# Module `unit`

With the `unit` module PyX makes available classes and functions for the specification and manipulation of lengths. As usual, lengths consist of a number together with a measurement unit, e.g., 1 cm, 50 points, 0.42 inch. In addition, lengths in PyX are composed of the five types “true”, “user”, “visual”, “width”, and “TeX”, e.g., 1 user cm, 50 true points, 0.42 visual + 0.2 width inch. As their names indicate, they serve different purposes. True lengths are not scalable and are mainly used for return values of PyX functions. The other length types can be rescaled by the user and differ with respect to the type of object they are applied to:

user length:
  used for lengths of graphical objects like positions etc.
visual length:
  used for sizes of visual elements, like arrows, graph symbols, axis ticks, etc.
width length:
  used for line widths
TeX length:
  used for all TeX and LaTeX output

When not specified otherwise, all types of lengths are interpreted in terms of a default unit, which, by default, is 1 cm. You may change this default unit by using the module level function

`unit.``set`(uscale=None, vscale=None, wscale=None, xscale=None, defaultunit=None)

  When uscale, vscale, wscale, or xscale is not None, the corresponding scaling factor(s) is redefined to the given number. When defaultunit is not None, the default unit is set to the given value, which has to be one of `"cm"`, `"mm"`, `"inch"`, or `"pt"`.

For instance, if you only want thicker lines for a publication version of your figure, you can just rescale all width lengths using

    unit.set(wscale=2)

Or suppose, you are used to specify length in imperial units. In this, admittedly rather unfortunate case, just use

    unit.set(defaultunit="inch")

at the beginning of your program.

## Class `length`

class `unit.``length`(f, type="u", unit=None)

  The constructor of the `length` class expects as its first argument a number f, which represents the prefactor of the given length. By default this length is interpreted as a user length (`type="u"`) in units of the current default unit (see `set()` function of the `unit` module). Optionally, a different type may be specified, namely `"u"` for user lengths, `"v"` for visual lengths, `"w"` for width lengths, `"x"` for TeX length, and `"t"` for true lengths. Furthermore, a different unit may be specified using the unit argument. Allowed values are `"cm"`, `"mm"`, `"inch"`, and `"pt"`.

Instances of the `length` class support addition and substraction either by another `length` or by a number which is then interpeted as being a user length in default units, multiplication by a number and division either by another `length` in which case a float is returned or by a number in which case a `length` instance is returned. When two lengths are compared, they are first converted to meters (using the currently set scaling), and then the resulting values are compared.

## Predefined length instances

A number of `length` instances are already predefined, which only differ in there values for `type` and `unit`. They are summarized in the following table

name | type | unit
--- | --- | ---
`m` | user | m
`cm` | user | cm
`mm` | user | mm
`inch` | user | inch
`pt` | user | points
`t_m` | true | m
`t_cm` | true | cm
`t_mm` | true | mm
`t_inch` | true | inch
`t_pt` | true | points
`u_m` | user | m
`u_cm` | user | cm
`u_mm` | user | mm
`u_inch` | user | inch
`u_pt` | user | points
`v_m` | visual | m
`v_cm` | visual | cm
`v_mm` | visual | mm
`v_inch` | visual | inch
`v_pt` | visual | points
`w_m` | width | m
`w_cm` | width | cm
`w_mm` | width | mm
`w_inch` | width | inch
`w_pt` | width | points
`x_m` | TeX | m
`x_cm` | TeX | cm
`x_mm` | TeX | mm
`x_inch` | TeX | inch
`x_pt` | TeX | points

Thus, in order to specify, e.g., a length of 5 width points, just use `5*unit.w_pt`.

## Conversion functions

If you want to know the value of a PyX length in certain units, you may use the predefined conversion functions which are given in the following table

function | result
--- | ---
`tom(l)` | `l` in units of m
`tocm(l)` | `l` in units of cm
`tomm(l)` | `l` in units of mm
`toinch(l)` | `l` in units of inch
`topt(l)` | `l` in units of points

If `l` is not yet a `length` instance but a number, it first is interpreted as a user length in the default units.