Autogenerate Path Visualisations

In the previous blog post I have described how to use GraphViz to describe a graph in a textual format and then generate an image from that.

This time, let’s do one more step and add some code so that the image gets generated automatically when the text file is updated.

For my project I use Guard, a Ruby gem that watches file system changes, and allows to define various reactions. Running unit tests whenever a file is saved is one example.

For my case, I let it generate a new image file, when a GraphViz file is saved. First of all, I installed the needed Ruby gems:

gem install guard guard-shell

With that in place, here’s the Guardfile I use to generate the images:

guard 'shell' do
  watch(/(.*\.gv)$/) { | m |
    dot_file = m[1]
    png_file = "#{File.basename(dot_file, '.*')}.png"
    command = "dot -Tpng -Gdpi=300 -o#{png_file} #{dot_file} && open #{png_file}"
    puts command
    puts `#{command}`

Note: In the code above I had to revert to enter (0xff06) instead of & (0x26) in order to prevent the syntax highlighting feature from turing a & into `&`… 🤷‍♂️

The part watch(/(.*.gv)$/) invokes the following code block when a file that matches the given regular expression changes. I am interested in files that match ‘*.gv’ in the directory that the Guardfile is stored in.

The line dot_file = m[1] extracts the file name from the parameter passed into the block, and uses it to create a _new_ file name for the image file that is going to be created.

Then the shell command is put together, logged, and finally called.

Component of the commandWhat it is/does
dot The command name that
translates the gv file into a PNG image
-TpngSet the output format to ‘png’
-Gdpi=300Set the resolution to 300 dpi
(gives a good resolution for printing)
-o#{png_file} Set the file name for the png file
#{dot_file}The name of the input file
&& open #{png_file}This appends another shell command to open the image file.
(I use this on my Mac to automatically (re-) open the file whenever it’s regenerated.)

In case you would like to experiment with this, I created as a starting point, including an example GraphViz file.

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