Draft Geometry Format

• Working name: SOG ("simple open geometry"), file extension .sog
• Purpose: to enable import of models from popular modelling packages
  • primarily 3DS MAX
  • consideration of other sources, including Lightwave (LWO), MultiGen (FLT) and Blender
• Design considerations
  • compact, binary format
  • chunk-based, using tokens with lengths, to increase the potential of forward and backward compatibility in case of changes to the format
  • well-matched to the capabilities and limitations of realtime 3D APIs
    • one material per mesh
  • should not require complicated code or significant CPU to parse and load
    • store vertex normals, not "smoothing groups"
  • should allow any number of geometry objects
  • should allow a hierarchy of parent-child relationships
• Scope decisions
  • should it attempt to represent an entire scene graph?  LOD nodes, container groups, transforms?
  • should it allow non-indexed primitives, or require the indices to be explicit?

Current (Evolving) Spec

Each chunk consists of a token (2 bytes), a length (2 bytes) and data (any length).
For data chunks, length is the number of bytes contained in the data.
For chunks which contain other chunks, length is the number of contained sub-chunks.
Strings data chunks include the terminating 0 byte.

Header: identifying 4 bytes "SOGF"

Hierarchy of chunks
optional chunks shown in italics
chunks are listed as NAME, type (length), contents

• VERSION, short (2), format version number, currently "1"
• NUM_MATERIALS, short (2), number of materials
  • MATERIAL, short (2), number of component chunks for this material
    • DIFFUSE, rgba(16)
    • SPECULAR, rgb(12)
    • AMBIENT, rgb(12)
    • EMISSION, rgb(12)
    • CULLING, bool (1)
    • LIGHTING, bool (1)
    • TRANSPARENT, bool (1)
    • IMAGE_FNAME, string
    • CLAMP, bool (1)
• NUM_GEOMETRIES, short (2), number of geometries
  • GEOMETRY, short (2), number of component chunks
    • GEOM_NAME, string
    • GEOM_ID, short (2), 0-based numeric id of this geometry
    • GEOM_PARENTID, short (2), numeric id of the parent geometry, or -1 if no parent
    • NUM_MESHES, short (2), number of meshes
      • MESH, short (2), number of component chunks (the first 8 are required)
        • VTX_FLAGS, short (2), bit values to indicate which vertex components are present
        • PRIM_TYPE, short (2), OpenGL primitive type (e.g. GL_TRIANGLE_STRIP)
        • MAT_INDEX, short (2), index of the material for this mesh
        • NUM_VERTICES, short (2), number of vertices
        • NUM_INDICES, short (2), number of indices
        • NUM_PRIMITIVES, short (2), number of primitives
        • VTX_COMPONENTS, short (2), number of chunks per vertex
        • VTX_ARRAY, array of vertices (length = number of vertices), data contains:
          • the vertices, in order, each composed of the following chunks:
            • VTX_POS, 3D vector (12)
            • VTX_NORMAL, 3D vector (12)
            • VTX_COLOR, rgb (12)
            • VTX_COORD1, 2d vector (8)
        • INDEX_ARRAY, array of shorts (length = 2 * number of indices), the index values
        • PRIM_LEN_ARRAY, array of shorts (length = 2 * number of primitives), if the primitives are variable-length, this states the number of indices for each primitive