Convert image to grayscale image (http://stackoverflow.com/questions/2265910/convert-an-image-to-grayscale)

grayscale.fs

open System
open System.Drawing
open System.Drawing.Imaging

// Direct translation of function taken from SO answer (http://stackoverflow.com/questions/2265910/convert-an-image-to-grayscale)
let makeGrayscale (img: Image) =
    let newImg = new Bitmap(img.Width, img.Height)
    use g = Graphics.FromImage newImg
    let attributes = new ImageAttributes()
    attributes.SetColorMatrix(
        new ColorMatrix(
            [| [|  0.3f;  0.3f;  0.3f;  0.f;  0.f |]
               [| 0.59f; 0.59f; 0.59f;  0.f;  0.f |]
               [| 0.11f; 0.11f; 0.11f;  0.f;  0.f |]
               [|   0.f;   0.f;   0.f;  1.f;  0.f |]
               [|   0.f;   0.f;   0.f;  0.f;  1.f |] |]))
    g.DrawImage(img, new Rectangle(0, 0, img.Width, img.Height), 0, 0, img.Width, img.Height, GraphicsUnit.Pixel, attributes)
    newImg

[<EntryPoint>]
let main argv = 
    makeGrayscale(Bitmap.FromFile(__SOURCE_DIRECTORY__ + "/original.jpg")).Save("grayscale.jpg")
    0