Invert PDF Document Colors

Product Version	Product	Author
2021.1.322	RadPdfProcessing	Martin Velikov

Description

This article describes how to invert the background and text colors within a PDF document by iterating its content.

Solution

The provided code snippets demonstrates how to import a PDF document and invert its background and text color by iterating its content.

Import PDF file and Invert its Background and Content Color

RadFixedDocument document; 
PdfFormatProvider provider = new PdfFormatProvider(); 
using (Stream input = File.OpenRead(path)) 
{ 
    document = provider.Import(input); 
} 
 
TextColor = RgbColors.White; 
BackGroundColor = RgbColors.Black; 
 
InvertBackgroundColor(document); 
InvertContentColor(document); 
 
string exportedDocument = "ExportedDocument6.pdf"; 
if (File.Exists(exportedDocument)) 
{ 
    File.Delete(exportedDocument); 
} 
 
using (Stream output = File.OpenWrite(exportedDocument)) 
{ 
    provider.Export(document, output); 
} 

As the PDF document has no background concept we are creating a RectangleGeometry using the page size and setting the desired color to it.

Invert Document's Background Color

private static void InvertBackgroundColor(RadFixedDocument document) 
{ 
    foreach (RadFixedPage page in document.Pages) 
    { 
        Graphics.Path path = new Graphics.Path(); 
        path.Fill = BackGroundColor; 
        Graphics.RectangleGeometry rectangle = new Graphics.RectangleGeometry(page.MediaBox); 
        path.Geometry = rectangle; 
        page.Content.Insert(0, path); 
    } 
} 

The following methods show how to invert the document elements' color by recursively iterating their content and setting the desired color using the relevant Fill property.

Invert Document Content`s Color

private static void InvertContentColor(RadFixedDocument document) 
{ 
    foreach (RadFixedPage page in document.Pages) 
    { 
        InvertCollectionColors(page.Content); 
    } 
} 
 
private static void InvertCollectionColors(ContentElementCollection contentElementCollection) 
{ 
    foreach (ContentElementBase contentElement in contentElementCollection) 
    { 
        if (contentElement is TextFragment textFragment) 
        { 
            textFragment.Fill = TextColor; 
        } 
        else if (contentElement is Graphics.Path path && path.Fill.Equals(TextColor)) 
        { 
            path.Fill = BackGroundColor; 
        } 
        else if (contentElement is Form form) 
        { 
            InvertCollectionColors(form.FormSource.Content); 
        } 
    } 
}