HP2XX, A HP-GL Converter
3.5 Dots and lines
Here are some basics about the generation of dots and lines within
hp2xx. I mention them, because there is something left to be
improved here...
Some HP-GL codes cause hp2xx to generate points rather than lines
of length zero. There is a subtle difference between both. Depending
on the current output format, special code for points will be generated,
and occasionally, a point will look different from a zero-length line.
Use `-m epic' for such an example.
Line thicknesses can vary. Especially for thick lines, the matter of
line caps (how lines are ended, e.g. with a round cap) becomes relevant.
(According to the HPGL/2 references, these issues are only relevant for
lines wider than 3.5 mm).
hp2xx does not yet offer complete support for these features -
currently your best bet is to use `-m eps' or `-m pdf', as
hp2xx will simply emit the corresponding linestyle commands of
those formats instead of having to calculate all the details itself.
While most line end cap types are supported even in the raster modes
(since version 3.4.3), this applies especially to the mitering options,
i.e. the way the junction is drawn.
To force a certain type of line caps, you can also use `-m eps', edit
the resulting Encapsulated PostScript file, look for a line with
setlinecap in it (near line 45), and select the line cap of your
choice by modifying the PostScript command setlinecap
accordingly. You can also use Metafont (via `-m mf') and replace
the picked pen "pencircle" by some other type. However, both methods
are far from convenient.
The internal rasterization done by versions of hp2xx before 3.4.3
was a simple process based on positioning squares (roughly corresponding
to the linewidth) by the Bresenham algorithm. Since 3.4.3, a modified
algorithm for drawing wide lines - based on an IBM Technical Bulletin
written by Alan Murphy - is employed, which also supports true endcap
drawing. The current code still does not support the various miter options,
e.g. drawing out the endpoints of two lines so that they meet in an exact
triangle or a blunt wedge. Unfortunately the current layout makes it hard
to implement such a feature, as the internal representation of the command
sequence is just another sequential list of lines without any indication
of intersections.
