ANIM (Metroid Prime)

This article is for the ANIM format used in Metroid Prime. See ANIM (File Format) for the other revisions of this format.

The ANIM format is used in conjunction with CINF + CSKR rigging to animate CMDL meshes.

There are two versions of the ANIM format in Metroid Prime (indicated by the file's first u32 value), one with compressed animations and one with uncompressed ones:

Version	Description
0x0	Quaternion Array Format (Uncompressed)
0x2	Rotation Vector Bitstream Format (Compressed)

Quaternion Array Format (Uncompressed)

The Quaternion Array Format (0x0) is a less common animation format in Metroid Prime. It only appears in a Ridley intro animation and the rotating pirate data artifact hologram above the Impact Crater, as well as some unused Samus animations.

The format stores uncompressed animations by mapping animation channels to bones and supplying rotations as an array of quaternion (WXYZ) keys. Optionally, translations may be specified for a subset of bones as an array of float3 keys.

Layout

Offset	Type	Element Count	Name	Notes
0x4	float	1	Duration	Time in seconds that the animation plays for
0x8	u32	1	Unknown 1
0xC	float	1	Key Interval	Time in seconds between keys of each bone channel (reciprocal of frame-rate)
0x10	u32	1	Unknown 2
0x14	u32	1	Key Count	Count of keys for each bone channel (keys are set for every frame of the animation)
0x18	u32	1	Unknown 3
0x1C	u32	1	Rotation Channel Index Count	Count of rotation channel indices in the next array. Always 100.
0x20	u8	Rotation Channel Index Count	Rotation Channel Index Array	This table maps each bone to a rotation channel, with the index-position corresponding to a Bone ID in the associated CINF, while the value corresponds to the index of a rotation channel. The value is 0xFF if the bone is not animated by this ANIM.
	u32	1	Translation Channel Index Count	Count of translation channel indices in the next array. Matches the number of valid rotation channels.
	u8	Translation Channel Index Count	Translation Channel Index Array	This table maps each rotation channel to a translation channel, with the index-position corresponding to an entry in the Rotation Channel Index Array above (disregarding 0xFF entries), while the value corresponds to the index of a translation channel. The value is 0xFF if the bone is not translated by this ANIM.
	u32	1	Quaternion Key Count	Count of elements in Quaternion Key Array below
	float4	Quaternion Key Count	Quaternion Key Array	The quaternion keys are interleaved as such (bone-major): |------------|------------|-----|------------| | bone0/key0 | bone0/key1 | ... | bone0/keyN | | bone1/key0 | bone1/key1 | ... | bone1/keyN | | .......... | .......... | ... | .......... | | boneN/key0 | boneN/key1 | ... | boneN/keyN | |------------|------------|-----|------------|
	u32	1	Translation Key Count	Count of elements in Translation Key Array below
	float3	Translation Key Count	Translation Key Array	The translation keys are interleaved the same way as the rotation keys. Any bone channels that were marked 0xFF in the Translation Channel Index Array are skipped.
	EVNT	1	EVNT Ref	ID for this ANIM's EVNT resource, or 0xFFFFFFFF if no events triggered

Rotation Vector Bitstream Format (Compressed)

The Rotation Vector Bitstream Format (0x2) is the most widely used animation format in Metroid Prime. The format uses a compressed bitstream to store keys that have been delta encoded and quantized to make for a very compact, compressed animation. The data is processed sequentially to reconstruct the uncompressed bone channel data as quaternions.

In addition to the bitstream quantization, the format includes a bitmap specifying which frames of the animation have keys added. Consequently, the runtime generates missing keys via interpolation (reducing animation size further).

Key Values

When decoding the animation, each bone's rotation (and possibly translation) values are initialized to the Initial * values in the bone channel descriptors. For every frame of every bone channel, the key value in the bitstream is added with the previous key value (to resolve the delta encoding).

Once resolved, rotation components are divided by Rotation Divisor, resulting in an axis-angle rotation vector. This rotation vector is converted to quaternion imaginary components with the W component derived via normalized-difference and a sign-bit packed into the bitstream:

q = π / 2.0 / rotation_divisor
X = sin(rx * q)
Y = sin(ry * q)
Z = sin(rz * q)
W = sqrt(MAX((1.0 - (X*X + Y*Y + Z*Z)), 0.0))
W = sign_bit ? -W : W

Translation components are multiplied by Translation Multiplier, yielding world units.

Layout

Offset	Type	Element Count	Name	Notes
0x4	u32	1	Scratch Size	Amount of memory that the animation system needs to allocate to process animation
0x8	EVNT	1	EVNT Ref	ID for this ANIM's EVNT resource, or 0xFFFFFFFF if no events triggered
0xC	u32	1	Unknown 1	Always 0x1
0x10	float	1	Duration	Time in seconds that the animation plays for
0x14	float	1	Interval	Time in seconds between frames (reciprocal of frame-rate, typically 30-fps)
0x18	u32	1	Unknown 2	Always 0x3
0x1C	u32	1	Unknown 3	Always 0x0
0x20	u32	1	Rotation Divisor	Rotation vectors are divided by this value before being applied
0x20	float	1	Translation Multiplier	Translation vectors are multiplied by this value before being applied
0x24	u32	1	Bone Channel Count	Count of bone channels present in ANIM
0x28	4	u32	Unknown 4	Always 0x1
0x2C	u32	1	Key Bitmap Length	Number of bits in Key Bitmap
0x30	u32	round_up(Key Bitmap Length / 32)	Key Bitmap	Word-packed bitmap. Each word is read from least-significant to most-significant bit. A set bit indicates the frame at the bit's position has keys for each bone channel. If the bit is unset, the keys must be generated via interpolation.
	u32	1	Bone Channel Count	Count of animated bone channels (matches the previous bone channel count)
	u32	1	Bone Channel Descriptor Count	Count of bone channel descriptors in Bone Channel Descriptor Table. Matches the Bone Channel Counts.
	Bone Channel Descriptor	Bone Channel Descriptor Count	Bone Channel Descriptor Table	Table describing animated bone channels
	u32	<remainder of file>	Key Bitstream	Word-packed bitstream laid out as described by the descriptor table. The bitstream is unpacked just like Key Bitmap (least-significant to most-significant). Key values are tightly-packed, quantized integers whose bit-depth is specified by the q_* values in the descriptor table. Each bone-channel key-set starts with a single bit providing the W sign-bit (1 for negative, 0 for positive). The key integers are signed using Two's complement representation. The keys are interleaved as such (key-major): |------------|------------|-----|------------| | bone0/key0 | bone1/key0 | ... | boneN/key0 | | bone0/key1 | bone1/key1 | ... | boneN/key1 | | .......... | .......... | ... | .......... | | bone0/keyN | bone1/keyN | ... | boneN/keyN | |------------|------------|-----|------------|

Bone Channel Descriptor

Offset	Type	Name	Notes
0x0	u32	Bone ID	Bone ID in the associated CINF for this bone channel
0x4	u16	Rotation Key Count	Number of rotation keys in the bitstream for this bone channel. If this value is 0, the remaining Rotation fields aren't present
0x6	s16	Initial Rotation X	Initial X value of bone's rotation vector
0x8	u8	Rotation Bits X	Number of bits allocated to bone's rotation delta X in bitstream
0x9	s16	Initial Rotation Y	Initial Y value of bone's rotation vector
0xB	u8	Rotation Bits Y	Number of bits allocated to bone's rotation delta Y in bitstream
0xC	s16	Initial Rotation Z	Initial Z value of bone's rotation vector
0xE	u8	Rotation Bits Z	Number of bits allocated to bone's rotation delta Z in bitstream
0xF	u16	Translation Key Count	If this bone channel has an animated translation, its value will match Rotation Key Count; otherwise the value will be 0. If the value is 0, the remaining Translation fields aren't present
0x11	s16	Initial Translation X	Initial X value of bone's translation vector
0x13	u8	Translation Bits X	Number of bits allocated to bone's translation delta X in bitstream
0x14	s16	Initial Translation Y	Initial Y value of bone's translation vector
0x16	u8	Translation Bits Y	Number of bits allocated to bone's translation delta Y in bitstream
0x17	s16	Translation Delta Z	Initial Z value of bone's translation vector
0x19	u8	Translation Bits Z	Number of bits allocated to bone's translation delta Z in bitstream
0x1A	End of descriptor