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:
- Creating_primitive_objects
- Determining_the_properties_of_primitive objects
- Changing_the_properties_of_primitive objects
also see:
Contents
Arbitrary convex polyhedra
An arbitrary convex polyhedron (arb) is a geometric solid that is completely enclosed by a set of 3-dimensional planes. Each has a set of straight-edged, flat faces/facets defined by those planes and their intersections. For example, a rectangular parallelpiped (rpp) is enclosed by six planes--two perpendicular/normal to the X-axis, two normal to the Y-axis, and two normal to the Z-axis.
The BRL-CAD geometry file format defines two types of arb object records:
- arb8s are specified by a set of eight vertices (V1 through V8) that collectively define up to six facets.
- arbns are specified by a set of intersecting planes, each defined by four coefficients.
Although any arb object could be defined and stored as an arbn, the arb8 record type is used to store all of the special-case arbs discussed below (that is, all except the explicitly arbn type).
See also: Creating and editing arbs
Rectangular parallelpiped (rpp)
As mentioned above, each rectangular parallelpiped (rpp) is enclosed by two planes normal to the X-axis (mathematically defined by X = Xmin and X = Xmax), two normal to the Y-axis (Y = Ymin and Y = Ymax), and two normal to the Z-axis (Z = Zmin and Z = Zmax). The opposing faces of any such arb are thus pairs of similar rectangles. If Xmax - Xmin = Ymax - Ymin = Zmax - Zmin, as they are for the default rpp created by MGED's make and create commands, the resulting ppp would be a cube with edges parallel to the three axes.
The eight vertices of an rpp are calculated from the six planar coordinates as:
- (Xmax, Ymin, Zmin)
- (Xmax, Ymax, Zmin)
- (Xmax, Ymax, Zmax)
- (Xmax, Ymin, Zmax)
- (Xmin, Ymin, Zmin)
- (Xmin, Ymax, Zmin)
- (Xmin, Ymax, Zmax)
- (Xmin, Ymin, Zmax)
Box
Special case of arb8
- Handled by
- in form(arb8)
- Arguments
- Vertex of first corner, direction vectors for height, width, and depth
Right angle wedge (raw)
Right Angle Wedge, special case of arb6
- Handled by
- in form(arb8)
- Arguments
- Vertex of first corner, direction vectors for height, width, and depth
arb8
Arbitrary straight-edged shape with 8 vertices.
- Handled by
- make in form create
- Arguments
- 8 Vertices in the following order: 1234 vertices for the front face, starting at bottom left, counterclockwise; then 5678 vertices for the rear face, starting at bottom left, counterclockwise.
- Example
in unitcube.s arb8 0 0 0 1 0 0 1 0 1 0 0 1 0 1 0 1 1 0 1 1 1 0 1 1
creates the unit cube (first vertex at the origin, extends for 1 unit in x, y and z direction).
arb7
Special case of arb8, except with point 8 merged into point 5, making the left face triangular
- Handled by
- make in form(arb8) create
arb6
Arbitrary straight-edged shape with 6 vertices, special case of arb8.
- Handled by
- make in form(arb8) create
- Arguments
- 6 Vertices in the following order: 1234 vertices for the front face, starting at bottom left, counterclockwise; then back edge is 5 on bottom, 6 on top. Top and bottom faces are triangles.
- Example
in arb6.s arb6 1 -1 -1 1 1 -1 1 1 1 1 -1 1 -1 0 -1 -1 0 1
arb5
special case of arb8.
- Handled by
- make in form(arb8) create
arb4
special case of arb8.
- Handled by
- make in form(arb8) create
arbn
Arbitrary solid bounded by N planes.
- The solid is constructed by N sets of plane coefficients and distance magnitudes, that define space which is ~outside~ the solid. The coefficients define a vector whose normal is a plane parallel to the face of the solid. The surface of the solid is at the perscribed distance along this vector.
The distances may be negative and are used when a face lies on the opposite side of the origin as the tip of its vector. An example is if the left side of a box lies on the positive X axis. In this case, because the left side is being defined, the vector points left (coefficients -1 0 0), but since the point is on the positive X axis its distance is opposite its vector and therefore negative.
- Handled by
- make in create
- Arguments
- Number of planes
- xyz direction vector and distance for each plane
- Example
- in arbn.s arbn 8 1 0 1 1 -1 0 0 1 0 1 0 1 0 -1 0 1 0 0 1 1 0 0 -1 1 0.5 0.5 0.5 1 -0.5 -0.5 -0.5 1
- Example with negative distance
- in arbn2.s arbn 6 1 0 0 100 -1 0 0 -10 0 1 0 200 0 -1 0 -10 0 0 -1 0 0 0 1 1.5
- ...is equivalent to...
- in rpp.s rpp 10 100 10 200 0 1.5
Ellipsoids
ell
Ellipsoid
- Handled by
- make in form create
- Arguments
- V
- vertex point, at the center
- vectors A B C describing the radii of the ellipses; A points front, B points right, C points up.
Example:
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
- make in form(ell) create
Arguments:
- V
- vertex point, at the center
- radius
ellg
Special case of ellipsoid
- Handled by
- in form(ell)
- Arguments
- two foci points, and axis length
ell1
Special case of ellipsoid
- Handled by
- in make form(ell) create
- Arguments
- vertex, vector A, radius of revolution
ehy
Elliptical hyperboloid
- Handled by
- make in form create
- Arguments
- vertex, perpendicular vectors Height and (A,r_1) major axis, (r_2) magnitude of vector B, (c) apex to asymptotes distance
epa
Elliptical paraboloid
- Handled by
- in make form create
Cones and Cylinders
tgc
Truncated general cone
- Handled by
- in make form create
- Arguments
- vertex, vectors H A B, magnitudes of vectors C D
rcc
Right circular cylinder, special case of tgc
- Handled by
- in make form(tgc) create
- Arguments
- vertex, height vector, radius
Example:
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
- 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
- in make form create
- Arguments
- vertex, perpendicular vectors for Height and B, (r) rectangular half width
tec
Truncated elliptical cone, special case of tgc
- Handled by
- in make form(tgc) create
- Arguments
- Vertex, vectors Height, A, B
trc
Truncated right circular cone, special case of tgc
- Handled by
- in make form(tgc) create
- Arguments
- Vertex, Height vector, radius of base and top
derived from 2d
extrude
Extrusion of a 2-d sketch
- Handled by
- in make form(?) create
- Arguments
- vertex, perpendicular vectors Height A B, sketch, K
revolve
Revolution of a 2-d sketch
- Handled by
- in
- Arguments
- vertex, revolution axis, vector in start plane, angle, sketch
dsp
- Handled by
- in create
- Arguments
- data type (f|o), datasource, count of length and width, interpolation type, cut direction, cell size, unit elevation
See also DSP tutorial.
ebm
extruded bit map
- Handled by
- in form create
- Arguments
- filename, width and height in cells, extrusion distance,
See also EBM 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.
hf
Height field
- Handled by
- none?
- Status
- depreciated, use dsp instead
Other solids
tor
Torus
- Handled by
- in make form create
- Arguments
- vertex, normal vector, radius of revolution, tube radius
eto
Elliptical torus
- Handled by
- 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
- in make create
- Arguments
- 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.
nmg
n-Manifold geometry solid (non-manifold geometry)
- Handled by
- make create
pipe
Hollow and solid pipes and wires
- Handled by
- in make create
- Arguments
- # points, for each point: location, inner and outer diameters, bend radius
ars
Arbitrary rectangular solid
- Handled by
- 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:
- The number of points per waterline (the number of vertices on each intermediate polygon)
- The number of waterlines (the number of intermediate polygons plus 2)
- X, Y, and Z for a starting point (the first waterline)
- for each interior polygon (an intermediate waterline)
- for each point on the polygon
- X, Y, and Z for the point on the polygon
- for each point on the polygon
- 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 : number of points per waterline (i.e. intermediate polygons have 4 vertices)
6 : number of waterlines (four intermediate polygons plus the two endpoints)
0 0 3 - the center of the top end of the bar
1 1 3 1 -1 3 -1 -1 3 -1 1 3 : a 2x2 square in the xy plane at z offset 3
1 1 1 1 -1 1 -1 -1 1 -1 1 1 : 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 : 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 : a 2x2 square at a 45 degree rotation from the first squares at z offset -3
0 0 -3 : the center of the bottom end of the bar
metaball
- Handled by
- in make form(*) create
- Arguments
- render method, threshold, number of points, location and field strength for each point (and blobbiness/goo factor)
nurb
Non-uniform rational b-spline
- Handled by
- none?
spline
surface splines
- Handled by
- ?
vol
volume / voxel
- Handled by
- 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.
bot
Bag of triangles
- Handled by
- in make create (not edit!)
- Arguments
- number of verticies, number of triangles, mode (1=surface 2=solid 3=plate), triangle orientation mode (1=unoriented 2=counter-clockwise 3=clockwise), each vertex, vertex index of each triangle
- See also
- BoT
poly
polysolid
- Handled by
- none?
- Status
- depreciated, use bot instead
Other
Sketch
2d outline
- Handled by
- make form(sketch editor) create
- See also
- sketch
grip
Grip -- support for joints, non-geometric (does not show in rt)
- Handled by
- in make form create
Arguments:
- C
- Center
- N
- normal vector
- L
- magnitude
half
halfspace
- Handled by
- in make form create
- Arguments
- 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.
binunif
Uniform-array binary object
- Handled by
- in create (not edit!)
- Arguments
- minor type (fdcsiLCSIL), data file, number of values
