I will talk about convert a colorized rgb image to a gray one using gnuplot in this article.
Amazing it is to edit pictures with gnuplot. Yes, it is a little amazing, this kind of works should left to picture editors in fact. But I will show you gnuplot can also do it well.
In a rgb image, the color of a point is marked with (r,g,b) where r,g and b can be and always are diffrent values. While for a gray one r,g and b must be the same. So to convert a rgb image to a gray one, we should convert (r,g,b) to (gray,gray,gray). Once we known how to do it, the problem becomes easy. Now we come to write our gnuplot script.
#Utility:Convert a rgb colorized image to a gray one
#Author:数声风笛离亭晚，我想潇湘君想秦！
#Email：qinjieli@gmail.com
#Usage:
##call "rgbtogray.gnuplot" inputfile outputfile
#parameters:
##inputfile: input filename (without ".png")
##outputfile: outputfilename (without ".png")
reset
inputfile="$0.png"
outputfile="$1.png"
plot inputfile binary filetype=png w rgbimage
xmax=GPVAL_DATA_X_MAX
xmin=GPVAL_DATA_X_MIN
ymin=GPVAL_DATA_Y_MIN
ymax=GPVAL_DATA_Y_MAX
set xrange [xmin:xmax]
set yrange [ymin:ymax]
#get the size of the picture
gray(r,g,b)=0.299*r + 0.587*g + 0.114*b
#The function which convert rgb color to gray.
#At first I choose the three weighting all to be 0.333,
#but a firend told me it will be better to use these values
set size ratio 1
# Set the scales so that the unit has the same length
#on both the x and y axes
#########################
#There should not be any margin in the output picture
set lmargin 0
set rmargin 0
set tmargin 0
set bmargin 0
unset key #The key, border and tics are all do not need
unset border
unset tics
#set terminal and output file name
set term png size (xmaxxmin),(ymaxymin)
set output outputfile
#Plot with gray color
plot inputfile\
u (gray(column(1),column(2),column(3))):\
(gray(column(1),column(2),column(3))):\
(gray(column(1),column(2),column(3)))\
binary filetype=png w rgbimage
Save the script as "rgbtogray.gnuplot", then it can be called like this:
gnuplot> call "rgbtogray.gnuplot" inputfile outputfile
The following is two groups of our input and ouput files. The initial input files are copied from
here and
here (Thanks the original picture author!).

Input file1 

Output file1 

Input file2 

Output file2 
Thanks a lot!
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