Step 1: Introduce a circular model (and a new texture to use it on).
Implementation detail: While collision between polygons and circles is pretty trivial, the ability to stretch and rotate entities freely means that 'circle' is really 'ellipse'. And collision between two arbitrary ellipses, while not unsolvable, is more computationally complex than standard polygon collision. Therefore, circles are approximated as 16-sided polygons ('hexadecagon', the internet suggests, and my spell checker complains of).
Step 2: For entities that act as triggers, spawn points, etc: allow us to set 'no texture' to the entity and exclude it from the rendering process in-game (but not in the editor). This is what they look like:
These triggers are always rendered on the top layer. They have every other property that entities have, namely script events, which enables us to make something happen when the user collides with the trigger zone, or...
Step 3: Introduced an 'update' script event that can be attached to entities. The 'update' script runs every logical frame of the game, or at an interval specified.
Step 4: Added a 'BounceVeloicty' parameter to the arguments passed through to the entity collision event. This could actually be calculated in the script, but it seems that using it will be so common that it's probably more efficient to calculate it from the engine and pass it through every time. The 'bounce velocity' is just what it sounds like--the direction an entity would travel if it were to bounce off of the entity it collide with (like a wall, or another ball).
Step 5: Add a little script, make a video. Enjoy!
Next up: Linking multiple sections (levels) to each other and transitioning between them.