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If you don't mind, here's my current understanding of how your image generation works - how close am I to correct?

Everything sounds about right. I'm not sure about fine details as this generator was made more than two years ago.

One thing I'm not clear on, is how many growth points initially are placed onto the card. 

As far as I remember, a single tracker is placed per time (or something like this). I tried to spawn all the trackers according to the density, but it looked bad: this way they didn't have chances to draw long curves because they bumped into each other too soon.

As I understand it, growth points actually spread outward in two directions (unlike Substrate).

To be honest I have no idea what Substrate is. Yes, curves grow in two directions. Otherwise there would be loose ends which could cause problems with building of closed contours.

(+1)

Ah, Substrate is a screensaver: https://www.youtube.com/watch?v=dCCVgBOVD0E It avoids the loose ends by starting seeds directly on existing lines. I was acting like you'd heard of it because I've seen it mentioned a couple times in the context of history of procedurally generated art. https://procedural-generation.tumblr.com/post/124657726768/substrate-2003-shortly-after-the-turn-of-the

Having played with the code more, now I understand that the objects are rendered in one at a time to give some appearance of depth. It does look like there's a setting for number of starting seeds which gets used on a few objects.

It might be interesting to produce some symmetry by having some objects which place two seeds at a time, symmetrically.  Maybe that would be a solution for getting more face-like things to appear.

Anyway, I'm enjoying poking around in your old project. :)