problematic Moire pattern in image produced with gnuplot pm3d and pdf output - image

Question

I'm plotting data using the command files discussed here:
gnuplot contour line color: set style line and set linetype not working
I want to provide different output options. PNG works well, as does the wxt terminal, however, these have fixed resolution, e.g. when I "zoom in" on the plot it gets grainier.
If I use pdf or pdfcairo for the terminal, the resulting file has a Moire pattern. The region in the image over which the Moire pattern is observed can be reduced by using increasing amounts of interpolation in the pm3d command. There are lots of points in the data set in the radial direction, but not many angular sets of data, so I need to interpolate more in that "direction". Using no interpolation results in a very grainy pm3d image, so I have been trying 0,20 to 20,20 or even 20,40. Unfortunately even lots of interpolation does not completely get rid of the Moire pattern, makes the file size HUGE (e.g. the PNG file is around 250kB but the pdf file is over 11MB) and as a result it renders very slowly. I've tried viewing these with Adobe Acrobat Reader 10.1.8 and GSview and the result is the same.
I am interested in using the pdf format because it is ubiquitous and can be zoomed in without excessive loss of detail (unlike PNG).
Below are a couple of screen shots that I captured of the pattern in the resulting pdf output at different levels of interpolation. The first image is the png file for reference as it shows no Moire pattern, 250kB file size.
Next, the pdf output without pm3d interpolation, 72kB file size:
Next, the pdf output using set pm3d map interpolate 0,20, file size 1861kB:
Next, the pdf output using set pm3d map interpolate 10,20, file size 5942kB:
Finally, the pdf output using set pm3d map interpolate 20,20, file size 11515kB:
Why are these Moire patterns generated for the pdf outputs? Is there away around this, that would allow me to still have a vector format that can be zoomed in without (much) loss of resolution?

Answer 1

This is not supposed to be a solution, but rather an explanation and a possible, although ugly workaround.
From time to time there are reports to the gnuplot mailinglists about this issue, but it seems to be related to the viewers. It has to do with the way gnuplot creates the surfaces plots. These are drawn as polygons, which are stitched together. The Moiré patterns you are showing come from wrong rendering between two polygons. That depends on the viewer, the viewer settings and the zoom factor.
The easiest example, to show that effect is the following Postscript file:
%!PS-Adobe-2.0
50 50 moveto 50 0 rlineto 0 50 rlineto -50 0 rlineto closepath 0 setgray fill
100 50 moveto 50 0 rlineto 0 50 rlineto -50 0 rlineto closepath 0 setgray fill
Save this file e.g. as moire.ps and view it, or convert it with ps2pdf and view it. With the Acrobat reader 9.5.1 I see the following:
The Acrobat Reader has a setting Preferences -> Page Display -> Enhance thin lines which can prevent this problem, but causes problems on other parts.
On my system (Debian), all viewers show this patterns, mupdf, firefox, ghostscript, pdftocairo, libpoppler` etc.
So, what to do? For myself I use the following workaround. I splot to a png with high resolution, and reread that file later with plot ... with rgbimage. Then you get your heatmap as bitmap, and the rest is vectorial. In most cases this is no problem, because in any way you have some measurement data with limited resolution, which you interpolate.
Based on the question gnuplot contour line color: set style line and set linetype not working, here is how you can implement it:
reset
set lmargin at screen 0.05
set rmargin at screen 0.85
set bmargin at screen 0.1
set tmargin at screen 0.9
set pm3d map interpolate 20,20
unset key
set cntrparam bspline
set cntrparam points 10
set cntrparam levels increment -6,-6,-24
set contour surface
set linetype 1 lc rgb "blue" lw 2
set linetype 2 lc rgb "blue"
set linetype 3 lc rgb "black"
set linetype 4 lc rgb "orange"
set linetype 5 lc rgb "yellow"
set palette rgb 33,13,10 #rainbow (blue-green-yellow-red)
set cbrange [-18:0]
unset border
unset xtics
unset ytics
set angles degree
r = 3.31
set xrange[-r:r]
set yrange[-r:r]
set colorbox user origin 0.9,0.1 size 0.03,0.8
##################### start changes ##############
set autoscale fix
RES_X = 2000
RES_Y = 2000
save('settings.tmp')
set lmargin at screen 0
set rmargin at screen 1
set bmargin at screen 0
set tmargin at screen 1
unset colorbox
set terminal pngcairo size RES_X, RES_Y
set output '3d-polar-inc.png'
splot 'new_test.dat' nocontour
unset output
load('settings.tmp')
# mapping of the coordinates for the png plotting later
X0 = GPVAL_X_MIN
Y0 = GPVAL_Y_MIN
DX = (GPVAL_X_MAX - GPVAL_X_MIN)/real(RES_X)
DY = (GPVAL_Y_MAX - GPVAL_Y_MIN)/real(RES_Y)
C0 = GPVAL_CB_MIN
DC = GPVAL_CB_MAX - GPVAL_CB_MIN
C(x) = (x/255.0) * DC + C0
# now plot the png
#set terminal pdfcairo size 10cm,10cm
#set output '3d-polar.pdf'
set terminal postscript eps color level3 size 10cm,10cm solid
set output '3d-polar-eps.eps'
set multiplot
set cbrange[GPVAL_CB_MIN:GPVAL_CB_MAX]
plot '3d-polar-inc.png' binary filetype=png \
origin=(X0, Y0) dx=DX dy=DY \
using (C($1)):(C($2)):(C($3)) \
with rgbimage, \
NaN with image t '' # hack for getting the colorbox
# plot the contours
unset surface
unset pm3d
splot 'new_test.dat' w l
###################### end changes #################
# now plot the polar grid only
set style line 11 lc rgb 'black' lw 2 lt 0
set grid polar ls 11
set polar
set logscale r 10
set rrange[10:20000]
unset raxis
set rtics format '' scale 0
#set rtics axis scale
set rtics (20,50,100,200,500,1000,2000,5000,10000,20000)
do for [i=-150:180:30] {
dum = r+0.15+0.05*int(abs(i/100))+0.05*int(abs(i/140))-0.05/abs(i+1)
set label i/30+6 at first dum*cos(i), first dum*sin(i) center sprintf('%d', i)
}
set label 20 at first 0, first -(log(20)/log(10)-1) center "20"
set label 100 at first 0, first -(log(100)/log(10)-1) center "100"
set label 200 at first 0, first -(log(200)/log(10)-1) center "200"
set label 1000 at first 0, first -(log(1000)/log(10)-1) center "1k"
set label 2000 at first 0, first -(log(2000)/log(10)-1) center "2k"
set label 10000 at first 0, first -(log(10000)/log(10)-1) center "10k"
set label 20000 at first 0, first -(log(20000)/log(10)-1) center "20k"
plot NaN w l
unset multiplot
unset output
With pdfcairo this gives a 1.7 MB pdf file, with epslatex level3 (this option is available in the 4.7 development version only) you get a 1.5 MB eps file, which can be converted with epstopdf to a 136 KB pdf file.
See also my answer to Big data surface plots: Call gnuplot from tikz to generate bitmap and include automatically? on TeX.SX.