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What is the easiest way to convert and save a 32-bit Bitmap to a 1-bit (black/white) .bmp file in C#?

Dmi
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2 Answers2

25

The easiest way to do achieve this by using the Clone()

using System.Drawing.Imaging;

var original = //your source image;
var rectangle = new Rectangle(0, 0, original.Width, original.Height);

var bmp1bpp = original.Clone(rectangle, PixelFormat.Format1bppIndexed);

As disclaim this will maybe not the fastest way to do this there are much faster way's to do this

WiiMaxx
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    this is definitely the easier way – SED Jul 29 '16 at 18:50
  • this works well and fast but brightness cannot be tuned. this is why I prefer https://stackoverflow.com/a/2574798/2323103 and adjust the 0.5 coefficient to whatever value I want – Couitchy May 14 '23 at 14:08
14

This code will get the job done:

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);
  byte[] scan = new byte[(w + 7) / 8];
  for (int y = 0; y < h; y++) {
    for (int x = 0; x < w; x++) {
      if (x % 8 == 0) scan[x / 8] = 0;
      Color c = img.GetPixel(x, y);
      if (c.GetBrightness() >= 0.5) scan[x / 8] |= (byte)(0x80 >> (x % 8));
    }
    Marshal.Copy(scan, 0, (IntPtr)((long)data.Scan0 + data.Stride * y), scan.Length);
  }
  bmp.UnlockBits(data);
  return bmp;
}

You can speed it up, if necessary, by using unsafe code to replace the GetPixel() method.

Hans Passant
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    If you're already locking the bits you might as well Marshal.Copy all the data to a byte[] and do it all at once. That'll save you the getpixel and the repeated marshal.copy overhead. – Ron Warholic Apr 04 '10 at 15:00
  • It's not that simple, the ImageFormat of the source image matters. – Hans Passant Apr 04 '10 at 15:08
  • Ron, that wouldn't work if you have a non-minimal stride in the source image unless you have the same in the destination image. (Stride >= width * bytesPerPixel) – Danny Varod Apr 04 '10 at 15:10