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:
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
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.
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.
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.
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.