We've already examined the effect of water in its gaseous form (as fog, mist, or steam); what about water in its liquid and solid states? Although it's not possible to re-create The Perfect Storm or The Day After Tomorrow without elaborate 3D and practical effects, compositing plays a pivotal role in re-creating rain and snow and in enhancing practical and computer-generated scenes.
Realistically, though, it's rare to create elaborate water effects without relying on some even more elaborate practical or computer-generated source. For this discussion, I'll assume that you're trying to complete shots only in After Effects, but I'll focus on techniques that are equally valid even if your particle and water animations are created elsewhere.
One area where After Effects' built-in features fall short is particle generation. The Particle Playground effect, which ships with the program and hasn't changed much since around version 3.0, is slow, crude, and cumbersome. I have yet to work with anyone who had the patience to coax realistic effects out of this plug-in.
If you're called upon to create rainfall or snowfall from scratch, consider the Particular plug-in from http://www.trapcode.com/. Not only does it outdo Particle Playground in features and ease of use, but also, if set up correctly, it obviates the need for creating precipitation in a dedicated 3D program.
Creating the Element
Particular contains all the controls needed to create a great precipitation element, but it also contains a lot of controls, period. Here's a brief attempt to outline a few of the most significant controls, followed by an example of how to use a final element.
A standard particle shape can be used, or customized particles, such as an irregular snowflake shape. There are several choices of particle emitters, but one great option is to use a spotlight, making the light layer's Transform controls available to establish the position and direction of the particles in 3D space (see Figure 23).
Figure 23 The axes belong to a light used as an emitter for Particular. It can be translated freely in 3D space.
The most important settings are found in the Emitter and Physics categories. Emitter settings establish the amount, velocity, and direction of particles, whereas Physics contains controls pertaining to the environment itself: gravity, air resistance, wind, turbulence, and spin.
The Visibility category contains controls affecting the depth of particles. You may find that, as with smoke, several planes will offer a better result than one big simulation; for example, allowing you to control the foreground separate from everything else.
Particular resides on a 2D layer, but the effect is 3D-aware, so if you add a camera to the composition, the particles will behave as if seen through that camera.
Compositing the Element
When the time comes to integrate falling rain or snow with a background plate, you can do better than a simple A-over-B comp; in fact, the key is to show the effect of these elements on the scene, rather than showing the elements themselves.
Raindrops and snowflakes are translucent, their appearance heavily influenced by the environment. Moreover, these individual bits of precipitation behave like tiny lenses that diffract light, defocusing and lowering the contrast of whatever is behind them, but also picking up the ambient light themselves. Therefore, on The Day After Tomorrow our crew found success with using the rain or snow element as a track matte for an adjustment layer containing a Fast Blur and a Levels effect.
In the image in Figure 24, blurriness is set very high (200), so that the area behind each individual raindrop or snowflake becomes a wash of color. Levels is applied with a slightly lowered (90%) Output White value, and a very high (80%) Output Black value. The precipitation is visible by its effect on the scene, lightening dark areas, darkening light ones, and adding diffusion throughout.
The best thing about this approach is that it works independently of the background appearance. There's no need to decide the color of the element for a given shot, and shots retain source colors, the precipitation having a similar influence on each shot in a sequence.
Figure 24 You must look closely to see the added snowfall in a still image with so many bright regions. (Source image courtesy of Eric E. Yang via Creative Commons.)