I'm making an indoor scene with a storm outside. I want the windows in my scene to have rain just bucketing over them. Ideally I would like some animated effect that can be composited over the background, which might be something like a bokeh image. How can I achieve this?
A polygon-rich approach
I gave it a go with a shower of particles. This may be a little bit too resource-intense for just a background feature though.
I created a simple "room" with
a double-layered window (two plane meshes);
a plane out in the sky, almost vertical, with a particle system;
two strong wind force fields that push the particles toward the window;
a simple icosphere as a dupliobject.
The building and the inner window layer are set as Collision objects with "Kill particles" activated.
The outer window is a Collision object with very high stickiness (no kill), and high damping/friction on particles.
The particle system has strong Normal velocity and low Gravity field weight, so that the rain drops don't fall off screen too quickly; "Object" > "
Icosphere" is selected as "Render" option, with "Emitter" un-checked.
(this preview is from an earlier version, with less particles and less friction on the surface)
By giving the icospheres and window layers a Principled shader with very high transmission; adding some orange streetlamps and violet "dusk" sun (and a sad Suzanne in the storm); with some DOF and motion blur: this is a very rough result:
(Only 7 frames)
Using nodes and a "Geometry" input, I made the raindrop's shader completely transparent (shadeless) when the "y" coordinate is less than y0 (the position of the window) to cut away the part of the sphere that "permeates" the first layer.
This answer's blend file
Even heavier for a close up shot (with metaballs)
If you want a close-up shot as in here, an additional Particle System with similar physics, but from a nearer plane (just near the camera) and with big meta-balls as DupliObjects, can help: