In the tutorial Expressions and External Documents, I showed some ideas on how to use external documents to drive content inside After Effects. The feedback that I’ve gotten from the community is that this technique can be a real timesaver in projects that require a lot of repetitive content, such as bumpers, lower thirds, interstitials, etc.

In the tutorial, with my own attempt at the idea, I fell short on a couple of challenges.  First, because After Effects uses different address structures for Windows and Mac OS X, it’s a little tricky to make one expression that works on both platforms.  Second, as I’ve later found out, the #include function was disabled in AE CS4.

Fortunately, scripting and expressions guru Lloyd Alvarez stepped in to fill in the gaps and provide a solution that works in both CS3 and CS4, as well as both platforms. Let’s take a look at the full Source Text expression:

if ($.os.indexOf("Mac") != -1)
myPath = "/Expressions/";
else
myPath = "file://c:\\\\Expressions\\";
myPath += "data.txt";
$.evalFile (myPath);
eval(thisComp.name);

Let’s take a look at what is going on here.

if ($.os.indexOf(“Mac”) != -1)

In this first line,  $.os is equal to the name of our current operating system. For example, on my computer this displays “Macintosh OS 10.5.6″.  indexOf() simply looks for the content in parentheses. If it finds this, like the letters “Mac”, it is equal to a value of 0. If it is not found, the value is -1. If you’ve not used it before, the term != means “does not equal”.

Therefore, this is an if/else condition that says “if the current OS contains the word ‘Mac’,then do the following. So, we have things set up for both Mac and Windows, and we can handle things accordingly.

The next line is what runs if the OS is Mac based, and it establishes the initial path of where to look for the external document.

myPath = “/Expressions/”;

If the OS is not a Mac, and is therefore Windows based, the path will be formatted for Windows.  Note that in Windows, two slashes: \\  need to be supplied to denote a directory, not just one.

else
myPath = “file://c:\\\\Expressions\\”;

Now that we have the path set up, let’s assume that the text document that has our data is called “data.txt”.  What can be done at this point is to add the path and filename together like this:

myPath += “data.txt”;

This is the equivalent of myPath = mypath + “data.txt”;

Then, an alternative to #include is $.evalFile, which fortunately works in CS3 and CS4 as a way to look at the contents of an external file. Then, just like in the tutorial, we can use eval() to have the comp name pass as the the value for the source text, as the last value in the expression. Assuming there is a variable in the data file that matches the comp name, it will be displayed as the source text.

$.evalFile (myPath);
eval(thisComp.name);

To have the expression wrapped inside try/catch to eliminate error messages (such as in the case of the text document not having the correct scene number), here is a version like that:

try{
if ($.os.indexOf(“Mac”) != -1)
myPath = “/Expressions/”;
else
myPath = “file://c:\\\\Expressions\\”;
myPath += “data.txt”;
$.evalFile (myPath);
eval(thisComp.name);
}catch(err){“Not Found”}

For some complete training on After Effects Expressions, and to support the site, take a look at my After Effects Expressions training series.expressions

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