Question

I’m trying to render a small Bitmap in memory with .NET that needs to be 16 bit Grayscale. The bitmap’s format is set to PixelFormat.Format16bppGrayScale. However, Bitmap.SetPixel takes a Color argument. Color in turn takes one byte for each of R, B, and G (and optionally A).

How do I specify a 16-bit gray scale value rather than an 8 bit value when drawing to my bitmap?

Answer 1

Regardless of the image format, SetPixel() is brutally slow. I never use it in practice.

You can set pixels much faster using the LockBits method, which allows you to quickly marshal managed data to the unmanaged bitmap bytes.

Here is an example of how that might look:

Bitmap bitmap = // ...

// Lock the unmanaged bits for efficient writing.
var data = bitmap.LockBits(
    new Rectangle(0, 0, bitmap.Width, bitmap.Height),
    ImageLockMode.ReadWrite,
    bitmap.PixelFormat);

// Bulk copy pixel data from a byte array:
Marshal.Copy(byteArray, 0, data.Scan0, byteArray.Length);

// Or, for one pixel at a time:
Marshal.WriteInt16(data.Scan0, offsetInBytes, shortValue);

// When finished, unlock the unmanaged bits
bitmap.UnlockBits(data);

Note that 16bpp grayscale appears to be unsupported by GDI+, meaning that .NET doesn’t help out with saving a 16bpp grayscale bitmap to a file or a stream. See http://social.msdn.microsoft.com/forums/en-US/csharpgeneral/thread/10252c05-c4b6-49dc-b2a3-4c1396e2c3ab