Creating 8-bit images

Question

I'm trying to create an 8-bit image with a solid background color. It seems like it should be pretty straight forward but the details on the file list it as 32-bit color depth. What am I missing?

    public void CreateImage()
    {
        var bmpOut = new Bitmap(300, 300);
        var g = Graphics.FromImage(bmpOut);
        g.FillRectangle(new SolidBrush(Color.Gray), 0, 0, 300, 300);

        var stream = new MemoryStream();
        bmpOut.Save(stream, GetPngCodecInfo(), GetEncoderParameters());

        bmpOut.Save(@"C:\image.png", GetPngCodecInfo(), GetEncoderParameters());
    }

    public EncoderParameters GetEncoderParameters()
    {
        var parameters = new EncoderParameters();
        parameters.Param[0] = new EncoderParameter(Encoder.ColorDepth, 8);

        return parameters;
    }

    public ImageCodecInfo GetPngCodecInfo()
    {
        var encoders = ImageCodecInfo.GetImageEncoders();

        ImageCodecInfo codecInfo = null;

        foreach (var imageCodecInfo in encoders)
        {
            if (imageCodecInfo.FormatID != ImageFormat.Png.Guid)
                continue;

            codecInfo = imageCodecInfo;
            break;
        }

        return codecInfo;
    }

Answer 1

Use this constructor to specify a pixel format : http://msdn.microsoft.com/en-us/library/3z132tat.aspx

Since you cannot create a Graphics from an indexed pixel format, you can only write raw pixels to a 8-bit image.

Bitmap bitmap = new Bitmap(32, 32, PixelFormat.Format8bppIndexed);
var bitmapData = bitmap.LockBits(new Rectangle(Point.Empty, bitmap.Size), ImageLockMode.ReadWrite, bitmap.PixelFormat);
Random random=new Random();
byte[] buffer=new byte[bitmap.Width*bitmap.Height];
random.NextBytes(buffer);
Marshal.Copy(buffer,0,bitmapData.Scan0,buffer.Length);
bitmap.UnlockBits(bitmapData);
bitmap.Save("test.bmp",ImageFormat.Bmp);

You can either use such code on WinForms : http://www.codeproject.com/Articles/17162/Fast-Color-Depth-Change-for-Bitmaps

Or if you can reference this class from WPF it will be much easier : http://msdn.microsoft.com/en-us/library/system.windows.media.imaging.formatconvertedbitmap(v=vs.85).aspx

Answer 2

You could also create the image at a higher bit rate and then convert it to 8 bits just before saving. This would allow you to use a Graphics context when creating the image. See this question for suggestions on how to convert to 8 bits:C# - How to convert an Image into an 8-bit color Image?

Answer 3

• ImageExtensions.cs

  using System.Runtime.InteropServices;
  using System.Linq;
  
  using System.Drawing.Imaging;
  using System.Drawing;
  using System;
  
  public static partial class ImageExtensions {
      public static ColorPalette ToGrayScale(this ColorPalette palette) {
          var entries=palette.Entries;
  
          for(var i=entries.Length; i-->0; entries[i]=entries[i].ToGrayScale())
              ;
  
          return palette;
      }
  
      public static Color ToGrayScale(this Color color, double[] luminance=null) {
          var list=(luminance??new[] { 0.2989, 0.5870, 0.1140 }).ToList();
          var channel=new[] { color.R, color.G, color.B };
          var c=(byte)Math.Round(list.Sum(x => x*channel[list.IndexOf(x)]));
          return Color.FromArgb(c, c, c);
      }
  
      public static Bitmap To8bppIndexed(this Bitmap original) {
          var rect=new Rectangle(Point.Empty, original.Size);
          var pixelFormat=PixelFormat.Format8bppIndexed;
          var destination=new Bitmap(rect.Width, rect.Height, pixelFormat);
  
          using(var source=original.Clone(rect, PixelFormat.Format32bppArgb)) {
              var destinationData=destination.LockBits(rect, ImageLockMode.WriteOnly, pixelFormat);
              var sourceData=source.LockBits(rect, ImageLockMode.ReadOnly, source.PixelFormat);
  
              var destinationSize=destinationData.Stride*destinationData.Height;
              var destinationBuffer=new byte[destinationSize];
  
              var sourceSize=sourceData.Stride*sourceData.Height;
              var sourceBuffer=new byte[sourceSize];
  
              Marshal.Copy(sourceData.Scan0, sourceBuffer, 0, sourceSize);
              source.UnlockBits(sourceData);
  
              destination.Palette=destination.Palette.ToGrayScale();
              var list=destination.Palette.Entries.ToList();
  
              for(var y=destination.Height; y-->0; ) {
                  for(var x=destination.Width; x-->0; ) {
                      var pixelIndex=y*destination.Width+x;
                      var sourceIndex=4*pixelIndex;
  
                      var color=
                          Color.FromArgb(
                              sourceBuffer[0+sourceIndex],
                              sourceBuffer[1+sourceIndex],
                              sourceBuffer[2+sourceIndex],
                              sourceBuffer[3+sourceIndex]
                              ).ToGrayScale();
  
                      destinationBuffer[pixelIndex]=(byte)list.IndexOf(color);
                  }
              }
  
              Marshal.Copy(destinationBuffer, 0, destinationData.Scan0, destinationSize);
              destination.UnlockBits(destinationData);
          }
  
          return destination;
      }
  }
  

Call bmpOut=bmpOut.To8bppIndexed(); before you it save to file.