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File Format Specification Index



Basic Description

ANI files hold the animated cursors for Win95 and WinNT. The file is a specialized RIFF . You should read the page on RIFFs first.

Basic File Format

An ANI file contains a RIFF Chunk with a HeaderID of 'ACON'. This RIFF Chunk must contain at least two subchunks: The header chunk 'anih' and a 'LIST' chunk containing one or more images. In addition to those it may contain a sequence chunk 'seq ' and a frame rate control chunk 'rate'.

Basic File Layout

Name ID
RIFF HeaderID = 'ACON'
  anih header chunk
  LIST HeaderID = 'fram'
   icon single frame
  seq (optional) specifies the display sequence of frames. Notice the space after the 'q'.
  rate (optional) specifies the display timing of frames

Structure of the 'anih' header chunk.

Name Size Description
HeaderSize 4 bytes size of this structure (=32)
NumFrames 4 bytes number of stored frames in this animation
NumSteps 4 bytes number of steps in this animation (may include duplicate frames, = NumFrames, if no 'seq '-chunk is present)
Width 4 bytes total width in pixels
Height 4 bytes total height in pixels
BitCount 4 bytes number of bits/pixel ColorDepth = 2BitCount
NumPlanes 4 bytes =1
DisplayRate 4 bytes default display rate in 1/60s (Rate = 60 / DisplayRate fps)
Flags 4 bytes currently only 2 bits are used
  reserved bits 31..2 unused =0
  SequenceFlag bit 1 TRUE: File contains sequence data
  IconFlag bit 0 TRUE: Frames are icon or cursor data
FALSE: Frames are raw data

Structure of the 'icon' chunks

Depending on the anih.Flags.IconFlag these chunks will contain either icon and cursor resources or raw images. Icons and cursors can be mixed within a file, but not together with raw images. If the file contains icons and cursors, they are included in their respective format, including their header. The first 4 bytes decide what format they are in. There are pages on both, ICO and CUR resources in this collection.
Raw images are BMP files, stored without their header, using the Width, Height, BitCount and NumPlanes information in 'anih' for decoding.
- Whether the BMP's color map should be included if BitCount <= 8 is unknown. I haven't seen a ANI with raw images yet.

Structure of the 'seq ' chunk

Notice the space after the 'q' in 'seq '. This chunk defines an order in which the frames are to be displayed, referring to the order in which they appear in the 'fram' - list. There may very well be a 'seq '-chunk without a 'rate' chunk. If there is no 'seq '-chunk in the file, frames will be displayed in the order in which they appear in the frame list. If an ANI file is used as MS-Windows' cursor, the sequence will loop. There is no way of influencing that behavior.
Name Size Description
  Rate 4 bytes Frame Number 0
repeated anih.NumSteps times

Structure of the 'rate' chunk

This chunk defines a display rate (in 1/60th sec) for every frame in the sequence. If there is no 'seq '-chunk in the file, it defines a display rate for every frame in the 'fram' - list.
Name Size Description
  Rate 4 bytes Display rate for frame 0
repeated anih.NumSteps times


I haven't seen any implementation of ANI software on other systems than MS-Windows95 and NT. But there is no reason why it can't be done, although it is questionable, why someone would want to do that. A full implementation would have to decode BMP, CUR and ICO structures. It doesn't have to include a full-sized RIFF decoder, since the structure of ANIs is fixed. See the RIFF, BMP, ICO and CUR page for additional information.

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 ANIs::
  • MS-Windows 95 (obviously not freeware)
  • MS-Windows NT 4 (ditto)
The following software is able to encode ANIs
  • ?

Online Resources


Paper Resources



This is not the official format documentation. Although we're doing our best to keep this information as accurate as possible, there is no way of checking all of it under all possible circumstances. We're not taking any responsibility for the results of these information or lack thereof. The 'trademarks, patents and royalties' section is here just for your convenience, and is in no way complete. Please send us a note, if you find any incorrect of missing information.