c＃.NET CORE使用ITextSharp为现有PDF添加透明图像 [英] c# .NET CORE adding image with transparency to existing PDF using ITextSharp

问题描述

我的目标是在现有pdf（非水印）的每个页面上添加公司徽标。

My goal is to add company logo to every page of an existing pdf(not watermark).

由于pdf文件和徽标细节，我只能放置pdf内容（不在下面）上面的徽标，徽标必须支持透明度。

Due to pdf file and logo specifics, I can only place the logo on top of the pdf content(not underneath) and the logo has to support transparency.

另一个限制是我必须使用.NET Core。

One more limitation is I have to use .NET Core.

发布这个答案，因为我找不到明确的解决方案。
建议/更正/改进是受欢迎的。

Posting this with an answer, because I could not find a clear solution. Suggestions/corrections/improvements are welcome.

希望有人觉得这很有用。

Hope someone finds this useful.

推荐答案

支持.NET Core的最新iTextSharp库是 iText7 ，但我不能合法使用它;既不是我的代码开源，也不是购买许可证是我的选择。因此我使用旧的第三方库：

The newest iTextSharp library to support .NET Core is iText7 however I cannot use it legitemately; neither making my code open source, nor purchasing the licence is an option for me. Therefore I use old, third party library:

Install-Package iTextSharp.LGPLv2.Core

我发布的最新版本是1.3.2

Latest version, the one I'm using, at the time of this post is 1.3.2

需要使用以下

using System;
using System.Drawing.Imaging;
using System.IO;
using iTextSharp.text;
using iTextSharp.text.pdf;

要获得pdf中的图像透明度，必须以正确的格式打开图像

To acheve image transparency in pdf, image has to be opened in a correct format

var preImage = System.Drawing.Image.FromFile(imagePath);
var image = Image.GetInstance(preImage, ImageFormat.Png);

添加图片时，选择要内嵌的图片也很重要

When adding the image, it is also important to not select the image to be inline

canvas.AddImage(image);//do not put .AddImage(image, true);

以下是所有代码

var imagePath = "logo.png";
var pdfPath = "edit_this.pdf";

//load pdf file
var pdfBytes = File.ReadAllBytes(pdfPath);
var oldFile = new PdfReader(pdfBytes);

//load image
var preImage = System.Drawing.Image.FromFile(imagePath);
var image = Image.GetInstance(preImage, ImageFormat.Png);
preImage.Dispose();

//optional: if image is wider than the page, scale down the image to fit the page
var sizeWithRotation = oldFile.GetPageSizeWithRotation(1);
if (image.Width > sizeWithRotation.Width)
    image.ScalePercent(sizeWithRotation.Width / image.Width * 100);

//set image position in top left corner
//in pdf files, cooridinates start in the left bottom corner
image.SetAbsolutePosition(0, sizeWithRotation.Height - image.ScaledHeight);

//in production, I use MemoryStream
//I put FileStream here to test the code in console application
using (var newFileStream = new FileStream("with_logo.pdf", FileMode.Create))
{
    //setup PdfStamper
    var stamper = new PdfStamper(oldFile, newFileStream);

    //iterate through the pages in the original file
    for (var i = 1; i <= oldFile.NumberOfPages; i++)
    {
        //get canvas for current page
        var canvas = stamper.GetOverContent(i);
        //add image with pre-set position and size
        canvas.AddImage(image);
    }

    stamper.Close();
}

此代码适用于本地文件。
在我的（真实世界）情况下，我收到pdf文件作为Base64字符串，从本地存储添加徽标，将其转换回Base64字符串并将其输出到网页上。

This code works with local files. In my (real world) case, I receive pdf files as Base64 string, add a logo from local storage, convert it back to Base64 string and output it on a web-page.

我强行打开图像（硬编码）因为我控制了徽标的扩展名。如有必要，您可以动态设置图像格式。

I open the image as PNG forcefully(hardcoded) because I control what extension does the logo have. If necessary you can dynamicaly set the image format.

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