Extensions.cs

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace HaarImageCropper
{
    public class Extensions
    {
        public static Bitmap CopyAsGrayscale(Image sourceImage)
        {
            Bitmap bmpNew = GetArgbCopy(sourceImage);
            BitmapData bmpData = bmpNew.LockBits(new Rectangle(0, 0, sourceImage.Width, sourceImage.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
            IntPtr ptr = bmpData.Scan0;
            byte[] byteBuffer = new byte[bmpData.Stride * bmpNew.Height];
            Marshal.Copy(ptr, byteBuffer, 0, byteBuffer.Length);
            float rgb = 0;
            for (int k = 0; k < byteBuffer.Length; k += 4)
            {
                rgb = byteBuffer[k] * 0.11f;
                rgb += byteBuffer[k + 1] * 0.59f;
                rgb += byteBuffer[k + 2] * 0.3f;
                byteBuffer[k] = (byte)rgb;
                byteBuffer[k + 1] = byteBuffer[k];
                byteBuffer[k + 2] = byteBuffer[k];
                byteBuffer[k + 3] = 255;
            }
            Marshal.Copy(byteBuffer, 0, ptr, byteBuffer.Length);
            bmpNew.UnlockBits(bmpData);
            return bmpNew;
        }

        private static Bitmap GetArgbCopy(Image sourceImage)
        {
            Bitmap bmpNew = new Bitmap(sourceImage.Width, sourceImage.Height, PixelFormat.Format32bppArgb);
            using (Graphics graphics = Graphics.FromImage(bmpNew))
            {
                graphics.DrawImage(sourceImage, new Rectangle(0, 0, bmpNew.Width, bmpNew.Height), new Rectangle(0, 0, bmpNew.Width, bmpNew.Height), GraphicsUnit.Pixel);
                graphics.Flush();
            }
            return bmpNew;
        }
    }
}