c# .NET CORE adding image with transparency to exi

Question

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

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.

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

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

Hope someone finds this useful.

Answer 1

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

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

Following usings are required

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

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);

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);

Here is all the code

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();
}

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.

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.