BRL-CAD Primitives
This article provides an overview of various types of geometric primitive objects that can be added to a BRL-CAD geometry file. The shape, size, location and orientation of each such object are defined by a set of parameters/properties that are specific to its type, which are discussed in the corresponding section below.
For general discussions on using MGED to create primitive objects, view their properties, and modify or move them, see:
also see:
Arbitrary convex polyhedra
An arbitrary convex polyhedron (arb) is a geometric volume that is completely enclosed by a set of 3-dimensional planes. Each has a set of straight-edged, flat faces outlined by the intersections of those planes. The intersection of each pair of planes is a line whose intersections with other planes defines a pair of vertices. The line segment between those two vertices is an edge of the polyhedron that is shared by two faces. Each vertex is common to an equal number (at least three) of faces and edges.
For example, a rectangular parallelepiped is enclosed by three orthogonal pairs of parallel planes. Their intersections define six faces, each with four edges and four vertices. There are a total of 12 edges (each shared by two faces) and 8 vertices (each shared by three faces and three edges).
The BRL-CAD geometry file format defines two types of records for such polyhedra:
Although any polyhedron can be defined and stored as an arbn, the arb8 record type is more commonly employed because it is simpler to work with and still accommodates most constructive solid geometry applications.
ARB8 Records
An arb8 record is specified by a set of eight {X, Y, Z} vertices designated V1 through V8, which need not all be unique. BRL-CAD uses such records to represent polyhedra having four, five or six faces:
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arb8 shapes have eight unique vertices. They represent hexahedra that have six quadrilateral faces sharing eight edges. In addition to simply specifying the {X, Y, Z} coordinates of those vertices, MGED provides easier ways to create the following specific types of hexahedra:
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3ptarb shapes represent right quadrilateral prisms, which are extruded quadrilaterals having parallel ends connected by four rectangular sides.
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box shapes represent parallelepipeds, whose faces comprise three pairs of equal parallelograms. Unlike a common box, those faces need not be rectangular—if they are, the enclosed volume is a rectangular parallelepiped.
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rpp shapes represent rectangular parallelepipeds (also known as cuboids and rectangular prisms), whose faces comprise three pairs of equal rectangles. If one pair of faces are squares, the volume is a square prism. If all of them are squares, the volume is a cube (geometrically, there cannot be just two pairs of square faces).
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arb7 shapes have seven unique vertices. They represent hexahedra that have four quadrilateral and two triangular faces sharing eleven edges. They can only be created by specifying the {X, Y, Z} coordinates of those vertices.
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arb6 shapes have six unique vertices. They represent triangular prisms and truncated tetrahedra, which are pentahedra* that have two triangular ends connected by three quadrilateral sides sharing nine edges. In addition to simply specifying the {X, Y, Z} coordinates of their vertices, MGED provides an easier way to create one specific type of hexahedron:
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raw (right angle wedge) shapes are triangular prisms whose ends are parallel to each other. Interestingly enough, they don’t seem to require any right angles. If the ends are perpendicular to the connecting edges, the shape is a right triangular prism and has rectangular sides. Presumably two of the rectangular sides of an actual right-angle wedge would also be perpendicular to each other.
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arb5 shapes have five unique vertices. They represent quadrahedra, which are pentahedra that have a quadrilateral base and four triangular sides sharing eight edges. If such a volume has a rectangular base it is a rectangular pyramid, one with a square base is a square pyramid.
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arb4 shapes have four unique vertices. They represent tetrahedra, which have four triangular faces sharing six edges. If all four triangles are equilateral, the shape is a regular tetrahedron.
ARBN Records
An arbn record is specified by N sets of intersecting planes, each defined by four coefficients:
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the {X, Y, Z} coefficients of the plane’s normal vector pointing outward from the center of the arbn shape, and
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the perpendicular distance of that plane from the origin.
As discussed in the Creating and editing arbn primitives article, BRL-CAD uses such primitives to represent polyhedra having any number of sides, edges and vertices.
Ellipsoids
ell
Ellipsoid
- Handled by
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make in form create
- Arguments
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- V
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vertex point, at the center
- vectors A B C
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describing the radii of the ellipses; A points front, B points right, C points up.
in ell.s ell 0 0 0 0 -1 0 1 0 0 0 0 1
sph
Sphere, special case of the ellipsoid, with vectors A B and C all the same magnitude (radius).
- Handled by
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make in form(ell) create
- Arguments
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- V
-
vertex point, at the center
- radius
ell1
Special case of ellipsoid
- Handled by
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in make form(ell) create
- Arguments
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vertex, vector A, radius of revolution
Cones and Cylinders
tgc
Truncated general cone
- Handled by
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in make form create
- Arguments
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vertex, vectors H A B, magnitudes of vectors C D
rcc
Right circular cylinder, special case of tgc
- Handled by
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in make form(tgc) create
- Arguments
-
vertex, height vector, radius
in rcc1.s rcc 0 0 0 1 1 1 0.5
rec
Right elliptical cylinder, special case of tgc
- Handled by
-
in make form(tgc) create
- Arguments
-
vertex, height vector, radius
rhc
Right hyperbolic cylinder
- Handled by
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in make form create
- Arguments
-
vertex, perpendicular vectors for Height and B, (r) rectangular half width, (c) apex to asymptote distance,
rpc
Right parabolic cylinder
- Handled by
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in make form create
- Arguments
-
vertex, perpendicular vectors for Height and B, (r) rectangular half width
derived from 2d
extrude
Extrusion of a 2-d sketch
- Handled by
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in make form(?) create
- Arguments
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vertex, perpendicular vectors Height A B, sketch, K
revolve
Revolution of a 2-d sketch
- Handled by
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in
- Arguments
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vertex, revolution axis, vector in start plane, angle, sketch
dsp
- Handled by
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in create
- Arguments
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data type (f|o), datasource, count of length and width, interpolation type, cut direction, cell size, unit elevation
See also Displacement (DSP) map primitive tutorial.
ebm
extruded bit map
- Handled by
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in form create
- Arguments
-
filename, width and height in cells, extrusion distance,
See also Inserting an ebm shape in MGED tutorial.
The extruded bitmap (also referred to as EBM) is a solid based on a greyscale bitmap. The bitmap is an array of unsigned char values, see bw(5), and is extruded by some distance. The EBM solid requires the dimensions of the bitmap file (height and width in bytes), an extrusion length, and a transformation matrix to position the EBM. Each byte in the bitmap file is treated as the base of a cell that is extruded by the specified extrusion length. If the value of the byte is nonzero, then that cell is considered solid.
Other solids
tor
Torus
- Handled by
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in make form create
- Arguments
-
vertex, normal vector, radius of revolution, tube radius
eto
Elliptical torus
- Handled by
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in make form create
- Arguments
-
vertex, normal vector, radius of revolution, vector C, (r_d) magnitude of semi-minor axis
part
Conical particle
- Handled by
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in make create
- Arguments
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vertex, height vector, radius at v, radius at h
The particle solid is a lozenge-shaped object defined by a vertex, a height vector and radii at both ends. The body of the particle is either a cylinder or a truncated cone, depending on the values of the radii. Each end of the particle is a hemisphere of the specified radius.
pipe
Hollow and solid pipes and wires
- Handled by
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in make create
- Arguments
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# points, for each point: location, inner and outer diameters, bend radius
ars
Arbitrary rectangular solid
- Handled by
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in make create
Solids of type 'ars' (Arbitrary Faceted Solids) are defined using "waterlines". The following figure consists of a start point, some number of intermediate polygons, and an ending point. Each of the intermediate polygons have the same number of vertices and the vertices are numbered 1 thru N. In addition to the intermediate polygons a line will be created that begins at the start point, goes through each polygon at its vertex numbered 1, and terminates at the end point. This is repeated for each polygon vertex 2 thru N. The start point, polygons, and end point are each a "waterline".
need an image here to illustrate the concept
the ars shape takes the following values as input:
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The number of points per waterline (the number of vertices on each intermediate polygon)
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The number of waterlines (the number of intermediate polygons plus 2)
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X, Y, and Z for a starting point (the first waterline)
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for each interior polygon (an intermediate waterline)
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for each point on the polygon
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X, Y, and Z for the point on the polygon
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X, Y, and Z for an ending point (the last waterline)
For example, the command:
in x.1 ars 4 6 0 0 3 1 1 3 1 -1 3 -1 -1 3 -1 1 3 1 1 1 1 -1 1 -1 -1 1 -1 1 1 1 0 -1 0 -1 -1 -1 0 -1 0 1 -1 1 0 -3 0 -1 -3 -1 0 -3 0 1 -3 0 0 -3
Will produce a square bar with a tapered 1/8 turn twist in the middle. Of course, more waterlines in the twist and more points per waterline would make the twist smoother.
The parameters to the above ars command can be dissected as:
- 4
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number of points per waterline (i.e. intermediate polygons have 4 vertices)
- 6
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number of waterlines (four intermediate polygons plus the two endpoints)
- 0 0 3
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the center of the top end of the bar
- 1 1 3 1 -1 3 -1 -1 3 -1 1 3
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a 2x2 square in the xy plane at z offset
- 3 1 1 1 1 -1 1 -1 -1 1 -1 1 1
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a 2x2 square oriented the same as the first but at z offset
- 1 1 0 -1 0 -1 -1 -1 0 -1 0 1 -1
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a 2x2 square at a 45 degree rotation from the first squares at z offset
- -1 1 0 -3 0 -1 -3 -1 0 -3 0 1 -3
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a 2x2 square at a 45 degree rotation from the first squares at z offset
- -3 0 0 -3
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the center of the bottom end of the bar
metaball
- Handled by
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in make form(*) create
- Arguments
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render method, threshold, number of points, location and field strength for each point (and blobbiness/goo factor)
brep
see NURBS
vol
volume / voxel
- Handled by
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in
- Arguments
-
filename, xyz dimensions of file (in voxels), lower and upper threasholds, xyz dimensions of a cell
The vol solid is defined by a 3-dimensional array of unsigned char values. The solid requires a file of these values, the extent of the file (in bytes) in each dimension, the size of each cell, and high and low thresholds. Any value in the file that is between the thresholds (inclusive) represents a solid cell.
Other
grip
Grip — support for joints, non-geometric (does not show in rt)
- Handled by
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in make form create
- Arguments
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- C
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Center
- N
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normal vector
- L
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magnitude
half
halfspace
- Handled by
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in make form create
- Arguments
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Normal, distance from origin
A half space is the portion of space on one side of a plane. It is represented by its boundary (the plane) and its outward-pointing normal vector.