Histogram Of An Image's "black Ink Level" By Horizontal Axis

I have a black and white image (or pdf) file, and want to get a histogram of the image's horizontal profile. That is, for each column in the image I want the sum of the grayscale

Solution 1:

I will give an answer in two acts, using two of my favorite free utilities: python and gnuplot.

As a fellow (computational) graduate student, my advice is that if you want to do things for free python is one of the most versatile tools you can learn to use.

Here's a python script that does the first part, counting the grayscale value (from 0 for white to 255 for black):

#!/usr/bin/pythonimport Image            # basic image processing/manipulation, just what we want

im = Image.open('img.png')       # open the image file as a python image objectwithopen('data.dat', 'w') as f: # open the data file to be writtenfor i inrange(im.size[0]):  # loop over columns
        counter = sum(im.getpixel((i,j)) for j inrange(im.size[1]))
        f.write(str(i)+'\t'+str(counter)+'\n')  # write to data file

Shockingly painless! Now to have gnuplot make a histogram*:

#!/usr/bin/gnuplotset terminal pngcairo size 925,900
set output 'plot.png'#set terminal pdfcairo#set output 'plot.pdf'set multiplot

## first plotset origin 0,0.025              # this plot will be on the bottomset size 1,0.75                 # and fill 3/4 of the whole canvasset title "Black count in XKCD 'Self-Description'"set xlabel 'Column'set ylabel "Black\ncount" norotate offset screen 0.0125

set lmargin at screen 0.15      # make plot area correct sizeset rmargin at screen 0.95      # width = 740 px = (0.95-0.15)*925 pxset border 0                    # these settings are just to make the dataset grid                        # stand out and not overlap with the tics, etc.set tics nomirror
set xtics scale 0.5 out 
set ytics scale 0

set xr [0:740]                  # x range such that there is one spike/pixel## uncomment if gnuplot version >= 4.6.0## this will autoset the x and y ranges#stats 'data.dat'#set xr [STATS_min_x:STATS_max_x+1]#set yr [STATS_min_y:STATS_may_y]

plot 'data.dat' with impulse notitle lc 'black'## second plotset origin 0,0.75               # this plot will be on topset size 1,0.25                 # and fill 1/4 of the canvasunset ylabel; unset xlabel      # clean up a bit...unset border; unset grid; unset tics; unset title

set size ratio -1               # ensures image proper ratio
plot 'img.png' binary filetype=png with rgbimage

unset multiplot         # important to unset multiplot!

To run these scripts, save them in the same directory with the image you want to plot (in this case the XKCD comic, which I saved as img.png). Make them executable. In bash this is

$ chmod 755 grayscalecount.py plot.plt

Then (if python+image module+gnuplot are all installed), you can run

$ ./grayscalecount.py$ ./plot.plt

On my computer, running Ubuntu 11.10 with gnuplot 4.4.3, I get this cool plot at the end:

**Side note*: there are a lot of different histogram plots gnuplot can make. I thought this style showed off the data well, but you can look into formatting your data for gnuplot histograms.

There are many ways to make python make plots itself or with gnuplot (matplotlib, pygnuplot, gnuplot-py), but I am not as facile with those. Gnuplot is awesomely scriptable for plotting, and there are lots of ways to make it play nicely with python, bash, C++, etc.