「WPF」画像ファイルをグレースケールへ変換

using System;
using System.IO;
using System.Windows.Media;
using System.Windows.Media.Imaging;

namespace WPFLibA;

public class Class1
{
    public static void Bgr32ToGray(string srcFile, string dstFile)
    {
        using var inputStream = new FileStream(srcFile, FileMode.Open, FileAccess.Read);
        var img = new BitmapImage();
        img.BeginInit();
        img.CreateOptions = BitmapCreateOptions.None;
        img.CacheOption = BitmapCacheOption.OnLoad;
        img.StreamSource = inputStream;
        img.EndInit();
        img.Freeze();

        int w = (int)img.PixelWidth;
        int h = (int)img.PixelHeight;
        int stride = (w * img.Format.BitsPerPixel + 7) / 8;
        int ch = img.Format.BitsPerPixel / 8;
        int bufSize = stride * h;
        byte[] inputBuf = new byte[bufSize];
        byte[] outputBuf = new byte[w * h];

        img.CopyPixels(inputBuf, stride, 0);
        
        for(int y = 0; y < h; y++)
        {
            for(int x = 0; x < w; x++)
            {
                int i = (stride * y) + (x * ch);
                int j =  w * y + x;
                
                int avg = 0;
                for (int k = 0; k < ch; k++)
                {
                    avg += inputBuf[i + k];
                }
                avg = avg / ch;

                outputBuf[j] = (byte)avg;
            }
        }
        
        BitmapSource result = BitmapSource.Create(
            w,
            h,
            img.DpiX,
            img.DpiY,
            PixelFormats.Gray8,
            BitmapPalettes.Gray256,
            outputBuf,
            w
        );

        using var outputStream = new FileStream(dstFile, FileMode.Create, FileAccess.Write);
        var encoder = new PngBitmapEncoder();
        var frame = BitmapFrame.Create(result);
        encoder.Frames.Add(frame);
        encoder.Save(outputStream);

    }
}

using WPFLibA;

public class Program1
{
    static void Main()
    {
        string colorFile = @"H:\Pictures\202307131103.PNG";
        string grayFile = @"H:\csharp\WPFLibA.Console\output.png";
        Class1.Bgr32ToGray(colorFile, grayFile);
    }
}