Search code examples
imageplotgnuplot

gnuplot: how to plot one 2D array element per pixel with no margins

I am trying to use gnuplot 5.0 to plot a 2D array of data with no margins or borders or axes... just a 2D image (.png or .jpg) representing some data. I would like to have each array element to correspond to exactly one pixel in the image with no scaling / interpolation etc and no extra white pixels at the edges.

So far, when I try to set the margins to 0 and even using the pixels flag, I am still left with a row of white pixels on the right and top borders of the image.

How can I get just an image file with pixel-by-pixel representation of a data array and nothing extra?

gnuplot script:

#!/usr/bin/gnuplot --persist

set terminal png size 400, 200

set size ratio -1
set lmargin at screen 0
set rmargin at screen 1
set tmargin at screen 0
set bmargin at screen 1

unset colorbox
unset tics
unset xtics
unset ytics
unset border
unset key

set output "pic.png"

plot "T.dat" binary array=400x200 format="%f" with image pixels notitle

Example data from Fortran 90:

program main
implicit none
integer, parameter :: nx = 400
integer, parameter :: ny = 200
real, dimension (:,:), allocatable :: T
allocate (T(nx,ny))

T(:,:)=0.500
T(2,2)=5.
T(nx-1,ny-1)=5.
T(2,ny-1)=5.
T(nx-1,2)=5.

open(3, file="T.dat", access="stream")
write(3) T(:,:)
close(3)

end program main

extra pixels

Solution

  • Some gnuplot terminals implement "with image" by creating a separate png file containing the image and then linking to it inside the resulting plot. Using that separate png image file directly will avoid any issues of page layout, margins, etc. Here I use the canvas terminal. The plot itself is thrown away; all we keep is the png file created with the desired content.

    gnuplot> set term canvas name 'myplot'
Terminal type is now 'canvas'
Options are ' rounded size 600,400 enhanced fsize 10 lw 1 fontscale 1 standalone'
gnuplot> set output '/dev/null'
gnuplot> plot "T.dat" binary array=400x200 format="%f" with image 
   linking image 1 to external file myplot_image_01.png
gnuplot> quit

$identify myplot_image_01.png
myplot_image_01.png PNG 400x200 400x200+0+0 8-bit sRGB 348B 0.000u 0:00.000