.LWO File Format – Data types and chunks

Data types

Everything is big-endian (most significant byte first).

Chunks and sub-chunks

Sub-chunks appear inside ENVL, CLIP and SURF. A sub-chunk’s meaning depends on where it is: CHAN in an envelope is not CHAN in a surface block. Readers skip chunks they do not know by their length.

LAYR – Layer

LAYR { number[U2], flags[U2], pivot[VEC12], name[S0], parent[U2] ? }

Starts a new layer; every chunk after it belongs to that layer until the next LAYR. Data before any layer goes into an arbitrary layer. Bit 0 of flags set means the layer is hidden. pivot is the layer’s pivot point. parent is the number of the default parent layer, or −1 (or missing) for none.

PNTS – Points

PNTS { point-location[VEC12] * }

The points of the current layer, numbered from 0; their number is the chunk length ÷ 12. A PNTS must come before the POLS, VMAP and VMAD chunks that refer to it. LightWave’s coordinate system is left-handed: +X right (east), +Y up, +Z forward (north). There are no explicit units; by convention the unit is the metre. Positions are relative to the layer’s pivot.

VMAP – Vertex map

VMAP { type[ID4], dimension[U2], name[S0], ( vert[VX], value[F4] # dimension )* }

Gives some of the points of the latest PNTS a vector of dimension values. A file may hold any number of vertex maps, each with a different type or name.

The description notes that other types will appear. As every ID is four bytes, RGB is written RGB , with a space.

POLS – Polygons

POLS { type[ID4], ( numvert+flags[U2], vert[VX] # numvert )* }

The polygons of the current layer. Each starts with a U2 whose low 10 bits are the vertex count (up to 1023) and whose top 6 bits are flags; the vertices follow as point numbers in the latest PNTS.

The meaning of the 6 flag bits depends on the type and is documented only for CURV: the two lowest flag bits (bits 10 and 11) mark the first and last points as continuity control points, and the four highest (bits 12 to 15) are extra high bits of the vertex count, allowing curves of up to 16,383 points. A polygon’s vertices start at a convex vertex and run clockwise as seen from its visible side; the normal is the cross product of the first and last edges.

TAGS and PTAG – Tags

TAGS { tag-string[S0] * }

A list of names, numbered from 0, that PTAG chunks assign to polygons.

PTAG { type[ID4], ( poly[VX], tag[U2] )* }

Gives polygons of the latest POLS a tag of one type, as an index into the latest TAGS. Not every polygon needs a tag of every type.

VMAD – Discontinuous vertex map

VMAD { type[ID4], dimension[U2], name[S0], ( vert[VX], poly[VX], value[F4] # dimension )* }

Introduced with LightWave 6.5. Like VMAP, but each value belongs to a point as used by one particular polygon, so a point can have different values in different polygons. This solves UV seams: where a VMAD has the same name as a VMAP, its values replace the map’s for the polygons it lists. The types TXUV, RGB , RGBA and WGHT are supported; others, such as MORF and SPOT, are kept but never used.

VMPA – Vertex map parameters

VMPA { UV-subdivision-type[I4], sketch-colour[I4] }

Special properties of a vertex map, from later versions of LightWave. The UV subdivision types are 0 linear, 1 subpatch, 2 subpatch with linear corners, 3 subpatch with linear edges, 4 subpatch with discontinuous edges.

Other chunks

The description gives the icon’s height as (chunk length − 4) ÷ width, which counts bytes rather than pixels; with three bytes per pixel the row count is a third of that.