HELP XptConvertUnit                             Jonathan Meyer, Nov 1990

    XptConvertUnit(num, from_unit, to_unit, direction, screen) -> num;


The -XptConvertUnit- procedure is used to scale numbers between
different units (specified as words). It includes 5 standard units (mm,
cm, inch, point, and pica) and one special unit (pixel). Converting
between one standard unit and another is just done using a
straightforward multiplication (the screen and direction arguments are
ignored).

However, converting between a standard unit and the pixel unit works by
determining the resolution of the provided screen in the specified
direction (measured in pixels per millimeter), and using this to perform
the conversion. Hence it is possible to determine how far apart two
objects are in "absolute" terms when you are provided their positions in
screen pixels. Similarly, it is possible to convert an absolute distance
into a number of screen pixels. This allows you to draw pictures which
in theory are accurate representations of the model involved (in
practice, there is a limit to the accuracy of such operations).

For example, to convert between picas and points, do:

    XptConvertUnit(1,"pica","point",false,false) =>
    ** 12.0

Converting between picas and pixels requires a <ScreenPtr> object
as returned by XtScreen. The following illustrates how XptConvertUnit
could be used to convert between picas and pixels:

    vars screen = XtScreen(widget);
    XptConvertUnit(1,"pica","pixel","x", screen) =>
    *** 111

Note that converting to pixels always produces an integer result -
pixels represent the smallest displayable unit on the screen, so this is
the desired result.

The direction argument is either "x", "y" or -false-. If -false- is
specified as the direction, then the average of screen x and y
resolution is used. This reduces accuracy, but is often more convenient.



--- C.x/x/pop/help/XptConvertUnit
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