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The ANI Format

RIFF provides only a generic chunk-based format for storing multimedia data. A more specific RIFF-based format (such as the WAVE format illustrated earlier by richo.wav) is required to name the multimedia data’s various chunks, define the data structures stored in the chunks, and identify the order (if necessary) in which the chunks appear in the RIFF file. For .ani files, this format is known as ANI.

ANI Internals

The ANI format fully defines an animated cursor. ANI is assigned form type ACON and identifies various mandatory and optional chunks. For example, the optional LIST/INFO chunk is often the first chunk located within the RIFF chunks of many .ani files. This chunk is illustrated in the following descriptive excerpt from Microsoft’s 6,712-byte stopwtch.ani file:

Chunk: RIFF
Chunk size: 6712
 Form type: ACON

 Chunk: LIST
 Chunk size: 68
  List type: INFO

  Chunk: INAM
  Chunk size: 10

  Chunk: IART
  Chunk size: 38

LIST/INFO contains INAM and IART subchunks. The animated cursor’s name is stored in the INAM subchunk; IART identifies the animation’s artist and often includes a copyright message. For example, stopwtch.ani stores StopWatch in INAM and Microsoft Corporation, Copyright 1993 in IART.

The next chunk contained within an .ani file’s RIFF chunk is anih. This mandatory ANI header chunk stores an ANIHEADER data structure in its data area and must precede all other chunks located within the RIFF chunk. The ANIHEADER data structure is conveniently expressed by the following C language structure type definition:

typedef struct
  DWORD cbSize;    // Data structure size (in bytes)
  DWORD nFrames;   // Number of images (also known as frames) stored in the file
  DWORD nSteps;    // Number of frames to be displayed before the animation repeats
  DWORD iWidth;    // Width of frame (in pixels)
  DWORD iHeight;   // Height of frame (in pixels)
  DWORD iBitCount;  // Number of bits per pixel
  DWORD nPlanes;   // Number of color planes
  DWORD iDispRate;  // Default frame display rate (measured in 1/60th-of-a-second units)
  DWORD bfAttributes; // ANI attribute bit flags

ANIHEADER’s cbSize field contains the structure’s size (in bytes) and is always set to 36. This field is followed by nFrames, which identifies the number of images (also known as frames) that are stored in the .ani file. All of these images are stored in Microsoft Icon Resource Format resources, in Microsoft Cursor Resource Format resources, or as raw bitmaps.

The nSteps field specifies the number of frames to be displayed prior to repeating the animation. This value often is set to the same value as found in the nFrames field, and the display order is first stored image through last stored image. However, if a seq chunk (which I discuss later) is present, this field identifies the number of values in this chunk’s array, and the order differs.

Following the nSteps field are iWidth, iHeight, iBitCount, and nPlanes. These four fields contain non-zero values if images are stored as raw bitmaps, and are useful for interpreting bitmap dimensions and color information. These fields contain zeroes if the images are stored in icon resources or cursor resources.

The iDispRate field identifies the default display rate for animation frames. In the absence of a rate chunk (which I discuss later), the value in this field is used to determine how long each frame in the animation remains on the screen. The value is expressed in 1/60th-of-a-second units, which are known as jiffies. If this value is zero, there’s no delay between successive frames.

The final bfAttributes field provides two bit flags that determine whether images are stored in icon or cursor resources (the least-significant bit is one) or as raw bitmaps (the least-significant bit is zero), and whether seq chunks are present (the next-to-least-significant bit is one) or absent (the next-to-least-significant bit is zero). This field and its predecessors are illustrated below:

Chunk: anih
Chunk size: 36
 cbSize = 36
 nFrames = 8
 nSteps = 45
 iWidth = 0
 iHeight = 0
 iBitCount = 0
 nPlanes = 0
 iDispRate = 8
 bfAttributes = 3

This descriptive anih chunk was excerpted from stopwtch.ani. It reveals that this file stores eight frames, each animation cycle consists of 45 steps, the default display rate is set to eight jiffies (each frame is displayed for 8/60ths of a second), each frame is stored in an icon or cursor resource, and a seq chunk is present.

The anih chunk can be followed by the optional rate chunk, which lets you determine how long each frame appears. This chunk stores a DWORD array of duration values (expressed as jiffies). ANIHEADER’s nSteps field identifies the number of entries in the array—each entry corresponds to one of the steps in an animation cycle (entry zero corresponds to the first step).

The following descriptive excerpt from stopwtch.ani reveals this file’s rate chunk. With two exceptions, each frame in the animation sequence is displayed for eight jiffies:

Chunk: rate
Chunk size: 180
 8, 16, 16, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8

The anih chunk can be followed by the optional seq chunk, which lets you specify a display order other than the default first-stored-image to last-stored-image order. You can even repeat images during an animation cycle. This chunk stores a DWORD array of frame indexes. The number of array entries is indicated by ANIHEADER’s nSteps field.

The following descriptive excerpt from stopwtch.ani reveals this file’s seq chunk. The sequence starts and ends with the first frame (at index zero):

Chunk: seq
Chunk size: 180
 0, 1, 0, 2, 3, 4, 5, 6, 7, 0, 2, 3, 4, 5, 6, 7, 0,
 2, 3, 4, 5, 6, 7, 0, 2, 3, 4, 5, 6, 7, 0, 2, 3, 4,
 5, 6, 7, 0, 2, 3, 4, 5, 6, 7, 0

The final chunk of interest within the RIFF chunk is LIST/fram. This chunk contains several icon subchunks, where each subchunk defines one image via a cursor or icon resource, or a raw bitmap. For example, stopwtch.ani contains eight subchunks, as revealed earlier via ANIHEADER’s nFrames field, and as revealed in the following excerpt:

Chunk: LIST
 Chunk size: 6196
  List type: fram

  Chunk: icon
  Chunk size: 766

  Chunk: icon
  Chunk size: 766

  Chunk: icon
  Chunk size: 766

  Chunk: icon
  Chunk size: 766

  Chunk: icon
  Chunk size: 766

  Chunk: icon
  Chunk size: 766

  Chunk: icon
  Chunk size: 766

  Chunk: icon
  Chunk size: 766

Each icon subchunk is implicitly assigned a zero-based index, which is determined by its order of appearance within the LIST/fram chunk. This index is used to locate the corresponding duration in a rate chunk’s DWORD array of durations, and may appear in the seq chunk’s DWORD array of frame indexes.

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