A more complex example

Here's a slightly more complex example that makes use of Jinja2 templates. Jinja is usually used to generate web pages, but it can also be used to manipulate batch files before they are actually run.

Here, we use it to loop through a set of filenames stored in a separate file, so we can run a PCOT document on each one. I'll go through the code line by line (more or less) and then show it all at the end.

Jinja basics

Jinja is a templating system. It uses special codes embedded in the batch file to manipulate it and change its contents, before the file is actually interpreted by the batch system.

• Codes of the form {%...%} are Jinja "statements", which control the flow of what Jinja is doing and set variables.

• Codes of the form {{...}} are "expressions" - Jinja calculates the expression's value and inserts it into the document. The expression is often just a variable name, and in this case Jinja will replace the code with that variable's value.

There's a lot more detail in the Jinja2 docs but bear in mind they are written very much from the point of view of web design.

The walkthrough

We start by setting up our input. We're going to use a single input - input 0 - and it's going to load and debayer an RGB image. This sets up input 0 to use the RGB method, and says that it should use Malvar-He-Cutler debayering:

inputs.0.rgb.debayer_algo = MHC

Still on input 0's RGB method, we tell it to use the GRGB debayer pattern. Remember that if we start with a leading ., that means we are setting a parameter in the subsystem in which we last set a parameter, in this case inputs.0.rgb:

.debayer_pattern = GRBG

We now add a new output (output 0) using the + operator, telling it that it should generate the output from a node called "sink":

outputs.+.node=sink

and we tell this output that's it's OK to overwrite existing files:

.clobber = y

So far, this should all be familiar to you from the rest of the batch file documentation. Now we'll use the Jinja templating system to load a list of files from another file called list.txt, and we'll loop through them. This is done with Jinja's {% for.. %} / {% endfor %} statements in association with the loadlist function, which loads lines of text from a file (stripping comments and blank lines):

{% for name in loadlist("list.txt") %}

Within the loop we create the input and output filenames, storing them as Jinja variables. We do this using {% set var = expression %} statements.

First, we create the input name by just copying the name variable.

    {% set input = name %}

We then generate the output name by using Jinja's built-in replace filter to replace the .png at with _corrected.png:

    {% set output = name | replace(".png","_corrected.png") %}

Now we can set the input filename within the input we set up earlier:

    inputs.0.rgb.filename = d:\PCOTdata\HRCdata\{{input}}

Naturally your full filename will be different; it doesn't have to be a full path, I'm just showing here what's possible. As noted above, putting a name inside {{..}} tells Jinja to substitute that variable's value into the batch file.

Similarly we tell output 0 where to put the result, which we've already specified will come from the sink node:

    outputs.0.file = {{output}}

Finally we actually do the work, using the batch system's print statement to output messages and the run statement to actually run the PCOT document with the modified settings:

    print {{input}} read
    run
    print {{output}} written

And finally finally, we end the loop we started earlier

{% endfor %}

Input file format

For reference, the input file might look something like this;

# some files (this is a comment)
file1.png
file2.png # another comment

Anything after a # is ignored by loadlist, as are blank lines and leading and trailing whitespace.

The whole batch file

{# Set the RGB input method on input 0, and tell it 
   to use the Malvar He Cutler debayering method. #}

inputs.0.rgb.debayer_algo = MHC

{# Still on input 0's RGB method, tell it to use the GRGB debayer pattern. #}

.debayer_pattern = GRBG

{# Add a new output (which will be output 0), and tell it to read
   from the node called "sink" #}

outputs.+.node=sink

{# Tell that output that's it's OK to overwrite existing files #}

.clobber = y

{# Now use Jinja to go over the filename list, which we load from
   another file. #}

{% for name in loadlist("list.txt") %}

        {# create the input and output filenames #}

    {% set input = name %}
    {% set output = name | replace(".png","_corrected.png") %}

        {# set the input filename in the input method #}

    inputs.0.rgb.filename = d:\PCOTdata\HRCdata\{{input}}

        {# set the output name in the output #}

    outputs.0.file = {{output}}

        {# run the PCOT document with those settings, printing out
           some logging information #}

    print {{input}} read
    run
    print {{output}} written

{% endfor %}