P3D Lod Faces

Alias 'Polygons'

LodFaces

struct
{
  ulong   nFaces;
  ulong   AllocationSize;   // see below
  LodFace LodFaces[nFaces];
}

LodFace

LodFace
{
  //////////// MLOD ////////////////////////
  char              TextureName[32]       // SP3X ONLY (ofp)
  ulong             FaceType;             // 3==Triangle or 4==Box
  PsuedoVertexTable PsuedoVertexTables[4];//
  ulong             FaceFlags;            // P3D Point and Face Flags
  Asciiz            TextureName;          // P3DM ONLY (arma)
  Asciiz            MaterialName;         // P3DM ONLY (arma)
  ////////// ODOL   ////////////////////////
  ulong             FaceFlags;            //ODOL7 ONLY see P3D Point and Face Flags
  short             TextureIndex;         //ODOL7 ONLY
  byte              FaceType;             // 3==Triangle or 4==Box
  ushort            VertexTableIndex[FaceType];
  //////////////////////////////////////////
}

There are always 3, or 4, vertices represented by the value in FaceType.

• 3 point vertices describe a triangle.
• 4 point vertices describe a rectangle.

Ultimately, the value points to a vertex table comprising of points, normals and uvset(s).

• Odol: The actual vertex wanted for each of the vertices of a triangle (eg) are carried in an index table within this structure. The so-called VertexTableIndex. This index table looks up the VertexTable of the lod.

• Mlod: The values of each vertex are supplied in a PsuedoVertexTable. Within which, are indexes to the respective points and normals. The PsuedoVertexTable has a fixed size of 4 vertices. The 4th is unused for triangles.

Indices themselves must be transformed as follows

• triangles  : 1st posn, 2nd posn, 0th posn.
• quadrangles : 1st, 2nd, 3rd, 0th

The TextureIndex is a zero based array. If set to -1, there are no textures

In Arma, the FaceFlags and Texture Index have been moved out to their own LodSections.

PsuedoVertexTable

PsuedoVertexTable
{
 ulong PointsIndex;
 ulong NormalsIndex;
 float U,V;
}

Polygon Vertex Order

For a visible 3-vertex polygon:

AB // clockwise order
C 
.
AC // CounterClock
B 

The same for 4-vertex polygon:

AB // clockwise
DC 
.
AD // CounterClock
BC 

Your 3D device will cull invisible polygons. An invisible polygon is a polygon that has the other direction order. For example, DirectX default setting is 'cull counterclockwise polygons', so only clockwise is visible.

Vertices must be reordered for clockwise vertex order (default for DirectX), and not changed for counterclockwise order:

for 3-vertices polygon: 
1. 1st vertice descriptor 
2. 3rd vertice descriptor 
3. 2nd vertice descriptor 
4. (not used, zero filled) 

for 4-vertices polygon: 
1. 1st vertice descriptor 
2. 4th vertice descriptor 
3. 3rd vertice descriptor 
4. 2nd vertice descriptor

AllocationSize

Because of the variable amount of face vertices (3 or 4), AllocationSize is used to

• skip the block.
• calculate memory requirements

It's value is relative to the first entry and is computed as follows

 AllocationSize= nFaces* (SizeofEach (FaceVertice)); 

In Memory, as allocated by the engine, each FaceVertice is:

Engine	FaceFlag	TextureIndex	FaceType	Indexes
On Disk	sizeof(ulong)	sizeof(short)	sizeof(byte)	sizeof(short)
OFP (V7)	sizeof(ulong)	sizeof(short)	sizeof(ulong)	sizeof(short)
Size	4+	2+	4+	2*FaceType	always 16 or 18
ARMA (V4x)			sizeof(short)	sizeof(short)
Size			2+	2*FacetType	always 8 or 10

This, is the MEMORY requirement. To calculate the offset to the sections struct

• V7 OffsetToSectionsStruct = AllocationSize- 3*nFaces;
• V4x OffsetToSectionsStruct = AllocationSize- nFaces;

The difference being between sizeof(byte) vs long and short respectively

Model File Formats