Surface (.sur) files are used for physics, i.e. collision detection. The format is somewhat convoluted and not friendly to parsing. Notable “features” are use of integers of unusual bit lengths and heavy use of offsets to denote structures.
|uint32||Signature||FourCC signature. Must always be 0x73726576 (vers).|
|float||Version||Must always be 2.0|
Following header are sequence of parts:
|uint32||PartID||FLCRC32 hash of object name in “Cmpnd/Part_*/Object name”|
|uint32||Sections||Number of sections|
partID is FLCRC32 hash of “Cmpnd/Part_*/Object name” or is 0 for .3db models.
Every part minimally has extent and mesh sections.
The order of section does not seem to matter but all Freelancer files follow the same order: non-fixed, extent, mesh, hardpoints. Therefore section counter always is between 2 and 4.
When present indicates this part is moveable. Root part for .cmp or null part for .3db should always have “not-fixed” section-flag. For other parts it should be present if associated compound object joint isn't of fixed type (Rev, Pris, etc). Section has no further data, just the tag.
|float||Minimum X||Minimum point coordinates|
|float||Maximum X||Maximum point coordinates|
Bounding box for part, probably used for AABB comparison. Min/max coordinates from mesh points.
Contains header, convex hulls, points buffer and BSP tree nodes, in exact this order.
|uint32||Size||Mesh section length|
|float||Center X||Sphere center offset|
|float||Inertia X||Inertia vector (unused?)|
|uint24||Nodes end offset||Offset to end of BSP tree|
|uint32||Nodes start offset||Offset to start of BSP tree|
|float||Unknown X||Unknown vector|
Immediately after header hulls are listed. There's neither counter nor length for them, so this block is is read until file pointer position matches hull offset + offset to point block.
|uint32||Offset to point block||Offset to points block|
|uint32||PartID/Node offset||Object name hash|
|uint24||Reference count||Number of refs in DWORDs|
|uint16||Faces count||Hull face count|
Followed by this block repeated faces count times:
|uint32||Edge A||Face edge|
Each face edge:
|uint16||Index||Index in points buffer|
|int15||Offset||DWORD offset to other edge|
|bit||Flag||True for wraps|
Each hull is expected to be convex, otherwise unpredictable behavior may occur during collisions, ranging from hits not being registered to outright crashing the game. It would appear hulls in Freelancer files are generated by quickhull algorithm. Some modeling software provides tools to generate convex hulls from array of points using this algorithm: MassFX/PhysX in 3Ds MAX, Convex Tool in Blender, etc. It is recommended to keep number of vertices as few as possible and having them homogeneously spread across mesh seem improve stability, a good number to aim for is 30-40 vertices per convex mesh. Generally avoid running convex tool over actual visible mesh, create much simplified geometry from mesh and use it instead. Even convex meshes may fail to work in Freelancer if they are too detailed or there are too many of them.
Typically mesh section will contain hull IDs matching that of the part containing mesh section, however additional hulls can be present for hardpoints and sometimes for fixed compound children too, resulting in duplicate hulls: a parent part having hulls for a child part (and those hulls IDs matching child part ID) while child having its own hulls as well. However this isn't always the case, not every hitbox in Freelancer model seemingly created with this behavior, some do and others not. It could be accounted for different exporter versions producing these duplicates or perhaps something to do with destructible parts. An example of duplicate parent-child hulls can be found in li_elite.sur, Root part containing hulls for wings as well, despite the fact wing parts have hulls of their own.
A single special hull (type 5) will encase all regular hulls (type 4) and hulls/shrinkwraps of all fixed children the associated compound object has. It would seem to be a case of a hull generated over all points in point buffer of a mesh.
However shrinkwrap hull will not be present if mesh has only one hull and associated compound object doesn't have fixed children.
Points are read until file pointer matches mesh section offset + nodes start offset.
|float||Position X||Point coordinates|
Tree nodes are read until file pointer match mesh section offset + nodes end offset.
|int32||Right child offset||Offset to right child|
|int32||Hull offset||Offset to hull|
|float||Center X||Bounding box center|
|float||Size||Bounding box size|
|uint8||Scale X||Size multiplier|
|uint32||Count||Humber of hardpoint IDs to follow|
Contains list of flcrc32s of hardpoint names. The list indicates which hardpoints will have their own hitbox overridden by hull of same ID provided in part meshes. Otherwise attached objects will retain their own hitboxes.
Vanilla models used boxes for HpWeapon hardpoints, hemispheres for HpTurret hardpoints, and some sort of cylinder for equipment hardpoints such as HpCM, HpThrust.
Here are some personal notes and observations regarding the format, which may or may not be correct.
For completeness sake despite opposite face index not being used it could be done in MAX by firing ray from triangle center opposite to face normal and see which face it lands on.
It's most likely that hulls within part matching it's child partID overrides any hulls of a child part. There are few “broken” hitboxes in Freelancer, for example or_elite.sur where hulls of wing parts (Or_port_wing_lod1 and Or_star_wing_lod1) are misplaced, yet Root has hulls for them at correct place.
Hulls of child partID are definitely used as can be observed with case of depot_lod.sur where tube walls hulls are stored within Root part yet have partID of two children (port_dock_lod1 and star_dock_lod1) and not Root.
For Lanceredit it might be useful to display hulls in color matching designated color of a part and use some special shading to hulls whose partID doesn't match part or any of its descendants (and their hardpoints too!) with a message in log telling which hull and where has no part to be associated with.