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I often plot graphs in gnuplot prompt shell, like this:

gunuplot> plot sin(x) with linespoints pointtype 3

and the figure showed up is great.

Today, I save the graph in a .png file, like this:

gnuplot> set term png
gnuplot> set output "output.png"
gunuplot> plot sin(x) with linespoints pointtype 3

Then, I open output.png with eog in Ubuntu, like this:

$ eog output.png

I found that, the output.png displayed in eog is not as good as the figure plotted in prompt shell.

Why is that? Do I need to adjust some settings before save the output.png?

PS

I found that a way around it, first, 

set term postscript
set output "output.ps"

then in linux shell,

$ convert output.ps output.jpg

This way some sort of solve the my problem.

Alcott
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  • Could you elaborate on `not as good as`? Maybe with a screenshot compared with the png you created? – Woltan Feb 01 '12 at 07:31
  • by `not as good as`, I mean the figure from prompt shell is plotted on `wxt`, and it has a very good view in terms of the line-width. But the figure in `output.png` presents a different view, that's, the line-width changed for the worse, but still I can understand the figure, but uncomfortable with the view. – Alcott Feb 01 '12 at 07:58
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    AFAIK every terminal in gnuplot looks a little different. You can check the output of each terminal with the gnuplot [test](http://www.gnuplot.info/docs_4.2/gnuplot.html#x1-34200047) function. – Woltan Feb 01 '12 at 08:06
  • Comment: you use a JPEG file as an example. You should probably never use JPEG for line plots unless they have photographs as backgrounds. JPEG is simply not designed to handle the sharp edges of line drawings. – djconnel Jan 10 '15 at 14:26

3 Answers3

86

The source of your problems with the PNG quality is most likely the missing antialiasing in the png terminal of Gnuplot. Since you give no screenshots, I'm not sure what you mean when talking about bad linewidth, but here's how it looks for me (on MacOS). This screenshot shows the output of gnuplot's native aquaterm output:

A plot using gnuplot's native aquaterm output

If we create a png using set term png, the lines become "jumpy" and pixellated:

The sine curve, plotted using the png terminal

However, there is a version of the png terminal that uses the Cairo libs for rendering, and that makes the output far more smooth and nicer. set term pngcairo gives this result:

The sine curve, plotted using the pngcairo terminal

You can use set terminal to check whether this terminal version is available for you. If it is, this should save you conversion work and also give better image quality than a JPG (which is not an ideal format for line art).

jstarek
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25

The default size of the PNG image generated gnuplot with the PNG terminal is 640x480 pixels. This resolution in certain cases may result in "pixelated" graphs which are not as nice as those produced on screen with the default (wxt) terminal.

You can change the resolution of the output image using the size option:

set terminal png size <x,y>

where x and y are the desired numbers of pixels along the horizontal and vertical axis, respectively. For example:

set terminal png size 1024,768

Please note that, images with larger resolution will result in proportionally larger files on disk. Alternatively you can try to use non-raster terminals like "post eps" or "pdf" if available on your machine, which may give you high quality scalable and (relatively) portable images without a large disk footprint.

Giannis
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10

Alternatively, if you want professional (publication-ready) quality images with gnuplot, you should have a look at the epslatex terminal. I have used it extensively for my thesis and my papers with very nice results, virtually no pixelation problems, portability when converting images to pdf, and almost all the capabilities of Latex.

Miguel
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