convertNV21ToRgb.fs

let nv21ToPPM() =
    let clampBetween0And255 v =
        if v < 0.0 then
            0.0
        else if v >= 255.0 then
            255.0
        else
            v

    let width = 1280
    let height = 720
    let bytes = IO.File.ReadAllBytes "NV21In.raw"
    let content = System.Text.StringBuilder(width * height * 3 * 4)
    let header = "P3\n" + width.ToString() + " " + height.ToString() + "\n255\n"
    content.Append(header) |> ignore
    
    let beginOfUVPlaneIndex = (width * height)

    for j in 0 .. (height - 1) do
        for i in 0 .. (width - 1) do
            let y = double bytes.[j * width + i]

            let delta =  (j / 2) * width + 2 * (i / 2)

            let cr = double bytes.[beginOfUVPlaneIndex + delta ] |> (fun a -> a - 128.0)
            let cb = double bytes.[beginOfUVPlaneIndex + delta + 1] |> (fun a -> a - 128.0)

            let r = (y + 1.40200 * cr);
            let g = (y - 0.34414 * cb - 0.71414 * cr);
            let b = (y + 1.77200 * cb);

            let r = r |> (clampBetween0And255 >> int)
            let g = g |> (clampBetween0And255 >> int)
            let b = b |> (clampBetween0And255 >> int)

            if i = 0 then
                content.Append(r.ToString() + " " + g.ToString() + " " + b.ToString()) |> ignore
            else 
                content.Append(" " + r.ToString() + " " + g.ToString() + " " + b.ToString()) |> ignore

        content.Append("\n") |> ignore |> ignore

    System.IO.File.WriteAllText("NV21.ppm", content.ToString())