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.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 | |||
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 = 0Every 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 = 0Every 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 = 0Every 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'.
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)Trademarks, Patents and Royalties
To my knowledge: none(please read the disclaimer)
Cross-Checking Software
This section is for programmers, who wish to cross-check their implementation with others. This is an incomplete list of programs, which are available as freeware / shareware / try-before-buy etc.The following software is able to decode ICOs::
- Windows itself (obviously not freeware)
- most resource construction sets (included in Windows Compilers)
- most resource construction sets (included in Windows Compilers)