.ANI File Format – Chunks

The form

Microsoft’s 1994 definition of the form, in the RIFF notation, with optional parts in square brackets:

RIFF( 'ACON'
      [ LIST( 'INFO' <info chunks> ) ]
      [ DISP( <display data> ) ]
      anih( <ANIHEADER> )
      [ rate( <DWORD>... ) ]
      [ 'seq '( <DWORD>... ) ]
      LIST( 'fram' icon( <icon file> )... )
    )

Chunk list

The anih chunk must come before rate, seq and the fram list, because a reader needs its counts to read them. Files from the usual editors put the chunks in the order of the form above. A reader should skip any chunk it does not know.

rate

An array of 32-bit unsigned values, one per step, cSteps of them. Each is the time to show that step, in jiffies of 1/60 second. When the chunk is present it replaces the header’s single default rate, jifRate, so each step can have its own timing.

seq

An array of 32-bit unsigned values, one per step, cSteps of them. Each is the number of the frame to show at that step, counting from 0 in the order of the icon chunks. A frame can appear at several steps, so a long animation can reuse a few frames. Without this chunk the steps show the frames in order, so cSteps should not be more than cFrames.

LIST fram and icon

A LIST chunk whose list type is fram holds cFrames chunks with the ID icon, one per frame. When the header’s AF_ICON flag is set, which it always is in practice, each icon chunk holds a complete ICO or CUR file: the 6-byte header, the directory and the images. A CUR frame carries its own hotspot, so animated pointers use CUR frames; an ICO frame has no hotspot.

A frame may hold several sizes or colour depths, like any icon file, and since Windows Vista a frame’s images may be PNG. The image offsets in the frame’s directory count from the start of the icon chunk’s data, not from the start of the ANI file.

Early third-party notes describe frames without AF_ICON as raw bitmaps. Microsoft’s definition does not include that form and Windows is not known to load it.

LIST INFO

The standard RIFF information list. Cursor editors write two of its chunks:

Each is a zero-terminated string in the system’s ANSI code page. Any other RIFF information chunk may appear; the full list is on the WAV LIST INFO page. Windows ignores this list when it loads a cursor.

DISP

The standard RIFF display chunk: a 32-bit Windows clipboard format number followed by data in that format, such as 1 for text or 8 for a device-independent bitmap. It lets a program show something for the file without understanding it. It is rare in ANI files, and a reader can skip it.

Example

A two-frame cursor, each frame shown for a quarter of a second, with no rate, seq or INFO: