.ICO

OS/2 colour icon (CI)

Introduction

The OS/2 colour icon is a BMP file with the type code CI that holds two bitmaps one after the other. The first is the same 1-bit pair of masks as a black and white icon, which decides where the icon is transparent. The second is a colour image that gives the colour of the rest.

Colour icons are usually named .ico, which on Windows means a different format. A file with several sizes is a bitmap array whose entries are CI images.

Software

  • ArcaOS, the current OS/2, whose Icon Editor opens and saves them
  • Deark, which converts them to PNG (Windows, macOS, Linux)

Structure

All the headers come first and the pixels after them. Each of the two file headers starts with CI and holds the offset of its own bitmap’s pixels. All numbers are little-endian.

  1. 0File header CI, size, hotspot and the offset of the masks14 bytes
  2. 14DIB header for the masks: width, double height, 1 bit per pixel12 to 64 bytes
  3. …Colour table for the masks, two entries6 or 8 bytes
  4. …Second file header CI again, with the offset of the colour image14 bytes
  5. …DIB header for the colour image, at the icon’s real height12 to 64 bytes
  6. …Colour table for the colour imagevaries
  7. …Masks 1-bit rows: the AND mask and the XOR mask one above the othervaries
  8. …Colour image rows, stored like a bitmap’s pixelsvaries
An OS/2 colour icon: two sets of headers, then two sets of pixels

The file header fields are the same as in a black and white icon, with the hotspot in the two fields a bitmap reserves.

Telling it apart

The file starts with CI (43 49). After the first DIB header and its two-entry colour table comes a second file header, also starting with CI. An OS/2 colour pointer is laid out the same way and differs only in its type code, CP. A Windows .ico file starts with 00 00 01 00.

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