3D Graphics Programming with QuickDraw 3D 1.5.4

Trimeshes

This section gives the 3DMF specification of the Trimesh object. Trimesh binary metafiles implement a simple type of compression, using a scheme described below.

Normally a Trimesh also has one or more AttributeArray subobjects. For details, see "Attribute Arrays" .

Labels

ASCII: TriMesh
Binary: tmsh ( = 0x746D7368 )

AUXILIARY DATA STRUCTURES

The following auxiliary data structures are used to specify trimeshes. They are mirror images of the similarly-named structures in the file QD3DGeometry.h. See 3D Graphics Programming With QuickDraw 3D 1.5 for a detailed description.



 
typedef struct TriMeshTriangleData {

    Uns32   pointIndices[3];

} TriMeshTriangleData;



 
typedef struct TriMeshEdgeData {

    Uns32   pointIndices[2];

    Uns32   triangleIndices[2];

} TriMeshEdgeData;



 
typedef struct BoundingBox {

    Point3D     min;

    Point3D     max;

    Boolean     isEmpty;

} BoundingBox;

Data format



 
Uns32               numTriangles

Uns32               numTriangleAttributeTypes

Uns32               numEdges

Uns32               numEdgeAttributeTypes

Uns32               numPoints

Uns32               numVertexAttributeTypes

TriMeshTriangleData         triangles[numTriangles]

TriMeshEdgeData             edges[numEdges]

Point3D                     points[numPoints]

BoundingBox                 bBox

Data COMPRESSION

The triangles and edges fields in a trimesh are compressed. These fields are arrays of indices into arrays of elements; the maximum value of the indices depends on the size of the array of elements. Hence, the maximum value of an index in the TriMeshTriangleData field pointIndices is equal to numPoints . The same is true for the pointIndices field of TriMeshEdgeData ; the maximum value of an index in the triangleIndices field is numTriangles . Compression is performed as follows:

If maximum value of index <= 0xFE, write index as Uns8
If maximum value of index > 0xFE and <= 0xFFFE, write index as Uns16
If maximum value of index > 0xFFFE, write index as Uns32

The triangleIndices field of TriMeshEdgeData has, in addition, the following special case. If a side of an edge does not have a triangle (as is the case if the edge is on a boundary), this is indicated in the trimesh data structure by the constant kQ3ArrayIndexNULL . Since this is a 32-bit quantity, you may need to compress it. Do this by writing kQ3ArrayIndexNULL as follows:

If maximum value of index <= 0xFE, write 0xFF
If maximum value of index > 0xFE and <= 0xFFFE, write 0xFFFF
If maximum value of index > 0xFFFE, write kQ3ArrayIndexNULL

Example

For an example of a trimesh text metafile, see "Attribute Arrays" .