iText 和 iTextSharp 有一些替换间接对象的方法。具体有PdfReader.KillIndirect()它按照它所说的去做PdfWriter.AddDirectImageSimple(iTextSharp.text.Image, PRIndirectReference)然后你可以用它来代替你杀死的东西。
在伪 C# 代码中，您可以执行以下操作：
var oldImage = PdfReader.GetPdfObject();
var newImage = YourImageCompressionFunction(oldImage);
PdfReader.KillIndirect(oldImage);
yourPdfWriter.AddDirectImageSimple(newImage, (PRIndirectReference)oldImage);
将原始字节转换为 .Net 图像可能很棘手，我将其留给您，或者您可以在此处搜索。马克有一个这里有很好的描述 https://stackoverflow.com/a/5953834/231316。另外，从技术上讲，PDF 没有 DPI 的概念，这主要是针对打印机的。在这里查看答案 https://stackoverflow.com/a/8245450/231316了解更多相关信息。
使用上述方法，您的压缩算法实际上可以做两件事：物理缩小图像以及应用 JPEG 压缩。当您物理缩小图像并将其添加回来时，它将占用与原始图像相同的空间，但可使用的像素更少。这将使您得到您认为的 DPI 降低。 JPEG 压缩是不言而喻的。
下面是一个针对 iTextSharp 5.1.1.0 的完整工作 C# 2010 WinForms 应用程序。它会获取桌面上名为“LargeImage.jpg”的现有 JPEG 并从中创建一个新的 PDF。然后它打开 PDF，提取图像，将其物理缩小到原始大小的 90%，应用 85% JPEG 压缩并将其写回 PDF。请参阅代码中的注释以获取更多解释。该代码需要更多的空/错误检查。还寻找NOTE您需要扩展以处理其他情况的注释。
using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;
using System.Windows.Forms;
using System.IO;
using iTextSharp.text;
using iTextSharp.text.pdf;
namespace WindowsFormsApplication1 {
public partial class Form1 : Form {
public Form1() {
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e) {
//Our working folder
string workingFolder = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);
//Large image to add to sample PDF
string largeImage = Path.Combine(workingFolder, "LargeImage.jpg");
//Name of large PDF to create
string largePDF = Path.Combine(workingFolder, "Large.pdf");
//Name of compressed PDF to create
string smallPDF = Path.Combine(workingFolder, "Small.pdf");
//Create a sample PDF containing our large image, for demo purposes only, nothing special here
using (FileStream fs = new FileStream(largePDF, FileMode.Create, FileAccess.Write, FileShare.None)) {
using (Document doc = new Document()) {
using (PdfWriter writer = PdfWriter.GetInstance(doc, fs)) {
doc.Open();
iTextSharp.text.Image importImage = iTextSharp.text.Image.GetInstance(largeImage);
doc.SetPageSize(new iTextSharp.text.Rectangle(0, 0, importImage.Width, importImage.Height));
doc.SetMargins(0, 0, 0, 0);
doc.NewPage();
doc.Add(importImage);
doc.Close();
}
}
}
//Now we're going to open the above PDF and compress things
//Bind a reader to our large PDF
PdfReader reader = new PdfReader(largePDF);
//Create our output PDF
using (FileStream fs = new FileStream(smallPDF, FileMode.Create, FileAccess.Write, FileShare.None)) {
//Bind a stamper to the file and our reader
using (PdfStamper stamper = new PdfStamper(reader, fs)) {
//NOTE: This code only deals with page 1, you'd want to loop more for your code
//Get page 1
PdfDictionary page = reader.GetPageN(1);
//Get the xobject structure
PdfDictionary resources = (PdfDictionary)PdfReader.GetPdfObject(page.Get(PdfName.RESOURCES));
PdfDictionary xobject = (PdfDictionary)PdfReader.GetPdfObject(resources.Get(PdfName.XOBJECT));
if (xobject != null) {
PdfObject obj;
//Loop through each key
foreach (PdfName name in xobject.Keys) {
obj = xobject.Get(name);
if (obj.IsIndirect()) {
//Get the current key as a PDF object
PdfDictionary imgObject = (PdfDictionary)PdfReader.GetPdfObject(obj);
//See if its an image
if (imgObject.Get(PdfName.SUBTYPE).Equals(PdfName.IMAGE)) {
//NOTE: There's a bunch of different types of filters, I'm only handing the simplest one here which is basically raw JPG, you'll have to research others
if (imgObject.Get(PdfName.FILTER).Equals(PdfName.DCTDECODE)) {
//Get the raw bytes of the current image
byte[] oldBytes = PdfReader.GetStreamBytesRaw((PRStream)imgObject);
//Will hold bytes of the compressed image later
byte[] newBytes;
//Wrap a stream around our original image
using (MemoryStream sourceMS = new MemoryStream(oldBytes)) {
//Convert the bytes into a .Net image
using (System.Drawing.Image oldImage = Bitmap.FromStream(sourceMS)) {
//Shrink the image to 90% of the original
using (System.Drawing.Image newImage = ShrinkImage(oldImage, 0.9f)) {
//Convert the image to bytes using JPG at 85%
newBytes = ConvertImageToBytes(newImage, 85);
}
}
}
//Create a new iTextSharp image from our bytes
iTextSharp.text.Image compressedImage = iTextSharp.text.Image.GetInstance(newBytes);
//Kill off the old image
PdfReader.KillIndirect(obj);
//Add our image in its place
stamper.Writer.AddDirectImageSimple(compressedImage, (PRIndirectReference)obj);
}
}
}
}
}
}
}
this.Close();
}
//Standard image save code from MSDN, returns a byte array
private static byte[] ConvertImageToBytes(System.Drawing.Image image, long compressionLevel) {
if (compressionLevel < 0) {
compressionLevel = 0;
} else if (compressionLevel > 100) {
compressionLevel = 100;
}
ImageCodecInfo jgpEncoder = GetEncoder(ImageFormat.Jpeg);
System.Drawing.Imaging.Encoder myEncoder = System.Drawing.Imaging.Encoder.Quality;
EncoderParameters myEncoderParameters = new EncoderParameters(1);
EncoderParameter myEncoderParameter = new EncoderParameter(myEncoder, compressionLevel);
myEncoderParameters.Param[0] = myEncoderParameter;
using (MemoryStream ms = new MemoryStream()) {
image.Save(ms, jgpEncoder, myEncoderParameters);
return ms.ToArray();
}
}
//standard code from MSDN
private static ImageCodecInfo GetEncoder(ImageFormat format) {
ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();
foreach (ImageCodecInfo codec in codecs) {
if (codec.FormatID == format.Guid) {
return codec;
}
}
return null;
}
//Standard high quality thumbnail generation from http://weblogs.asp.net/gunnarpeipman/archive/2009/04/02/resizing-images-without-loss-of-quality.aspx
private static System.Drawing.Image ShrinkImage(System.Drawing.Image sourceImage, float scaleFactor) {
int newWidth = Convert.ToInt32(sourceImage.Width * scaleFactor);
int newHeight = Convert.ToInt32(sourceImage.Height * scaleFactor);
var thumbnailBitmap = new Bitmap(newWidth, newHeight);
using (Graphics g = Graphics.FromImage(thumbnailBitmap)) {
g.CompositingQuality = CompositingQuality.HighQuality;
g.SmoothingMode = SmoothingMode.HighQuality;
g.InterpolationMode = InterpolationMode.HighQualityBicubic;
System.Drawing.Rectangle imageRectangle = new System.Drawing.Rectangle(0, 0, newWidth, newHeight);
g.DrawImage(sourceImage, imageRectangle);
}
return thumbnailBitmap;
}
}
}
