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Create Indexed Color Image in C#

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This snippet shows how to create indexed color bitmap in C#. The code also shows how to access and modify pixel data using LockBits.

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  1. using System;
  2. using System.Drawing;
  3. using System.Drawing.Imaging;
  4. using System.Runtime.InteropServices;
  5.  
  6. namespace IndexedImage
  7. {
  8.     class IdxImage
  9.     {
  10.         int[] _red;
  11.         int[] _green;
  12.         int[] _blue;
  13.         public IdxImage()
  14.         {
  15.             fire();
  16.         }
  17.  
  18.         public Image GetImage()
  19.         {
  20.             //allocate new bitmap
  21.             //width: paletteSize, height: 16
  22.             Bitmap img = new Bitmap(paletteSize(), 16, PixelFormat.Format8bppIndexed);
  23.             ColorPalette pal = img.Palette;
  24.  
  25.             //fill image palette with color information
  26.             fillPalette(ref pal);
  27.  
  28.             //setup images
  29.             img.Palette = pal;
  30.             BitmapData data = img.LockBits(new Rectangle(0, 0, img.Width, img.Height),
  31.                 ImageLockMode.ReadWrite, img.PixelFormat);
  32.  
  33.             byte[] rowPixels = new byte[img.Width];
  34.             for (int k = rowPixels.Length - 1; k >= 0; k--)
  35.                 rowPixels[k] = (byte)k;
  36.  
  37.             IntPtr scan0 = data.Scan0;
  38.             for (int h = 0; h < data.Height; h++)
  39.             {
  40.                 Marshal.Copy(rowPixels, 0, scan0, rowPixels.Length);
  41.                 scan0 = new IntPtr(scan0.ToInt64() + data.Stride);
  42.             }
  43.  
  44.             img.UnlockBits(data);
  45.  
  46.             return img;
  47.         }
  48.  
  49.         private void fillPalette(ref ColorPalette pal)
  50.         {
  51.             int N = Math.Min(Math.Min(_red.Length, _green.Length), _blue.Length);
  52.             for (int k = 0; k < N; k++)
  53.                 pal.Entries[k] = Color.FromArgb(_red[k], _green[k], _blue[k]);
  54.         }
  55.         private int paletteSize()
  56.         {
  57.             if (_red == null)
  58.                 return 0;
  59.             return Math.Min(Math.Min(_red.Length, _green.Length), _blue.Length);
  60.         }
  61.  
  62.         /// <summary>
  63.         /// Example palette: Fire
  64.         /// </summary>
  65.         private void fire()
  66.         {
  67.             _red = new int[]{ 0, 0, 1, 25, 49, 73, 98, 122, 146, 162, 173, 184, 195, 207, 217, 229, 240, 252, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255 };
  68.             _green = new int[]{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 14, 35, 57, 79, 101, 117, 133, 147, 161, 175, 190, 205, 219, 234, 248, 255, 255, 255, 255 };
  69.             _blue = new int[] { 0, 61, 96, 130, 165, 192, 220, 227, 210, 181, 151, 122, 93, 64, 35, 5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 35, 98, 160, 223, 255 };
  70.         }
  71.     }
  72. }

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