[Rd] Artefacts in (screen viewed) PDF output

[Roger Bivand]

On Wed, 2 Aug 2006, Paul Murrell wrote:

> Hi
> 
> 
> Roger Bivand wrote:
> > This issue is probably to do with on-screen viewing of PDF files written
> > from R (2.3.1, Windows XP, RHEL 4), not with how the files are produced.  
> > So the question is mainly to ask whether others have seen similar
> > behaviour, and whether a remedy is known.
> > 
> > When neighbouring polygons are written with the same fill colour, and with
> > no border line colouring, PDF files show traces of probably unstroked
> > "lines" or probably interstices when viewed on-screen in at least acroread
> > (7.0) on both Windows XP and RHEL 4 (though not xpdf 3.0 on RHEL 4). This
> > is intrusive when many neighbouring polygons share fill colour, for
> > example on election party share maps, where borders are suppressed for
> > clarity. An example is:
> > 
> > library(maps)
> > us <- map("state", fill=TRUE, plot=FALSE)
> > pdf("borders.pdf")
> > plot(us, type="n", axes=FALSE, asp=1)
> > polygon(us, col="blue", border=NA)
> > dev.off()
> > 
> > Using polygon(us, col="blue", border="transparent") gives the same result. 
> > Curiously, the same is also observed with postscript() and external 
> > conversion to PDF (epstopdf), although viewing the EPS file on RHEL 4 in 
> > ggv does not show any artefacts up to 400%.
> > 
> > My feeling is that the output files are correct but that acroread is 
> > introducing interstices in rendering to screen - I do not have a printer 
> > with high enough resolution to check properly, but I believe that 
> > acroread-printed output does not have the artefacts. They are however 
> > visible when acroread is used in presentation mode.
> > 
> > Any insight would be very useful.
> 
> 
> I have seen this sort of thing happens when viewing PDF or PostScript
> onscreen *with antialiasing turned on*.  Most viewers allow you to turn
> off antialiasing (some even allow you to turn it off just for lines and
> images, but not for text).  Does that help in your case?
> 

Sorry for the delay in replying. On both Windows XP/Acroread 7.0 and RHEL
4/Acroread 7.0, Edit -> Preferences -> Page Display -> Smooth line art
(off) removes the artefact. On a number of printers printing from Acroread
7.0, the artefact is not present. Acroread has on/off ticks for smooth
text, smooth line art, and smooth images, where the smooth line art tick
is the one that is being over-enthusiastic in this case. 

Thanks,

Roger.

> Paul
> 

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no