# BRL-CAD Primitives

These are primitive objects that can be created in mged.

Objects can be created in any of the following ways: (note: are there more? get? load from file?)

- make
- the mged make command creates the object with default dimensions
- in
- the mged in command interactively prompts for dimensions not already supplied as arguments
- form
- the graphical primitive editor form (* some objects not fully supported)
- create
- the graphical create menu

When an object is selected from the create menu, you are prompted for a name, and then dropped into the primitive editor form; however, if the objec type has no form, create will do about the same as make. Some derivative objects do not have their own form, and the primitive editor will use the base object's form. Most parameters (including ones not editable from a form) have special items on the edit menu.

## Contents

# 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

- Handled by
- make in create
- Arguments

- Number of planes
- xyz direction vector and normal 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

## box

Special case of arb8

- Handled by
- in form(arb8)
- Arguments

- V
- vertex of first corner
- direction vectors for height, width, and depth

## rpp

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

### ehy

Elliptical hyperboloid

- Handled by
- make in form create

### ell1

Special case of ellipsoid

- Handled by
- in make form(ell) create

### epa

Elliptical paraboloid

- Handled by
- in make form create

# Cones and Cylinders

## tgc

Truncated general cone

- Handled by
- in make form create
- Arguments

## rcc

Right circular cylinder, special case of tgc

- Handled by
- in make form(tgc) create
- Arguments
- vertex ,

## 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 cone

- Handled by
- in make form(tgc) create
- Arguments
- Vertex, Height vector, radius of base and top

# Other

## grip

Grip

- Handled by
- in make form create

Arguments:

- C
- Center
- N
- normal vector
- L
- magnitude

## 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

## half

halfspace

- Handled by
- in make form create
- Arguments
- Normal, distance from origin

## part

Conical particle

- Handled by
- in make create
- Arguments
- vertex, height vector, radius at v, radius at h

## nmg

n-Manifold geometry solid

- Handled by
- make create

## pipe

- 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