LIGHTWAVE 3D LAYERED OBJECT FILE FORMAT

                          by Stuart Ferguson
                           revised  10/4/95


Introduction

  The "Layered Object" LWLO file format is used by Modeler to save an
  editing session which includes multiple layers of data, some active
  and some in the background.  This IFF format file is nearly
  identical to the normal LWOB format object file, so this document
  assumes that the reader is familiar with the LWOB format.

  The IFF form-type is LWLO for "LightWave Layered Object" and it
  contains the same PNTS, SRFS, POLS, CRVS and SURF chunks as LWOB
  files.  Unlike the LWOB format, however, the LWLO file may contain
  any number of PNTS, POLS and CRVS chunks, with each one describing
  the contents of a single layer.  Layers are described by the LAYR
  chunk.


LAYR Chunk

  An LAYR chunk must precede each set of PNTS, POLS and CRVS data
  chunks and indicates to which layer those data belong.  An LAYR
  chunk consists of two unsigned short values and a string.  The first
  is the layer number which should be from 1 to 10 to operate
  correctly in Modeler.  This restriction may be lifted in future
  versions of the format.  The second value is a bitfield where only
  the lowest order bit is defined and all others should be zero.  This
  bit is one if this is an active layer and zero if it is a background
  layer.  The string which follows is the name of the layer and should
  be null-terminated and padded to even length.


Schematic Example

  The file is read just like any LWOB format file except that each time
  a LAYR chunk is encountered, the subsequent PNTS, POLS and CRVS data
  chunks are redirected to the indicated layer.  The SRFS and SURF
  chunks are read normally.  Below is a simplified example which shows
  a typical file and the order of reading.

    FORM LWLO       -- tag identifies file type and size
        SRFS ...    -- surface name chunk gives name mappings
                        for subsequent POLS chunks
        LAYR        -- first layer starts
            0003        layer 3 which is active and called "noname"
            0001
            6E 6F 6E 61 6D 65 00 00
        PNTS ...    -- data in layer 3
        POLS ...
        LAYR        -- second layer starts
            0006        layer 6, background, called "Foo"
            0000
            46 6F 6F 00
        PNTS ...    -- data in layer 6
        POLS ...
        CRVS ...
        SURF ...    -- surface description
                    -- end implied by file length