ICO (file format)

The ICO file format is an image file format for computer icons in Microsoft Windows. An ICO file holds one or more images at multiple sizes and color depths, from which Windows selects and scales the most appropriate one for a given use. A Windows application's own icon — shown on the desktop, in the Start menu, or in File Explorer — is stored in the same format inside its executable as an icon resource, rather than as a separate file.

The CUR file format is an almost identical format for non-animated cursors in Microsoft Windows. The two differ only in the bytes that identify the file and in a hotspot recorded in the CUR header — the pixel offset, in x,y coordinates, from the top-left corner of the cursor image to the point at which the user is actually pointing.

The ANI file format is used for animated Windows cursors. ANI is a RIFF container; each frame within it is a complete CUR-format image and may itself contain images at multiple sizes.

First introduced as monochrome images in Windows 1.0, the format acquired its directory-based multi-image structure in Windows 3.0 and support for larger sizes and higher color depths in Windows 95; Windows Vista added 256×256 images and PNG compression. Each image is stored either as a DIB or as a complete PNG file, and a file typically bundles several so the best match for the requested size and color depth can be chosen.

History

Icons introduced in Windows 1.0 were monochrome; in Windows 1.x and 2.x, ICO and CUR files were raw binary images with no directory wrapper — each file contained a single such image, at 64×64 pixels for icons and 32×32 for cursors.