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Basic Description

Windows uses ICO files to display its Icons. There can (and for 
compatibility's sake should) be more than 1 picture in an ICO file. 
For general purpose ICOs there should be at least one 32x32 
image using the 16 Windows colors.


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Basic File Format

Name          Size        Description

Reserved      2 byte      =0
Type          2 byte      =1
Count         2 byte      Number of Icons in this file
Entries       Count * 16  List of icons
 {Width       1 byte      Cursor Width (16, 32 or 64)
  Height      1 byte      Cursor Height (16, 32 or 64 , most commonly = Width)
  ColorCount  1 byte      Number of Colors (2,16, 0=256)
  Reserved    1 byte      =0
  Planes      2 byte      =1
  BitCount    2 byte      bits per pixel (1, 4, 8)
  SizeInBytes 4 byte      Size of (InfoHeader + ANDbitmap + XORbitmap)
  FileOffset  4 byte      FilePos, where InfoHeader starts
  } repeated Count times

InfoHeader    40 bytes    Variant of BMP InfoHeader
  Size        4 bytes     Size of InfoHeader structure = 40
  Width       4 bytes     Icon Width
  Height      4 bytes     Icon Height (added height of XOR-Bitmap and 
                          AND-Bitmap)
  Planes      2 bytes     number of planes = 1
  BitCount    2 bytes     bits per pixel = 1, 4, 8
  Compression 4 bytes     Type of Compression = 0
  ImageSize   4 bytes     Size of Image in Bytes = 0 (uncompressed)
  XpixelsPerM 4 bytes     unused = 0
  YpixelsPerM 4 bytes     unused = 0
  ColorsUsed  4 bytes     unused = 0
  ColorsImportant 4 bytes  unused = 0

Colors        NumberOfColors * 4 bytes  Color Map for XOR-Bitmap
 {Red         1 byte      red component
  Green       1 byte      green component
  Blue        1 byte      blue component
  reserved    1 byte      =0
  }repeated NumberOfColors times

XORBitmap     see below   bitmap
ANDBitmap     see below   monochrome bitmap


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Raster Data encoding

The XOR Bitmap is stored as 1-bit, 4-bit or 8-bit uncompressed 
Bitmap using the same encoding as BMP files. The AND Bitmap 
is stored in as 1-bit uncompressed Bitmap. 

Pixels are stored bottom-up, left-to-right. Pixel lines are padded 
with zeros to end on a 32bit (4byte) boundary. Every line will 
have the same number of bytes. Color indices are zero based, 
meaning a pixel color of 0 represents the first color table entry, a 
pixel color of 255 (if there are that many) represents the 256th 
entry.


Raster Data encoding for 1bit / black & white XORbitmaps and for the 
AND-bitmap

BitCount = 1 Compression = 0 
Every byte holds 8 pixels, its highest order bit representing the 
leftmost pixel of those. There are 2 color table entries. Some 
readers will ignore them though, and assume that 0 is black and 1 
is white. If you are storing black and white pictures you should 
stick to this, with any other 2 colors this is not an issue. Remember 
padding with zeros up to a 32bit boundary (This can be up to 31 
zeros/pixels!) 


Raster Data encoding for 4bit / 16 color XORbitmaps

BitCount = 4 Compression = 0 
Every byte holds 2 pixels, its high-order 4 bits representing the left 
of those. There are 16 color table entries. These colors do not have 
to be the 16 MS-Windows standard colors. Padding each line with 
zeros up to a 32bit boundary will result in up to 28 zeros = 7 
'wasted pixels'.


Raster Data encoding for 8bit / 256 color XORbitmaps

BitCount = 8 Compression = 0 
Every byte holds 1 pixel. There are 256 color table entries. 
Padding each line with zeros up to a 32bit boundary will result in 
up to 3 bytes of zeros = 3 'wasted pixels'.


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Portability

Since ICO Files are a derivatives of BMPs they should be quite 
easily converted into two BMPs, one holding the (possibly 
colored) XOR Image and one holding the monochrome AND 
Image. Due to the XOR-AND-scheme which may not apply to 
other OS, these files are not very portable, and were never 
intended to be. 2byte and 4byte entries are stored in Intel order 
(Least significant byte first)