Watch Dogs HKX Collision Format (WD1/WD2 — Ubisoft's Havok fork)

Cross-reference: WDL (2017.2 Dunia wrapper) leak-vs-retail bytes, build tags, TAG0/TCM0 chunking, TCRF/TCID compendium, PTCH/ITEM, type catalog → ../watch_dogs_legion/hkx_format_wdl.md Cross-reference: Standard Havok packfile format (FO4 uses regular Havok SDK) → reference/havok/hkx_format.md

The .hkx files alongside .xbg models contain Havok physics collision data for Watch Dogs 1 and Watch Dogs 2. Ubisoft used a custom fork of Havok to generate custom collision meshes — standard Havok tools cannot parse them (confirmed by FrankMK04: "the commercial SDK doesn't work for rendering them"). He reverse-engineered the format from the game's graphics engine DLL.

FrankMK04 (Discord, 8/18/26): "It's interesting to know that Ubisoft modified the SDK and removed several classes just to generate its own collision meshes, which is why other tools couldn't understand or analyze its structure. But now 'HKX has been hacked.' For now, it has a few bugs with complex models like vehicles (easy to fix, but it takes time) and with models that don't use convex meshes (first the complicated stuff, then the easy stuff) because basic colliders are easier to find."

• WD1 (2014): Uses Havok 2012 SDK, modified by Ubisoft (old packfile format) — see packfile-format.md
• WD2 (2016): Uses Havok 2015.1 SDK, modified by Ubisoft (TAG0 format) — see ../watch_dogs_2/wd2-tag0-collision-format.md
• WDL (2020): Uses Havok 2017.2 SDK with Dunia wrapper — see ../watch_dogs_legion/hkx_format_wdl.md

File Roles

Extension	Role
.hkx	Havok collision mesh (physics body)
.hkr	Havok ragdoll (skeleton constraints)
.ci	Collision index — maps collision meshes to entity slots

HKX + CI handshake: The .hkx file contains the actual collision geometry. The .ci (collision index) file maps which .hkx to use for each entity slot. They must match — a mismatched CI/HKX pair causes missing or broken collision.

File Counts

Game	.hkx Files	Notes
WD1	23,278	Buildings, vehicles, characters
WD2	8,195	Locations, vehicles, characters

Binary Structure

Header (16 bytes)

+0x00 u32 HeaderField0 (typically 0x89)
+0x04 u32 HeaderCrc
+0x08 u32 TagfileTotalSize (entire file size)
+0x0C u32 Reserved/unknown

Section Loop (starts at offset 0x10)

Each section is an hkChunk (8 bytes header + payload):

+0x00 u32 sizeAndFlags (24-bit size + flags, little-endian)
+0x04 u32 tag (FourCC, little-endian)
+0x08 byte[] payload (size bytes)

The sizeAndFlags field: size = value & 0xffffff, isSubChunk = (value & 0x40000000) != 0.

Section Tags (FourCC)

Tag	Constant	Description
TAG0	CompileFourCC("TAG0")	Header marker (809976148)
SDKV	CompileFourCC("SDKV")	Havok SDK version string
DATA	CompileFourCC("DATA")	Serialized object data
INDX	CompileFourCC("INDX")	Index data
TYPE	CompileFourCC("TYPE")	Type definitions
TSTR/TST1	CompileFourCC("TSTR")	Type name strings
FSTR/FST1	CompileFourCC("FSTR")	Field name strings
SCFG	—	Ubisoft-specific section

DATA Section

Contains serialized Havok objects:

• Type count (u32) + type entries (class_id + byte_count + data)
• Item count (u32) + item entries (type_idx + flags + offset + count)

HavokLib vs HavokDisrupt

HavokLib (PredatorCZ)

• C++ library, GPL-3.0
• Supports Havok 5.0.0 - 2017
• Handles hkaSkeleton, hkaAnimation, hkxEnvironment
• Python wrapper available
• Standard packfile format

HavokDisrupt (FrankMK04)

• C#/.NET tool (WPF)
• Watch Dogs 2 specific
• Handles HkxConvexShape (quantized/full-precision)
• Supports OBJ injection (HKX ↔ OBJ)
• Custom collision mesh format

Key Types (from HavokDisrupt)

HkxConvexShape

Variant HkxShapeVariant Quantized or FullPrecision
ShapeItemIndex int32
VertexHeaderItemIndex int32
AabbMin Vector3
AabbMax Vector3
ConvexRadius float32
QuantScale Vector3
Vertices List<Vector3>
QuadIndices List<int32>

HkxChildTransform

ChildItemIndex int32
Transform Matrix4x4
Scale Vector3

Ubisoft Modifications

1. Removed several Havok classes for custom collision mesh generation
2. Added HkxConvexShape with Quantized/FullPrecision variants
3. Used QuantScale + AabbMin/AabbMax for quantized vertex compression
4. The TypeId field maps to modified Havok class IDs

References

• HavokLib — C++ packfile library
• HavokDisruptWD2 — FrankMK04's HKX resolver for WD2 (reverse-engineered from game DLL, beta; bugs with complex vehicles and non-convex meshes)
• Havok 2013 SDK — original headers
• format_new.cpp in HavokLib — packfile parsing reference