Skyrim Havok Animation Reference

Cross-reference: BSA v104/v105, NIF format, BSA tools → XeNTaX Bethesda knowledge §1–3

Skyrim (LE/SE/AE) drives its animation system with Havok packfile (.hkx) files built against the Havok 2010.2.0 SDK, the same generation of the engine used by Fallout 3/New Vegas. This document distills the animation file types, the pack/unpack pipeline, and the community tooling — most of it from the hkxcmd README and the Havok packfile format notes in reference/havok/hkx_format.md. It is written from the formats and the tools, not from any single implementation.

Havok Version & File Types

Skyrim's .hkx files are Havok packfile binaries (magic 0x57e0e057 0x10c0c010). The packfile version and pointer width differ between the releases: LE ships 32-bit packfiles (4-byte pointers), while SE/AE ship 64-bit packfiles (8-byte pointers), packfile version 8 — version 11 and 64-bit pointers apply to Fallout 4, not Skyrim. Always parse pointer width from the header (bytes_in_pointer), not from the game title.

Extension	Contents	Notes
.hkx	Havok packfile binary	Animation clips, skeletons, behavior graphs — same container, different payloads
skeleton.nif	NetImmerse/Gamebryo rig	The mesh-side bone hierarchy; what NifSkope and the 3ds Max importer read
skeleton.hkx	hkaSkeleton packfile	Havok-side skeleton with reference pose, parent indices, bone names
animation\*.hkx	hkaAnimation packfile	Spline/interleaved compressed animation data
behavior\*.hkx	hkbBehaviorGraph project data	State machines, variables, events, modifiers; not readable by SDK tooling
.kf	Gamebryo keyframe	Interchange format for 3ds Max round-trips
.bsa	Bethesda archive	Bundled .nif/.hkx/.kf — must be unpacked before tooling

Where Animation Files Live

Animation data ships inside .bsa archives; you must unpack the BSAs first (BSA Browser, Archive2, or similar) before any tool can touch them. The archive format itself differs by release: LE ships BSA v104 (0x68) with zlib compression, SE ships BSA v105 (0x69) with LZ4 compression (UESP "Skyrim Mod:Archive File Format"). Within the extracted meshes tree the layout is consistent:

meshes\actors\<actor>\character assets\skeleton.hkx (Havok skeleton)
meshes\actors\<actor>\character assets\skeleton.nif (rig)
meshes\actors\<actor>\animation\*.hkx (animation clips)

Concrete example from the hkxcmd README (the chicken tutorial):

meshes\actors\ambient\chicken\character assets\skeleton.hkx
meshes\actors\ambient\chicken\animation\getup.hkx

Behavior graph files sit in parallel behavior folders (e.g. the human 0_master.hkx master graph); the exact per-actor layout varies, but all of them are Havok Behavior payloads.

Animation Pipeline

The standard pipeline converts packed HKX → XML HKX (for Havok tools) and HKX → Gamebryo KF (for 3ds Max), then back. All commands take an input folder (or file) and recurse into subfolders; without an output folder, output goes next to the input with an -out suffix.

Step	Command	Purpose
1	hkxcmd convert <in> [out]	Packed HKX → XML HKX (loadable in Havok tools)
2	hkxcmd exportkf <in> [out]	HKX → Gamebryo .kf for 3ds Max / NifSkope
3	hkxcmd convertkf <skeleton.hkx> <anim.kf> <out.hkx>	KF → HKX using a skeleton's bone layout and bind pose; Win32 packed binary by default, -v:XML for XML output
4	hkxcmd dumptext <skeleton.hkx>	Dump bone names to text (useful for the Havok export pipeline)
5	hkxcmd retarget <...>	Re-map animation to another skeleton's bone layout — does not work correctly

End-to-end workflow:

1. Unpack the BSAs so .hkx files are loose on disk.
2. Convert hkxcmd convert "Skyrim - Animations" → XML HKX.
3. Preview in Havok Preview Tool (see below).
4. Export KF hkxcmd exportkf → .kf files.
5. Import into 3ds Max via the niftools importer (Netimmerse/Gamebryo (*.nif, *.kf)). Import the skeleton.nif explicitly before the mesh if animations come in corrupt.
6. Edit, add the start/end note tracks, export as "Single KF w/o NIF".
7. Convert back hkxcmd convertkf skeleton.hkx export.kf export.hkx.
8. Validate the result in NifSkope.

Note-Track Convention

KF duration is carried by note tracks on the root node (NPC Root [Root]), and exportkf writes them automatically. When exporting from 3ds Max by hand you must replicate them:

• Add a Note Track to the root node.
• Add a note at time 0 with text start.
• Add a note on the final frame with text end.

exportkf (v1.3+) computes frames as frame_increment = duration / (nframe - 1); convertkf rounds frames back, so the note positions define the clip's timeline on both sides of the round-trip.

Havok Preview Tool Workflow

The Preview Tool (from the Havok Content Tools 2010.2.0 install) can display Skyrim animations, but only the converted XML forms — not the packed originals:

1. Convert skeleton.hkx and the animation .hkx with hkxcmd convert.
2. File | Open... → the converted skeleton XML.
3. File | Add... → the converted animation XML.

"Wrong platform for packfile" means you selected the original packed skeleton, not the converted one. Use the -out folder's files.

The Preview Tool also chokes on very large XML dumps — dragon.hkx, for instance, fails to load even after conversion.

Known Limitations

The community tooling is partial, and the gaps are structural, not bugs:

Limitation	Detail
Behavior files unreadable	The Havok SDK only ships Physics and Animation support; Bethesda's hkbBehaviorGraph project data (state machines, variables) cannot be read — "ignore any errors about files it cannot convert"
Bone mapping assumed 1:1	hkxcmd assumes a 1:1 skeleton-to-animation-track mapping because the mapping info is not present in the files; this is "obviously not always a correct assumption"
Non-uniform scale lost in KF	Havok supports non-uniform scale; Gamebryo does not, so .kf export drops that information
No visibility/camera tracks	.kf export has no handling for visibility or camera tracks, which Skyrim uses
retarget broken	The retarget command is documented as not working correctly
Round-trip is lossy	exportkf + convertkf does not reproduce the original .hkx (hkxcmd issue #1)
SDK lock-in	hkxcmd targets the 2010.2.0 SDK; the question of a 2012.2 SDK build is still open (issue #2), and skeleton.hkx replacement workflows remain unsolved (issues #4, #5)

Packfile Format Notes

For binary-level work, reference/havok/hkx_format.md documents the packfile container (header, section table, fixups). Key Skyrim deltas from that doc:

• Packfile version 8 (SE/AE) vs 11 (FO4); LE is 32-bit → 4-byte pointers, SE/AE are 64-bit → 8-byte pointers.
• Contents version string is the hk_2010.2.0-r1 era for Skyrim, versus hk_2014.1.0-r1 for FO4.
• hkaSkeleton in the Skyrim era predates the HK2012+ partitions member seen in FO4 files.

Tools

Tool	Source	Role
hkxcmd 1.4	figment/hkxcmd	Convert packed↔XML HKX, export/convert KF, dump bones
NifSkope	niftools/nifskope	NetImmerse .nif/.kf editor & validation
HKXPack	Dexesttp/hkxpack	Pack/unpack HKX↔TagXML (the FO4 guide's editor of record); targets 2014.1, 2010.2 noted
Havok Content Tools 2010.2	Intel (free, registration required)	Preview Tool + SDK for the 2010.2 format
3ds Max + niftools exporter	niftools.org	Import/export .nif/.kf for editing
Havok Behavior Tool	Creation Kit wiki	Behavior graph editing — wiki is down for backend maintenance (last modified 2024-02-07)

Community Resources

Resource	What it is	Notes
Modding.wiki — Skyrim	Community knowledge base (Nexus Mods)	/users, /developers (Creation Kit, settlements, documented crashes, plugin differences) and /glossary

Sources

• figment/hkxcmd — Havok Command Line Tools for Skyrim (README)
• hkxcmd README.txt
• hkxcmd issues
• niftools/nifskope
• Havok Behavior Tool — Creation Kit wiki
• Havok Content Tools (Intel)
• UESP — Skyrim Mod:Archive File Format
• Dexesttp/hkxpack
• reference/havok/hkx_format.md — packfile container format (FO4/Skyrim SE header facts)