How can I create a 16-bit grayscale PNG in C#?

Question

I'm trying to generate a height map for Cities: Skylines and it imports them as 16-bit grayscale PNGs.

I created a Bitmap with PixelFormat.Format16bppGrayScale, but trying to save it via Bitmap.Save, passing in ImageFormat.Png, results in an ExternalException with

"Additional information: A generic error occurred in GDI+."

Furthermore, loading a Bitmap from one of these 16-bit grayscale PNG files opens the file as Format32bppArgb, I assume silently dropping half the data.

How can I work with 16-bit grayscale PNG images in .Net/C# ?

Answer 1

Update: Magick.NET-Q16-AnyCPU has moved to https://github.com/dlemstra/Magick.NET.

The NuGet package for Magick.NET-Q16-AnyCPU https://magick.codeplex.com/ worked perfectly. Here's my code:

namespace heightmap_generator {
internal class Program {
    private static int GRID_SIZE = 1081;

    private static void Main(string[] args) {
        using (MagickImage image = new MagickImage(@"C:\Users\Brent\AppData\Local\Colossal Order\Cities_Skylines\Addons\MapEditor\Heightmaps\benchmark.png")) {
            image.Draw(
                new DrawableFillColor(new MagickColor(ushort.MaxValue / 2, ushort.MaxValue / 2, ushort.MaxValue / 2)),
                new DrawableRectangle(0, 0, GRID_SIZE, GRID_SIZE)
            );
            var drawables = new List<IDrawable>();
            for (var y = 0; y < GRID_SIZE/50;  ++y) {
                float t = y / (GRID_SIZE / 50.0f);
                t = t*t*(3 - 2*t); //cubic hermite spline h01
                ushort v = (ushort)(ushort.MaxValue * (5 + (t-1)*0.3) / 10);
                if (y == GRID_SIZE/50 - 1) {
                    v = (ushort) (ushort.MaxValue*0.501);
                }
                drawables.Add(new DrawableFillColor(new MagickColor(v, v, v)));
                for (var x = 0; x < GRID_SIZE; ++x) {
                    if (x == GRID_SIZE/2) {
                        var v2 = (ushort) (v + ushort.MaxValue/1024);
                        drawables.Add(new DrawableFillColor(new MagickColor(v2, v2, v2)));
                        drawables.Add(new DrawableColor(x, GRID_SIZE/2 - y, PaintMethod.Point));
                        drawables.Add(new DrawableColor(x, GRID_SIZE/2 + y, PaintMethod.Point));
                        drawables.Add(new DrawableFillColor(new MagickColor(v, v, v)));
                    } else {
                        drawables.Add(new DrawableColor(x, GRID_SIZE/2 - y, PaintMethod.Point));
                        drawables.Add(new DrawableColor(x, GRID_SIZE/2 + y, PaintMethod.Point));
                    }
                }
            }
            image.Draw(drawables);
            image.Write(new FileStream(@"C:\Users\Brent\AppData\Local\Colossal Order\Cities_Skylines\Addons\MapEditor\Heightmaps\benchmark3.png", FileMode.Create));
        }
    }
}

}