How to extract quantization matrices from a JPEG in C#

Question

Is it possible to extract the quantization matrices of a JPG file in C#? I have found the libjpeg.NET but I cannot figure out how to retrieve the QTs matrices. Please find below my code.

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using BitMiracle.LibJpeg;

namespace ConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {

            string file = @"PATH_TO_FILE";
            JpegImage img = new JpegImage(file);    
            Console.Read();            
            // Ideally, there should be some img.GetQuantizationMatrix() method    
        }
    }
}

Answer 1

If you just want the quantization tables (and nothing else), it is a simple matter to scan the JPEG stream for the DQT marker (FFDB-followed by a 2-byte length) and extract the values.

You do not need to decode the image to get this information.

Answer 2

Thanks to the suggestion by @user3344003, I managed to read the JPEG header to extract quantization matrix information. I followed the file layout spec presented in this web page.

void Main()
{   
    string folder = @"PATH_TO_FILE";
    var files = Directory.GetFiles(folder, "*jpg", SearchOption.AllDirectories);

    byte dqt = 0xDB;

    foreach (string file in files)
    {
        file.Dump();

        byte[] s = File.ReadAllBytes(file);

        for (int i = 0; i < s.Length-1; i++) {
            byte b1 = s[i];
            byte b2 = s[i+1];

            if (b1 == 0xff && b2 == 0xdb) {

                int field_length = s[i + 2] + s[i + 3];
                int matrix_length = field_length - 3;
                int qt_info = s[i + 4];
                ("QT info" + qt_info).Dump();
                ("QT Matrix").Dump();
                byte[] mat = new byte[matrix_length];

                for (int k = 0; k < matrix_length; k++) {
                    mat[k] = s[i+5+k];
                }
                Print8x8Matrix(mat);    
            }
        }
    }
}
public static void Print8x8Matrix(byte[] bytes)
{
    string s = "";
    for (int i= 0; i < bytes.Length; i++) {
        s += bytes[i] + " ";
        if (i % 8 == 0)
        {
            s.Dump();
            s="";
        }
    }

}