString()
Throw New NotImplementedException("Decoding this filter has not been implemented")
Case Else
Throw New ApplicationException("Unknown filter found : " & filter)
End Select
''//At this points the byte array should contain a valid JPEG byte data, write to disk
My.Computer.FileSystem.WriteAllBytes(Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Desktop), I & ".jpg"), bytes, False)
End If
End If
Next
Me.Close()
End Sub
End Class
【讨论】：
必须将整个页面呈现为图像。否则，您将获得“文本对象”（文本的单个单词/字母）和水印对象（叠加图像），它们始终是页面的不同/单独部分。
【讨论】：