Magic Number: PGEOMETRY
Point/Prim Counts: NPoints # NPrims #
Group Counts: NPointGroups # NPrimGroups #
Attribute Counts: NPointAttrib # NVertexAttrib # NPrimAttrib # NAttrib # 

In each of these cases, the # represents the number of the element described. Groups are named and may be defined to contain either points or primitives. Each point or primitive can be a member of any number of groups, thus membership is not exclusive to one group.

Attributes in GPD have been generalized. Attributes can be assigned per point, per vertex, per primitive or on the detail. Therefore, the number of attributes is declared at the top of the file. Later, each of these attributes will be defined in full.

Internally, there are “dictionaries” to define the attributes associated with each element. These dictionaries define the name of the attribute, the type of the attribute and the size of the attribute. Also, the default value of the attribute is stored in the dictionary.

When the dictionary is saved, each attribute (in a specific order) is defined. The definition is basically as follows:

Name Size Type Default

For example, the attribute name for normals is “N”, so the attribute definition would look like:

N 3 float 0 0 0

…specifying the attribute name “N”, that there are three elements in this attribute and the type is float. The default value would be (0, 0, 0)

Following the element definition is the attribute data associated with the element. There are braces delineating the attribute data. The attribute data appears in the order that the dictionary for the element was defined.

For example, a dictionary might look like:

PointAttrib
Cd 3 float 0 0 0 # Color attrib., 3 floats, default 0 0 0
Alpha 1 float 1 # Alpha attribute, 1 float, default 1
N 3 float 0 0 0 # Normal attribute
uv 2 float 0 0 # Texture coordinate 

The data for the point might look like:

0 0 0 1 (1 0 0 1 0 0 1 .5 .5)
^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^ Position Attributes 

The point would have:

Cd = (1, 0, 0)
Alpha = 1
N = (0 0 1)
uv = (.5, .5) 

The types of attribute data supported are: integer, float, string and index. The “string” type is stored as a 32 character string since each attribute must have a fixed length. The integer and float types are pretty self-explanatory. The index attribute type is used for specifying things like material. It contains a list of strings which are indexed by integer values. Thus the storage for an index attribute is an integer. In the definition of the index attribute, the attribute values are defined as well.

mat 1 index 3 marble gold crystal_glass3

The default value for all index attributes is -1 indicating that the attribute has not been assigned.

If there are point attributes, the attribute dictionary is saved before the definition of the points.

Dictionary Name: PointAttrib
Dictionary Data: -- Attribute Definition -- 

Following the attribute dictionary, is the point data for the points. Each point is stored with four components (x, y, z, w). The positions are not true homogeneous coordinates. To get the homogeneous coordinate, simply multiply each x, y, and z by w.

If (and only if) there is attribute data, the attribute data is defined following the point position. The attribute data is enclosed in parentheses “()”.

If (and only if) there are vertex attributes, the attribute dictionary is found here.

Following the vertex attribute dictionary is the primitive attribute dictionary (if and only if there are attributes for primitives).

Since every primitive may have local information which needs to be saved, the format of every primitive is different. In general, the format is:

PrimKey <local_information> [attributes]

Here, the local_information is primitive specific.

As part of the local information, a vertex or multiple vertices are specified. Each vertex is saved in the same format, which is:

point_number attribute_data

The point numbers start at 0 and go through number of points - 1. If there is vertex attribute data, the data is delimited by parenthesis (). If there is primitive attribute data, it is delimited by brackets [].

Each primitive has a unique identifier. The current primitives and their identifiers are:

Polygon: "Poly"
NURBS Curve: "NURBCurve"
Rational Bezier Curve: "BezierCurve"
Linear Patch: "Mesh"
NURBS Surface: "NURBMesh"
Rational Bezier Patch: "BezierMesh"
Ellipse/Circle: "Circle"
Ellipsoid/Sphere: "Sphere"
Tube/Cone: "Tube" Metaball "MetaBall"
Meta Super-Quadric: "MetaSQuad"
Particle System:  "Part"
Paste Hierarchy: "PasteSurf" 

The primitive keys are case sensitive. For example:

VertexAttrib
uv 3 float 0 0 0
PrimitiveAttrib
Cd 3 float 0 0 0
Poly 3 < 0 (1 0.5 0) 1 (0 0 0) 2 (0 1 0) [1 1 0 .5] 

…would specify a closed polygon (see below) which has three vertices referencing points 0, 1 & 2. Each vertex has 3D texture coordinates specified in (), the polygon has Color and Alpha specified in []. The color is yellow, with 50% alpha coverage.

When there are two or more consecutive primitives of the same type, this is specified as a run of primitives. In this case, the following should appear in the file:

Run # PrimKey

Where # is the number of primitives in the run. In this case, the following primitives are not saved with the PrimKey identifier since it is implicit in the run.

#Vtx OpenClose Vertex_List

Where…

#Vtx OpenClose Basis Vertex_List

The basis definition for both NURBS and Bezier primitives starts with:

Keyword Order

Where:

Keyword Order EndCondition Knots

#K = #Vtx + Order - 2

Keyword Order Knots

#Cols #Rows UWrap VWrap connectivity

Where:

The connectivity is ignored in many cases, but is critical for operations like sweeping or conversion to polygons.

Triangulated and Alternate meshes are structured like this:

#Cols #Rows UWrap VWrap connectivity UBasis VBasis

#Cols, #Rows, UWrap, VWrap, connectivity, Vertex_List are the same as for Mesh.

UBasis / VBasis are the same as for NURBS / Bezier Curve.

Profiles is an optional list of profile curves (curves on surfaces). The structure of the profiles section is very similar to that of the main geometry, including a header section, points, primitives, point and primitive groups. The differences are that this section doesn’t contain any attributes and has only four primitive types: polygon, NURBS curve, Bezier curve, and Trim Sequence.

The profile header is “Profiles:”. It is followed by “none” if there are no profiles. If there are profiles, the profile section has the following structure:

ProfileKey <local_information>

point_number

Polygon: "Poly"
NURBS Curve: "NURBCurve"
Rational Bezier Curve: "BezierCurve" 

Poly 3 < 0 1 2

Run # ProfileKey

TrimRegion [natural] #Profiles <profile_number ustart uend>...

TrimRegion 2 0 1 0 5 -3.5 8

GroupName Type NElements BitMask ElementList

Vertex_Info Matrix33

There is always only one vertex for a Circle. The 3 x 3 matrix contains scaling and rotation transformations about the center of the circle. Sheared circles are thus allowed.

Vertex_Info Matrix33

There is always only one vertex for a Sphere. The 3 x 3 matrix contains scaling and rotation transformations about the center of the sphere. Sheared spheres are thus allowed.

Vertex_Info Taper Closure Matrix33

There is always only one vertex for a Tube/Cone. The vertex lies in the center of the tube (along the axis connecting the centers of the top and bottom circles/ellipses). The taper value affects the radius of the top circle. A regular tube has a taper value of 1. A cone’s taper is 0. The closure - “closed” or “open” - indicates whether the tube is end-capped. The 3 x 3 matrix contains scaling and rotation transformations about the center of the tube. Sheared tubes are thus allowed.

Vertex_Info Kernel_Function Weight Matrix33

There is always only one vertex for a metaball. The kernel function is one of: wyvill, quartic, blinn, or links. The 3 x 3 matrix contains scaling and rotation transformations about the center of the metaball. Sheared metaballs are thus allowed.

Vertex_Info XY_Exponent Z_Exponent Kernel_FunctionWeight Matrix33

There is always only one vertex for a meta super-quadric. The exponents are float values. The kernel function is one of: wyvill, quartic, blinn, or links. The 3 x 3 matrix contains scaling and rotation transformations about the center of the metaball. Sheared metaballs are thus allowed.

Part_Count Vertex_List

Where Part_Count is the number of particles in the system.

The Detail Attribute Dictionary is saved after the Primitives and before the group information.

The Point groups are saved first, followed by the primitive groups. There is no identifier indicating the groups. The format for a group depends on whether it is ordered or unordered:

GroupName Type NElements BitMask ElementList

This is meant for saving information such as metaball expressions and surface hierarchies. Currently this section contains only the delimiting tokens, one per line:

beginExtra endExtra 

For now the Extra body is empty because all the metaballs are merged (“add”-ed implicitly) and there is no support for surface hierarchies.