Converting Colored PDF Document Images to GrayScale in .NET Standard

Environment

Version	Product	Author
2025.3.806	RadPdfProcessing	Desislava Yordanova

Description

Convert colored images in PDF documents using Telerik PdfProcessing in an ASP.NET Core (.NET 9) environment.

Solution

To convert colored PDFs to grayscale in ASP.NET Core (.NET 9), follow these steps:

1. Use the Telerik PdfProcessing library to load and manipulate PDF content.
2. Replace the image processing logic with a custom implementation using System.Drawing.

Here is the complete code example:

using System;
using System.Diagnostics;
using System.Drawing;
using System.IO;
using Telerik.Windows.Documents.Fixed.FormatProviders.Pdf;
using Telerik.Windows.Documents.Fixed.Model;
using Telerik.Windows.Documents.Fixed.Model.Objects;
using Telerik.Windows.Documents.ImageUtils;

class Program
{
    static void Main(string[] args)
    {
        // Initialize ImagePropertiesResolver
        ImagePropertiesResolver defaultImagePropertiesResolver = new ImagePropertiesResolver();
        Telerik.Windows.Documents.Extensibility.FixedExtensibilityManager.ImagePropertiesResolver = defaultImagePropertiesResolver;

        string inputPath = "colored.pdf"; // Path to the input PDF
        string outputPath = "grayscale.pdf"; // Path for the grayscale output

        ConvertPdfToGrayscale(inputPath, outputPath); 

        Process.Start(new ProcessStartInfo() { FileName = outputPath, UseShellExecute = true });
    }

    /// <summary>
    /// Converts all content in a PDF document to grayscale.
    /// </summary>
    public static void ConvertPdfToGrayscale(string inputPath, string outputPath)
    {
        PdfFormatProvider provider = new PdfFormatProvider();
        RadFixedDocument document = provider.Import(File.ReadAllBytes(inputPath), TimeSpan.FromSeconds(10));

        foreach (RadFixedPage page in document.Pages)
        {
            foreach (ContentElementBase element in page.Content)
            {
                Image img = element as Image;
                if (img != null)
                {
                    ImageSource source = img.ImageSource;
                    EncodedImageData encodedImageData = source.GetEncodedImageData();
                    byte[] original = encodedImageData.Data;

                    Bitmap grayBitmap = ConvertToGrayscale(original);

                    // Save the grayscale bitmap to a temporary file
                    string tempImagePath = "temp_grayscale_image.jpeg";
                    grayBitmap.Save(tempImagePath, System.Drawing.Imaging.ImageFormat.Jpeg);

                    // Update the image source in the PDF
                    using (FileStream fs = File.Open(tempImagePath, FileMode.Open))
                    {
                        ImageSource grayscaleImageSource = new ImageSource(fs);
                        img.ImageSource = grayscaleImageSource;
                    }
                }
            }
        }

        // Save the modified PDF to the output path
        if (File.Exists(outputPath))
        {
            File.Delete(outputPath);
        }

        File.WriteAllBytes(outputPath, provider.Export(document, TimeSpan.FromSeconds(10)));
    }

    /// <summary>
    /// Converts a byte array of image data to grayscale.
    /// </summary>
    public static Bitmap ConvertToGrayscale(byte[] imageBytes)
    {
        using (var ms = new MemoryStream(imageBytes))
        {
            using (var originalImage = new Bitmap(ms))
            {
                Bitmap grayBitmap = new Bitmap(originalImage.Width, originalImage.Height);

                for (int y = 0; y < originalImage.Height; y++)
                {
                    for (int x = 0; x < originalImage.Width; x++)
                    {
                        Color originalColor = originalImage.GetPixel(x, y);
                        int grayValue = (int)(originalColor.R * 0.3 + originalColor.G * 0.59 + originalColor.B * 0.11);
                        Color grayColor = Color.FromArgb(grayValue, grayValue, grayValue);
                        grayBitmap.SetPixel(x, y, grayColor);
                    }
                }
                return grayBitmap;
            }
        }
    }
}

See Also

• Converting Colored PDF Documents to GrayScale in .NET Framework