How to Use Base URLs & Asset Encoding

Chaknith Bin

January 25, 2023

Updated October 20, 2024

IronPDF is a useful tool for generating PDF documents in .NET projects.

A common use of this library is “HTML to PDF” rendering - where HTML is used as the design language for rendering a PDF document. A logical question is: how can we use CSS stylesheets and Image files in our HTML to PDF Conversion?

How to Use HTML with CSS and Images

Download IronPDF for HTML to CSS to Image Support
Specify BaseUrlOrPath parameter to use external images in HTML
Configure the correct src in MVC to show both in web and output PDF
Specify BaseUrl property to work with custom header and footer
Check the output PDF

Rendering a PDF from HTML String with Image and CSS Assets

When working with HTML String to PDF conversion, it's important to set a BaseUrlOrPath parameter for assets such as CSS, JavaScript files, and images. The BaseUrlOrPath specifies the base URL from which all assets will be loaded relative to.

This can be a web URL starting with 'http' to load remote assets or a local file path to access assets on your disk. Setting the BaseUrlOrPath correctly ensures that the assets are loaded correctly during the conversion process.

:path=/static-assets/pdf/content-code-examples/how-to/base-urls-baseurl.cs

using IronPdf;

// Instantiate ChromePdfRenderer
ChromePdfRenderer renderer = new ChromePdfRenderer();

string baseUrl = @"C:\site\assets\";
string html = "<img src='icons/iron.png'>";

// Render HTML to PDF
PdfDocument pdf = renderer.RenderHtmlAsPdf(html, baseUrl);

// Export PDF
pdf.SaveAs("html-with-assets.pdf");

Imports IronPdf

' Instantiate ChromePdfRenderer
Private renderer As New ChromePdfRenderer()

Private baseUrl As String = "C:\site\assets\"
Private html As String = "<img src='icons/iron.png'>"

' Render HTML to PDF
Private pdf As PdfDocument = renderer.RenderHtmlAsPdf(html, baseUrl)

' Export PDF
pdf.SaveAs("html-with-assets.pdf")

$vbLabelText $csharpLabel

MVC application

In an MVC application, specifying the file path of an image can be challenging. To ensure that the image can be found by IronPDF and displayed correctly on the website, the baseUrl of IronPDF and src="" attribute on HTML string need to be configured correctly.

With the file hierarchy shown below set

baseUrlOrPath to @"wwwroot/image"
src attribute to "../image/Sample.jpg"

wwwroot
└── image
    ├── Sample.jpg
    └── Sample.png

For example:

:path=/static-assets/pdf/content-code-examples/how-to/base-mvc.cs

// Instantiate ChromePdfRenderer
ChromePdfRenderer renderer = new ChromePdfRenderer();

// Render HTML to PDF
PdfDocument pdf = renderer.RenderHtmlAsPdf("html.Result", @"wwwroot/image");

' Instantiate ChromePdfRenderer
Dim renderer As New ChromePdfRenderer()

' Render HTML to PDF
Dim pdf As PdfDocument = renderer.RenderHtmlAsPdf("html.Result", "wwwroot/image")

$vbLabelText $csharpLabel

<img src="../image/Sample.jpg"/>
<img src="../image/Sample.png"/>

<img src="../image/Sample.jpg"/>
<img src="../image/Sample.png"/>

HTML

Before proceeding

File Path Formats That Do Not Work

The following formats work well when viewing on Chrome browser, but point to the wrong folder directory when used in an MVC app. These formats still work with IronPDF if baseUrlOrPath is provided in the RenderHtmlAsPdf method:

<img src="image/footer.png"/>  
<img src="./image/footer.png"/>

<img src="image/footer.png"/>  
<img src="./image/footer.png"/>

HTML

On the other hand, these formats work well with MVC app, but when it comes to file path, these formats do not work well in IronPDF:

<img src="/image/footer.png"/>  
<img src="~/image/footer.png"/>

<img src="/image/footer.png"/>  
<img src="~/image/footer.png"/>

HTML

HTML Headers and Footers with Images and Assets

When we render HTML headers and footers to a new or existing PDF, they are treated as standalone HTML documents and do not inherit the BaseURL from the PDF itself.

We should set a BaseURL from which assets may be loaded:

:path=/static-assets/pdf/content-code-examples/how-to/base-header-footer.cs

using IronPdf;
using System;

// Instantiate ChromePdfRenderer
ChromePdfRenderer renderer = new ChromePdfRenderer();

// Add header
renderer.RenderingOptions.HtmlHeader = new HtmlHeaderFooter()
{
    MaxHeight = 20,
    HtmlFragment = "<img src='logo.png'>",
    BaseUrl = new Uri(@"C:\assets\images\").AbsoluteUri
};

Imports IronPdf
Imports System

' Instantiate ChromePdfRenderer
Private renderer As New ChromePdfRenderer()

' Add header
renderer.RenderingOptions.HtmlHeader = New HtmlHeaderFooter() With {
	.MaxHeight = 20,
	.HtmlFragment = "<img src='logo.png'>",
	.BaseUrl = (New Uri("C:\assets\images\")).AbsoluteUri
}

$vbLabelText $csharpLabel

HTML File to PDF with CSS, JS and Image Assets

When rendering an HTML file to PDF, all assets will be assumed local to that file.

:path=/static-assets/pdf/content-code-examples/how-to/base-html-file.cs

using IronPdf;

// Instantiate ChromePdfRenderer
ChromePdfRenderer renderer = new ChromePdfRenderer();

// Render HTML file to PDF
PdfDocument pdf = renderer.RenderHtmlFileAsPdf("C:\\Assets\\TestInvoice1.html");

// Export PDF
pdf.SaveAs("Invoice.pdf");

Imports IronPdf

' Instantiate ChromePdfRenderer
Private renderer As New ChromePdfRenderer()

' Render HTML file to PDF
Private pdf As PdfDocument = renderer.RenderHtmlFileAsPdf("C:\Assets\TestInvoice1.html")

' Export PDF
pdf.SaveAs("Invoice.pdf")

$vbLabelText $csharpLabel

In the above example, all JS, CSS and image files will be loaded from the C:\Assets folder on the disk - the same directory that the HTML file is located.

For convenience, you may use CustomCssUrl in ChromePdfRenderOptions for Additional Stylesheets to specify an additional stylesheet that is only used for .NET PDF rendering if you so desire. For example:

:path=/static-assets/pdf/content-code-examples/how-to/base-html-file-baseurl.cs

using IronPdf;

// Instantiate ChromePdfRenderer
ChromePdfRenderer renderer = new ChromePdfRenderer();

// Set additional CSS url
renderer.RenderingOptions.CustomCssUrl = "./style.css";

// Render HTML file to PDF
PdfDocument pdf = renderer.RenderHtmlAsPdf("<h1>Hello World</h1>");

// Export PDF
pdf.SaveAs("tryCss.pdf");

Imports IronPdf

' Instantiate ChromePdfRenderer
Private renderer As New ChromePdfRenderer()

' Set additional CSS url
renderer.RenderingOptions.CustomCssUrl = "./style.css"

' Render HTML file to PDF
Dim pdf As PdfDocument = renderer.RenderHtmlAsPdf("<h1>Hello World</h1>")

' Export PDF
pdf.SaveAs("tryCss.pdf")

$vbLabelText $csharpLabel

Please note

The ChromePdfRenderOptions.CustomCssUrl property is currently only functional when rendering from an HTML string to a PDF document using the RenderHtmlAsPdf method.

Image Asset Encoding

Image assets can be encoded directly into the HTML file or string, which can help avoid some of the frustrating issues of images not being found. To do this we can utilize the use of base64:

First obtain the binary data of the image either by reading the image file or receiving it through a network request.
Use the Convert.ToBase64String method in Microsoft .NET to convert the binary data to base64.
Construct the image tag in HTML using "data:image/svg+xml;base64," before the base64 data. You may have noticed that the image type is being specified before the base64 data. Visit the MDN Web Docs on Image Types and Formats for more information on image format types.

:path=/static-assets/pdf/content-code-examples/how-to/add-images-to-pdfs-base64-image.cs

using IronPdf;
using System;
using System.IO;

ChromePdfRenderer renderer = new ChromePdfRenderer();

// Import image file binary data
byte[] binaryData = File.ReadAllBytes("ironpdf-logo-text-dotnet.svg");

// Convert the binary data to base 64
string imgDataUri = Convert.ToBase64String(binaryData);

// Embed in HTML
string html = $"<img src='data:image/svg+xml;base64,{imgDataUri}'>";

// Convert HTML to PDF
PdfDocument pdf = renderer.RenderHtmlAsPdf(html);

// Export the PDF
pdf.SaveAs("embedImageBase64.pdf");

Imports IronPdf
Imports System
Imports System.IO

Private renderer As New ChromePdfRenderer()

' Import image file binary data
Private binaryData() As Byte = File.ReadAllBytes("ironpdf-logo-text-dotnet.svg")

' Convert the binary data to base 64
Private imgDataUri As String = Convert.ToBase64String(binaryData)

' Embed in HTML
Private html As String = $"<img src='data:image/svg+xml;base64,{imgDataUri}'>"

' Convert HTML to PDF
Private pdf As PdfDocument = renderer.RenderHtmlAsPdf(html)

' Export the PDF
pdf.SaveAs("embedImageBase64.pdf")

$vbLabelText $csharpLabel

Chaknith Bin

Chat with engineering team now

Software Engineer

Chaknith works on IronXL and IronBarcode. He has deep expertise in C# and .NET, helping improve the software and support customers. His insights from user interactions contribute to better products, documentation, and overall experience.