Trelis uses a unitless coordinate system. For example, the command brick x 10 creates a cube 10 units wide, but Trelis does not know whether those 10 units are 10 inches, 10 meters, 10 microns or 10 miles. The Aprepro Units() function facilitates the use of a unit system in Trelis' unitless environment.

The Aprepro Units(svar) function takes a single string parameter which identifies the desired unit system. If the specified unit system is recognized, then a set of variables are defined to facilitate working in that unit system. Dimensions can be multiplied by an appropriate unit variable to convert between various dimensions. For example, the statement #{Units("in-lbf-s")} defines variables useful when working in a coordinate system where one Trelis unit is one inch. The command brick x {1*ft} will create a cube 12 units wide, and the command brick x {1*m} will create a cube 39.37 units wide, the number of inches in 1 meter.

It is important to note that the Units() function does nothing more than define a set of useful variables. The function does not change the dimensions of existing geometry, nor does it change the scale of geometry imported from a file. For example, the following commands create two cubes, the first being 12 units wide and the second being 1 unit wide. The first cube remains 12 units wide, even after the second call to the Units() function:

brick x 1*ft
brick x 1*ft

The Units() function returns a zero-length string if it is successful. If the Units() function fails (usually because the specified unit system was not recognized), a non-zero-length string containing an error message is returned.

The unit systems currently supported by the Units() function are: si, cgs, cgs-ev, shock, swap, ft-lbf-s, ft-lbm-s, in-lbf-s. For each of these unit systems, the following variables are defined by the Units() function:

Table 1. String Variables




Base Time Unit


Base Length Unit


Base Acceleration Unit


Base Mass Unit


Base Force Unit


Base Velocity Unit


Base Volume Unit


Base Density Unit


Base Energy Unit


Base Power Unit


Base Pressure Unit


Base Temperature Unit


Base Angle Unit

Time Variables

Length Variables

Acceleration Variables

Force Variables

Mass Variables

Velocity Variables

Volume Variables

Density Variables

Power Variables

Energy Variables

Pressure Variables

Temperature Variables

Angular Variables