.NET (Core and Framework)

<System.Runtime.CompilerServices.Extension()> _
Public Function ToGrayScale(ByVal img As Image, ByVal method As Methods) As Image
   If Not [Enum].IsDefined(GetType(Methods), method) Then
      Return Nothing
   End If

   ' set the grayscale function to use for conversion
   Dim gsfunc As Func(Of Color, Color) = Nothing
   Select Case method
      Case Methods.Average : gsfunc = AddressOf Average
      Case Methods.CorrectingForTheHumanEye_Gimp : gsfunc = AddressOf Gimp
      Case Methods.CorrectingForTheHumanEye_BT_601 : gsfunc = AddressOf BT_601
      Case Methods.CorrectingForTheHumanEye_BT_709 : gsfunc = AddressOf BT_709
      Case Methods.Desaturation : gsfunc = AddressOf Desaturation
      Case Methods.DecompositionMax : gsfunc = AddressOf DecompositionMax
      Case Methods.DecompositionMin : gsfunc = AddressOf DecompositionMin
      Case Methods.SingleColorRed : gsfunc = AddressOf SingleColorRed
      Case Methods.SingleColorGreen : gsfunc = AddressOf SingleColorGreen
      Case Methods.SingleColorBlue : gsfunc = AddressOf SingleColorBlue
   End Select

   ' create a 32bppArgb bitmap from source image
   Dim bm As New Bitmap(img.Width, img.Height, Imaging.PixelFormat.Format32bppArgb)
   Dim g As Graphics = Graphics.FromImage(bm)
   g.DrawImageUnscaled(img, Point.Empty)
   g.Dispose()

   Dim bmdata As System.Drawing.Imaging.BitmapData

   ' convert 1 row of pixels at a time
   Dim pixelcolors(0 To bm.Width - 1) As Int32
   For row As Int32 = 0 To bm.Height - 1

      ' lock the row in memory
      bmdata = bm.LockBits(New Rectangle(0, row, bm.Width, 1), _
                           Drawing.Imaging.ImageLockMode.ReadWrite, _
                           bm.PixelFormat)

      ' get row data as array integer color
      System.Runtime.InteropServices.Marshal.Copy(bmdata.Scan0, pixelcolors, 0, bm.Width)

      ' convert each pixel color to grayscale using selected method
      For col As Int32 = 0 To bm.Width - 1
         pixelcolors(col) = gsfunc(Color.FromArgb(pixelcolors(col))).ToArgb()
      Next col

      System.Runtime.InteropServices.Marshal.Copy(pixelcolors, 0, bmdata.Scan0, bm.Width)

      bm.UnlockBits(bmdata) ' release memory lock
      bmdata = Nothing

   Next row

   Return bm

End Function

public static Image ToGrayScale(this Image img, Methods method)
{
    if (!(Enum.IsDefined(typeof(Methods), method)))
    {
        return null;
    }

    // set the grayscale function to use for conversion
    Func<Color, Color> gsfunc = null;
    switch (method)
    {
        case Methods.Average:
            gsfunc = Average;
            break;
        case Methods.CorrectingForTheHumanEye_Gimp:
            gsfunc = Gimp;
            break;
        case Methods.CorrectingForTheHumanEye_BT_601:
            gsfunc = BT_601;
            break;
        case Methods.CorrectingForTheHumanEye_BT_709:
            gsfunc = BT_709;
            break;
        case Methods.Desaturation:
            gsfunc = Desaturation;
            break;
        case Methods.DecompositionMax:
            gsfunc = DecompositionMax;
            break;
        case Methods.DecompositionMin:
            gsfunc = DecompositionMin;
            break;
        case Methods.SingleColorRed:
            gsfunc = SingleColorRed;
            break;
        case Methods.SingleColorGreen:
            gsfunc = SingleColorGreen;
            break;
        case Methods.SingleColorBlue:
            gsfunc = SingleColorBlue;
            break;
    }

    // create a 32bppArgb bitmap from source image
    Bitmap bm = new Bitmap(img.Width, img.Height, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
    Graphics g = Graphics.FromImage(bm);
    g.DrawImageUnscaled(img, Point.Empty);
    g.Dispose();

    System.Drawing.Imaging.BitmapData bmdata = null;

    // convert 1 row of pixels at a time
    Int32[] pixelcolors = new Int32[bm.Width];
    for (Int32 row = 0; row < bm.Height; row++)
    {

        // lock the row in memory
        bmdata = bm.LockBits(new Rectangle(0, row, bm.Width, 1), System.Drawing.Imaging.ImageLockMode.ReadWrite, bm.PixelFormat);

        // get row data as array integer color
        System.Runtime.InteropServices.Marshal.Copy(bmdata.Scan0, pixelcolors, 0, bm.Width);

        // convert each pixel color to grayscale using selected method
        for (Int32 col = 0; col < bm.Width; col++)
        {
            pixelcolors[col] = gsfunc(Color.FromArgb(pixelcolors[col])).ToArgb();
        }

        System.Runtime.InteropServices.Marshal.Copy(pixelcolors, 0, bmdata.Scan0, bm.Width);

        bm.UnlockBits(bmdata); // release memory lock
        bmdata = null;

    }

    return bm;

}

Private Function BT_601(ByVal c As Color) As Color
   Dim gs As Int32 = CInt((c.R * 0.299) + (c.G * 0.587) + (c.B * 0.114))
   Return Color.FromArgb(c.A, gs, gs, gs)
End Function

private static Color BT_601(Color c)
{
    Int32 gs = Convert.ToInt32((c.R * 0.299) + (c.G * 0.587) + (c.B * 0.114));
    return Color.FromArgb(c.A, gs, gs, gs);
}