Rain simulation

As a generative animation exercise (and also just because, um, rain is nice-?), I decided to code me up some good ol’ precipitation. Considerations included: degree of realism, adaptability and performance. Specifically, animating a single layer of tear-shaped drops falling uniformly in perfectly straight lines is relatively easy to do, and certainly has its place within the right art style. On the other hand, ultra-realism would entail thousands of varied and varying particles, deeply distributed in 3D space – something that would better suit pre-rendering than realtime animation, or at the very least, something other than Flash. But that would be no fun at all. I opted for a low-ish number of convincing enough, but inaccurately shaped, particles (apparently raindrops are actually round, mushroom-top blobs – who knew?!), with fairly realistic motion and three distinct layers for the parallactic goodness. I tried to keep everything as configurable as possible, so fall speed, wind strength, blustery-ness, particle graphic, drop size variation and viewport size can be adjusted easily. Reuse or lose! – as they say. Well, they ought to start saying that.
On with the rain!:

You may notice that in the front and backmost layers, the drops are a little blurred. I wanted to give a narrow depth of field effect, and had initially applied a BlurFilter to their containing sprites, but that hit performance hard – so I drew a separate drop symbol for their layers, softening the perimeter with a radial gradient down to transparent, which is just as cheap performance-wise as a solid drop.

Thinking a little more about the worth of such an effect, I realised that I would likely want the rain to interact with whatever other objects might be in the scene. And so I added an umbrella:

It didn’t make much sense for the umbrella to stop drops across all layers, but the effect wasn’t very noticeable with so many other drops falling past it, so I removed them for this demo. I also added some physics-y hinged swinging based on the umbrella’s sideways movement. A splash animation plays at the exact intersecting point of drop and canopy – but I only later realised that, since around 75 drops make contact every second, I could easily have randomised the locations of the splashes and simplified the collision detection. Still! – honesty is the best policy, and when it’s only drizzling, accuracy of the contact point is more relevant.

So now I have almost everything I need for my ‘KEEP THE LOST KITTIES DRY’ minigame. No need to thank me just yet, world.

I drew the visual assets in the Flash CS3 IDE, compiled my classes in the wonderfully lightweight FlashDevelop, and animated the drops using Tweener. That’s all!


I'm Adam Vernon: front-end developer, free-time photographer, small-hours musician and general-purpose humanoid.