It was a bright cold day in April...

You asked me once, what was in Post 101. I told you that you knew the answer already. Everyone knows it. The thing that is in Post 101 is... well, pretty cool. All cultural references aside, the content of this amusingly-numbered post is in fact rather spiffy, and comes in the manifestation of an update on PyIgnition's progress. To summarise, it's been going rather spiffily. So spiffiliy indeed that I decided it warranted a post. So! To make haste t'wards the very tip of this post's point, I shall cast in your general direction some video footage of PyIgnition, post mutatio:

PyIgnition alpha 3 from Animatinator on Vimeo.

So as you can see, the promised physics system is very much in place. Particles can now deflect off objects thanks to a combination of what is effectively an inverse gravity (but using an inverse cube law instead of inverse square to ensure objects aren't visibly repelled from a distance), and a preceding intersection resolution stage. The latter is a fairly simple process which firstly works out whether any particles are inside obstacles and then shifts them back out to appropriate places. The two procedures are defined as functions for collision obstacles (as InsideObject() and GetResolved() respectively), so the code handling the overall process is just a couple of lines:

if (not obstacle.OutOfRange(particle.pos)) and (obstacle.InsideObject(particle.pos)):
____particle.pos = obstacle.GetResolved(particle.pos)

(The first part of the if statement is just checking if the particle is within reasonable range of the obstacle before it checks for an intersection.) I also added a 'bounce' multiplier to the repulsive force which is mainly set to a value below 1.0, as the force often tends to be a bit big and results in particles gently drifting towards an obstacle only to be propelled away so quickly that the program would need to take the effects of special relativity into account in order to process them correctly. In all of the demos shown this was set to a standard value of 0.2, which causes only a slight bounce when particles collide with obstacles.

So, that concludes this decade's blog update. See you next time, when I'll hopefully be bringing news about the work-in-progress new-and-vastly-improved keyframe system!