Editing Graphics development with vplot (section)

===Low level output routines===
(only called by certain generic routines)
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int lost = YES;
dev.plot(x, y, draw)
int x, y, draw;
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Move or draw to device coordinate (x,y).
Draw=0 for move, 1 for draw.

This routine MUST declare the global flag "lost". It is the responsibility
of all the device routines to set this to YES whenever anything happens
that may cause genvector's idea of the "current device pen position"
to be wrong.  (For example, printing an error message, doing hardware text,
filling a polygon, etc.) Generic routines do not present a difficulty, since
they can't output anything directly to the device.

When lost is set to YES, it is guaranteed that the next call to dev.plot
will by a move, and not a draw.

When genvector's idea of the current position is correct again (for example
when dev.plot has been called) lost should be set to NO. THIS IS IMPORTANT;
genvector only looks at the value of lost, it doesn't set it. Leaving
lost YES all the time will greatly slow down plotting by forcing one move
for each draw!

Generic routines: nulldev
Link in nulldev here as a placeholder if you didn't use genvector as
your vector routine.

<syntaxhighlight lang="c">
dev.startpoly(npts)
int npts;

dev.midpoly(x,y)
int x, y;

dev.endpoly(last)
int last;
</syntaxhighlight>
Polygon-drawing routines called only by genarea.

Dev.startpoly is called once at the beginning of every polygon.
Npts gives the number of points in the polygon. After dev.startpoly
has been called, then dev.midpoly is called once for each point in
the polygon. (x,y) gives the device coordinate of the vertex.

Dev.endpoly is called once at the end of each polygon. Genarea may fragment
one polygon into several. If this polygon just defined is the last polygon
in a set of polygons that were fragmented from one, then last will be 1.
If there are more fragments in this set to go, last will be 0. It is
guaranteed that no other routines but these three will be called after
the first dev.startpoly call and before dev.endpoly(last=YES) is called.

The genarea algorithm will fail for certain very complicated crossed
polygons, unless the multiple polygons fragmented from the one original
one are shaded as a unit. Some devices such as the Tek4105 allow this
to be done. That is why the "last" flag is provided. There is no
great harm in shading each polygon as it comes. Occasionally interior
voids will be filled twice instead of not filled at all, that's all.

If the device can fill with an arbitrary pattern, it should do so.
The pattern is available to these routines in the same way as described
under dev.area. If the device cannot fill with a user-defined pattern,
filling solidly with the current color is the next best thing.

Generic routines: nulldev.c
If you did not use genarea as your dev.area routine, then none of
these routines are ever called so you just need to put in nulldev
as a placeholder.