# 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.

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:

# Arbs

Objects with an arbitrary number of points and flat faces...

## 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

## box

Special case of arb8

- Handled by
- in form(arb8)
- Arguments
- Vertex of first corner, direction vectors for height, width, and depth

## 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

## rpp

Rectangular Parallelpiped, special case of arb8

- Handled by
- make in form(arb8) create
- Arguments
- xmin xmax ymin ymax zmin zmax

# 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