Image Tools

installation | display | identify | convert | composite | potrace | rsvg-convert | gnuplot | montage | graphviz | ditaa | ffmepg

This is a crash course in creating and modifying images at the command line.


Mac OS X with Homebrew:

$ brew install ditaa ffmpeg gnuplot graphviz imagemagick librsvg potrace


$ sudo apt install ditaa ffmpeg gnuplot graphviz imagemagick potrace


When editing images at the command line, it is convenient to have a quick way to examine the results.

On Mac OS X use qlmanage -p:

$ qlmanage -p onepiece_crop.jpg

On a operating system where X Windows is running, use display, part of the ImageMagick tool suite:

$ display onepiece_crop.jpg

These techniques usually don't work when working on a remote server, but remember that python makes it easy to serve up all the files in a directory via http:

$ python3 -m http.server 8000


Let's start with this image:


We can use identify to get the dimensions of the image:

$ identify onepiece.jpg
onepiece.jpg JPEG 324x486 324x486+0+0 8-bit sRGB 151KB 0.000u 0:00.000


convert is part of the ImageMagick suite of tools. It takes the input file and the output file as command line arguments. The output file is a copy of the input file with the specified changes applied to it. A hyphen - can be used to read from standard input or write to standard output.

The convert command recognizes over 200 option flags. The mogrify command recognizes almost all of the same flags. It behaves like convert, except that it modifies the input file.

convert can make an image smaller:

$ echo $(( 324 / 2 ))x$(( 486 / 2 ))

$ convert -resize 162x243 onepiece.jpg half.jpg

Actually, we don't have to compute the new dimensions:

$ convert -resize '50%' onepiece.jpg half.jpg

We can crop it:

$ convert -crop 273x294+51+0 onepiece.jpg onepiece_crop.jpg

The argument to the -crop flag is WIDTHxHEIGHT+X+Y, where X andY give the location of the upper left corner of the crop region in the coordinate system whose origin is the upper left corner of the original image.

To reflect the image around the vertical center line:

$ convert -flop onepiece.jpg onepiece_flop.jpg

We can also reflect the image around the horizontal center line:

$ convert -flip onepiece.jpg onepiece_flip.jpg

We can rotate the flopped image clockwise 270°:

$ convert -rotate 270 onepiece_flop.jpg onepiece_flop_rotate270.jpg

The output format is controlled by the suffix of the file. Hence to convert a JPG to a GIF:

$ convert onepiece.jpg onepiece.gif

If the output format is JPG or PNG, you can control the compression level with the -quality flag. Give it a value between 100 (no compression) and 0 (maximum compression):

$ ls -l onepiece.jpg 
-rw-r--r--@ 1 clark  staff  151209 Sep 12 18:38 onepiece.jpg

$ convert -quality 50 onepiece.jpg onepiece_low.jpg

$ ls -l onepiece_low.jpg 
-rw-r--r--@ 1 clark  staff  40871 Sep 12 19:45 onepiece_low.jpg

Convert to a black-and-white image:

$ convert onepiece.jpg -colorspace gray onepiece_gray.jpg

Now let's switch to this image:


The original image of the dachshund was 195k. Before I uploaded it I used convert to reduce the size 43k. The number after the -quality flag can be set even lower for lossier compression:

$ convert -strip -quality 50 dachshund_orig.jpg dachshund.jpg

We can perform two operations at once. For example, to crop the dog's head and zoom in:

$ convert -crop 300x317+670+50 -resize 800x617 dachshund.jpg head.jpg

We can use convert to place text over an image:

$  convert head.jpg -font Arial -pointsize 72 -fill white -draw "text 80,550 'wut'" comp4.jpg

This lists the available fonts:

$ convert -list font

Draw a white rectangle, so we can make the text black:

$ convert head.jpg -fill white -draw "rectangle 60,490 210,570" \
  -font Arial -pointsize 72 -fill black -draw "text 80,550 'wut'" comp5.jpg

Here is an example of the graphics primitive available to the convert command:

$ convert -size 300x210 xc:transparent \
  -fill red -stroke red -draw "circle 150,130 150,180" \
  -fill white -stroke black -draw "rectangle 120,30 220,130" \
  -stroke green -strokewidth 3 -draw "line 50,30 200,180" \
  -fill blue -stroke blue -draw "polygon 240,30 240,110, 150,110" \

Here is an example of using Bézier curves to draw a figure:

$ convert -size 300x300 xc:transparent \
  -fill pink -stroke pink \
  -draw "path 'M143,279C145,246,8,135,8,96C10,9,134-3,138,89 \
    c0-39,22-64,46-64c33-0,70,35,70,61C255,156,140,246,143,279z'" \


To put one image over another in the z-axis direction use composite. To put one image over another in the y-axis direction, use montage.

Place the dachshund's head on top of the original image:

$ composite head.jpg dachshund.jpg comp.jpg

We can use the -geometry flag to control the position of the head:

$ composite -geometry +400+0 head.jpg dachshund.jpg comp1.jpg

We can use convert to make the head semi-transparent. We save the new head file as a PNG, since JPG doesn't have an alpha channel.

$ convert head.jpg -alpha set -channel a -evaluate set 50% +channel semi_transparent.png
$ composite semi_transparent.png dachshund.jpg comp2.jpg

Here is a masking example. The convert -alpha Off -compose CopyOpacity command is the one doing the masking.

$ convert -crop 300x317+670+50 -resize 300x300 dachshund.jpg head2.jpg
$ convert -size 300x300 xc:white white.png
$ convert -size 300x300 xc:pink pink.png
$ composite head2.jpg -geometry -20+0 white.png head3.jpg
$ convert head3.jpg heart_mask.png -alpha Off -compose CopyOpacity -composite heart_head.png
$ composite heart_head.png pink.png comp7.png
$ convert -crop 270x290+0+0 -resize 270x290 comp7.png comp8.png


potrace converts images to a vector graphics format.

potrace requires a bitmap input. Use convert to create a .bmp file from another format:

$ convert onepiece.jpg onepiece.bmp

Then create an SVG file:

$ potrace -b svg -k 0.65 -t 6 onepiece.bmp

$ ls

The -k flag controls how dark an input pixel must be to be classified as black. It can be anywhere between 0.00 and 1.00, with the default value at 0.50. With 0.65, the value we have chosen, more input pixels are classified as black.

The -t flag suppresses elements that derive from a small number—in this case 6—of input pixels.


The --color '#ff0000' flag wouldchange the color from black to red. The "white" parts of the image are actually transparent. Non-black areas which are surrounded by black areas can have their color set using the --fillcolor flag.

Sharpening the image with convert can help preserve outlines:

$ convert -sharpen 0x2 onepiece.jpg onepiece_sharp.jpg
$ convert onepiece_sharp.jpg oncepiece_sharp.bmp
$ potrace -b svg -k 0.65 -t 6 onepiece_sharp.bmp

Here is the sharpened jpg, and the resulting svg in which we have preserved an outline of the subject's arms:

onepiece_sharp.jpg onepiece_sharp.svg


To convert an svg back to a pixel format:

$ rsvg-convert onepiece_sharp.svg > onepiece_sharp.png


gnuplot is an interactive program which can be used to plot a function:

$ gnuplot
> set terminal png size 400,300
> set output "sin.png"
> plot sin(x)
> quit

gnuplot can be used in a noninteractive way with the -e flag. Separate statements with semicolons:

$ gnuplot -e 'set terminal png size 400,300; set output "sin.png"; plot sin(x)'

It is also possible to put statements in a file and run them like a script. Note the technique for passing in an argument on the command line:

$ cat sin.gnuplot
set terminal png size 400,300
set output path
plot sin(x)

$ gnuplot -e "path='sin.png'" sin.gnuplot

It is possible to replace the "sin(x)" in the legend with a different string, or to remove the legend entirely:

> plot sin(x) title "sine function"

> plot sin(x) notitle

It is possible to set the lengths of the x and y axes:

$ gnuplot
> set terminal png size 400,300
> set output "sin_range.png"
> plot [-7.0:7.0] [-1.2:1.2] sin(x)
> quit

Multiple functions can be plotted:

$ gnuplot
> set terminal png size 400,300
> set output "superimpose.png"
> plot sin(x),cos(x)
> quit

gnuplot can be used to plot data. A histogram is a good way to summarize univariate data. This example shows the number of words by word length in a dictionary:

$ awk '{ print length($0)}' /usr/share/dict/words > words.txt

$ gnuplot
> set terminal png size 400,300
> set output "words.png"
> binwidth=1
> bin(x,width)=width*floor(x/width)
> plot "words.txt" using (bin($1,binwidth)):(1.0) smooth freq with boxes
> quit

The most general way to plot bivariate data is with a scatterplot.

$ gnuplot
> set terminal png size 400,300
> set output "scatter.png"
> plot "" with points
> quit

The file is just generated data.

When reading a file of data, gnuplot ignores lines that start with #. It expects the columns to be whitespace delimited. If there are more than two columns of data, a using X:Y clause can be added to the plot statement to specify the number of x column and the y column.

The man page for gnuplot is brief, but the online documentation is extensive. Use the help command to see general documentation, or help FUNC to see documentation on a specifics function:

$ gnuplot
> help
> help plot

If there is a functional relationship in the data, such as time series data, then a line graph

For this example we are using a web server access log. The entries are one per line and look like this: - - [07/Sep/2014:06:55:23 +0000] "GET /numerical-analysis HTTP/1.1" 200 71155 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/36.0.1985.143 Safari/537.36" - - [07/Sep/2014:06:55:24 +0000] "GET /favicon.gif HTTP/1.1" 304 0 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/36.0.1985.143 Safari/537.36"

We pull out the timestamps and count the number per hour:

$ awk '{print substr($4, 2, 14)}' access.log > hours.txt

$ awk '{cnt[$1] +=1 } END {for (i in cnt) print i, cnt[i]}' < hours.txt | sort > by_hour.txt

Here is the result: by_hour.txt

Then we plot it:

$ gnuplot
> set terminal png size 400,300
> set output "by_hour.png"
> set xdata time
> set timefmt "%d/%b/%Y:%H"
> plot "by_hour.txt" using 1:2 with lines
> quit

The set timefmt command uses the same notation as strftime from the C standard library.


The dates on the x-axis are crowded and hard to read. We can fix it by adding these two statements to the gnuplot session before we call plot.

> set format x '%m/%d'
> set xtics '08/Sep/2014:00', 2 * 24 * 60 * 60, '14/Sep/2014:00'

The set format x sets the format of the tics on the x-axis. The set xtics command controls where they occur. The first and last timestamp must be specified in the format of the set timefmt command. The middle argument is the distance between tics in seconds.



montage is part of the ImageMagick tool suite. Use it to paste multiple images together into a single image:

montage -geometry +0+0 words.png scatter.png montage.png

To stack the images on top of each other, use the -tile option to control the number of input images per row:

$ montage -tile 1x -geometry +0+0 words.png scatter.png stack.png

Also convert foo.png bar.png +append for left-to-right.

And convert foo.png bar.png -append for top-to-bottom.


dot is part of the Graphviz suite of tools.

dot takes as input files which contain the DOT graph description language.

Here is an undirected graph:

graph {
     a -- b -- c;
     b -- d;

Here is how to run dot:

$ dot -Tpng < /tmp/ > /tmp/graph.png

This is a directed graph:

digraph {
     a -> b -> c;
     b -> d;

How to set attributes of the vertices and edges:

 graph {
     a [label="Foo"];
     b [shape=box];
     a -- b -- c [color=blue];
     b -- d [style=dotted];

There are more atttributes which can be used to customize individual nodes and edges or the whole graph. See man dot.

Some of the other command line tools you get when installed Graphviz:

  • circo
  • fdp
  • neato
  • patchwork
  • sfdp
  • twopi

The commands work on the same input files; they use different strategies for arranging the nodes.

dot works well when the graph is a hierarchy or DAG. The algorithm is possibly described here.

The nodes are sorted by the upper limit on the number of parents. Nodes without any parents are root nodes and appear at the top of the page.

An example of using an invisible edge to push a node down a rank:

digraph {
     a -> b -> c;
     b -> d;

     c -> d [style=invis];

Using rankdir to change the orientation of the graph:

digraph {
     a -> b -> c;
     b -> d;

Other possible rankdir values are RL and BT. rankdir can be set from the command line:

$ dot -Tpng -Grankdir=LR < > foo.png


ditaa can convert ASCII art into a diagram:

$ cat diagram.txt
  +-------------------+          +---------------------+
  |                   |          |                     |
  |                   |          |                     |
  |    The Source     |--------->|     The Target      |
  |                   |          |                     |
  |                   |          |                     | 
  +-------------------+          +---------------------+

$ ditaa diagram.txt diagram.png

In Emacs, use M-x picture-mode to edit ASCII art.

ditaa also interprets these characters:

line notation
* small circle
: turn line to dashes
= turn line to dashes
\ round corner
/ round corner
box notation
cFFF hex code for color
{d} document (ragged bottom)
{s} storage (cylinder)
{io} I/O (parallelogram)

For example:

  /-------------------\          +---------------------+
  |                   |          | cF00 {s}            |
  |                   |          |                     |
  |    The Source     |-=----*-->|     The Target      |
  |                   |          |                     |
  |                   |          |                     |
  \-------------------/          +---------------------+


Get information about a video such as duration is seconds and width and height of the frames in pixels:

$ ffprobe foo.mpg

Dump frames from a movie at a rate of 25 per second:

$ ffmpeg -i foo.mpg %4d_foo.jpg

Dump frames from a movie at a rate of 5 per second. Use them to construct an animated gif which loops:

$ ffmpeg -i foo.mpg -r 5 %4d_foo.jpg
$ convert -delay 20 -loop 0 *_foo.jpg foo.gif

This will splice videos that are the same codec together, but for mpegs at least, there are artifacts at the joins:

$ cat list.txt
file 'first.mpg'
file 'second.mpg'
file 'third.mpg'

$ ffmpeg -f concat -i list.txt -c copy output.mpg

Supposing the original dimensions are 352x240. This crops a half-sized, centered video:

$ ffmpeg -i foo.mpg -filter:v "crop=176:120:88:60" crop_foo.mpg


  • mvg file
  • greenscreen effect
  • animated gif: pan of static image
  • animated gif from several static images
  • splice animated gifs
  • ffmpeg: extract audio/replace audio
  • ffmpeg: cut a scene from a clip
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