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.