[R] Postscript query: plotting long vectors

[Prof Brian Ripley]

I don't think that's entirely correct: as far as I know doing that resets 
the line-type pattern (and I have just checked the PostScript Reference 
manual).

Level 1 Postscript interpreters often have a limit of about 1500 segments 
in a path, but level >=2 (all recent ones) are supposed to have no limit.

It seems that your suggested change would help a few PostScript 
interpreters but damage the output for the rest.  It is better that the 
end-user splits the polylines up.


On Thu, 29 May 2003, Stephen Eglen wrote:

> Hi, 
> 
> I have a query about the maximum length of vector that can be plotted
> in one go in a postscript driver.  Try the following code (in 1.7.0;
> version details below):
> 
> t <- seq(from=0, to=4*pi, length=200000)
> y <- sin(t)
> postscript(file="o.ps")
> plot(t, y, type="l")
> dev.off()
> 
> If I view the postscript file o.ps in "gv", it takes many seconds
> before eventually the axes appear, but then only one vertical line is
> drawn within the plot area -- there is no sine curve.  (this is on a
> fast dual processor linux machine with 2Gb RAM.)  This is clearly a
> postscript problem, rather than a R problem, since reducing the length
> of t down to something like 2000 solves the problem.  By looking at
> the file o.ps it looks like the line is drawn by one "rlineto" call
> per point, followed eventually by a "stroke" after the last point.
> I'm guessing that the postscript interpreter simply cannot remember so
> many points in the path before it gets to the stroke.
> 
> The example above is artificial, but this problem appeared with a real
> data set this morning.  The fix was to replace the single call to
> plot() with many calls to line(), breaking the t and y vectors into
> more manageable chunks; in this way, each postscript path was
> manageable and we got the plot.
> 
> I tried plotting the same long vectors in gnuplot by first writing
> them from R:
> 
> write.table(cbind(t,y), sep="\t", file="eg.dat", row.names=F, col.names=F,
>             quote=F)
> 
> and then in gnuplot:
> 
> set term postscript
> set output "gnuplot.ps"
> plot "eg.dat" wi lines
> 
> This came out fine; in gnuplot.ps every 400 lines during the plot it
> outputs "currentpoint stroke M" (M is defined to moveto).  I had a
> look at the gnuplot source (gnuplot-3.7.3/term/post.trm) and found
> that it does keep count of the length of the current postscript path:
> e.g. in the function PS_vector(x,y) we see (line 1122):
> 
> 	if (ps_path_count >= 400) {
> 		fprintf(gpoutfile,"currentpoint stroke M\n");
> 		ps_path_count = 0;
> 	}
> 
> so every 400 points it draws the line so far and then continues.
> (Matlab .ps files also seem to have regular "MP stroke".
> 
> I had a quick look in the corresponding R code src/main/devPS.c and
> could not see any counter.  Would it be worth adding such a counter
> and periodic line output to PS_Polyline?

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595