.ICO

Windows icon

Introduction

ICO is the icon format of Microsoft Windows. One file holds the same picture at several sizes and colour depths, such as 16, 32, 48 and 256 pixels square, and Windows picks the best match for where the icon is shown. Web sites use it for favicon.ico, the small icon in a browser tab.

Mouse cursors use the same layout with the extension .cur; their directory entries give the hotspot, the point that clicks. Since Windows Vista, each image in the file can be a complete PNG as well as the older bitmap form.

Software

Structure

An ICO file starts with a 6-byte header: two zero bytes, the type (1 for an icon, 2 for a cursor) and the number of images, all little-endian. A 16-byte directory entry for each image follows, giving its width and height (0 meaning 256), colour count, planes and bit count (or the hotspot in a cursor), its size in bytes and its offset in the file.

Each image is either a PNG file or a device independent bitmap like those in BMP files, starting at the BITMAPINFOHEADER with no file header. In the bitmap form the stated height is doubled, because the colour pixels are followed by a 1-bit transparency mask of the same size.

  1. 0Header reserved, type, image count6 bytes
  2. 6Directory one entry per image: size, colours, bytes, offset16 bytes each
  3. …Images each a PNG, or a bitmap with its AND maskvaries
An ICO file: header, directory and images

See also

Other formats that use the same extension: