C#/GDI - Create 1bpp mask from image.

Question

How do you create a 1 bit per pixel mask from an image using GDI in C#? The image I am trying to create the mask from is held in a System.Drawing.Graphics object.

I have seen examples that use Get/SetPixel in a loop, which are too slow. The method that interests me is one that uses only BitBlits, like this. I just can't get it to work in C#, any help is much appreciated.

Answer 1

Try this:

using System.Drawing;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;

...

   public static Bitmap BitmapTo1Bpp(Bitmap img) {
      int w = img.Width;
      int h = img.Height;
      Bitmap bmp = new Bitmap(w, h, PixelFormat.Format1bppIndexed);
      BitmapData data = bmp.LockBits(new Rectangle(0, 0, w, h), ImageLockMode.ReadWrite, PixelFormat.Format1bppIndexed);
      for (int y = 0; y < h; y++) {
        byte[] scan = new byte[(w + 7) / 8];
        for (int x = 0; x < w; x++) {
          Color c = img.GetPixel(x, y);
          if (c.GetBrightness() >= 0.5) scan[x / 8] |= (byte)(0x80 >> (x % 8));
        }
        Marshal.Copy(scan, 0, (IntPtr)((int)data.Scan0 + data.Stride * y), scan.Length);
      }
      bmp.UnlockBits(data);
      return bmp;
    }

GetPixel() is slow, you can speed it up with an unsafe byte*.

Answer 2

Do you mean LockBits? Bob Powell has an overview of LockBits here; this should provide access to the RGB values, to do what you need. You might also want to look at ColorMatrix, like so.

Answer 3

In the Win32 C API the process to create a mono mask is simple.

• Create an uninitialzied 1bpp bitmap as big as the source bitmap.
• Select it into a DC.
• Select the source bitmap into a DC.
• SetBkColor on the destination DC to match the mask color of the source bitmap.
• BitBlt the source onto the destination using SRC_COPY.

For bonus points its then usually desirable to blit the mask back onto the source bitmap (using SRC_AND) to zero out the mask color there.