Awesome

Note: I'm working on a more solid and universal volumetrics rendering solution, but it's still WIP (e.g. doesn't support directional lights): VolumetricLighting

Light shafts

A light shafts (aka light scattering, aka volumetric shadows) effect for Unity.

video

spot light directional and spot light

Performance: in 1080p on GTX580 about 1.0-1.5ms for a full screen light, down to 0.2ms if the light is smaller or partially occluded.

Download

Check out this repo into a subfolder of your Unity project (visible meta files), e.g. Assets/LightShafts/

Light shafts should work on Windows (DX9 and DX11) and OSX.

Version 2. Verified in Unity versions 4.5.5f1 - 5.6.0f3.

Usage

Add the LightShafts.cs script to a directional light or spot light and tweak the settings.

In general volumetric lighting is a very expensive effect. This implementation tries to make it affordable by avoiding slow raymarching for every screen pixel. A smaller number of raymarching samples in important places is chosen instead (red pixels in images below) and the final lighting is interpolated from those.

sampling

It's important to tweak the effect's quality settings to get as few red (expensive) samples as possible. Other settings are important for performance too.

Start out by tweaking the size (directional light) or spot angle and volume start and end (spot light) to get the yellow box/frustum in the scene view tightly around your target area.
Set the culling mask to only include objects which need to cast volumetric shadow (doesn't matter if the shadowmap mode is static, as then the shadowmap is only rendered at startup).
Enable show samples (DX11 only for now).
Tweak shadowmap resolution to be as low as possible, but still be able to make out the detail in silhouettes of shadow casters.
Samples across rays - that many samples - and rays - around the light, when looking at it.
Samples along rays - that many potential samples along each ray, but they only become actual samples if they encounter a difference in depth or are forced by the force samples every setting.
Depth threshold - from camera's perspective, light shafts change intensity wherever there's a bigger depth difference. Make sure this setting creates silhouettes of red pixels around objects where it matters.
Force samples every - even if there's no abrupt change in depth, light shafts' intensity still changes along it's length somewhat and that gradient needs to be sampled. Set to a higher value if you can (the goal is still to have as few red samples as possible).

Colored light shafts

To get the effect of light tinted by stained glass, enable the colored checkbox and set the color filter layer mask to whatever layer contains your colored objects. Those objects will be rendered to a buffer using a forward rendering camera, so sometimes it might be better to create duplicates with a shader not using lighting, just outputting saturated color. The rays will be tinted along their entire length.

Cookies on spot lights are multiplied in as well, and also affect performance.

LightShafts.cs vs SunShafts.js

The SunShafts.js effect in standard assets performs a (depth-aware) radial blur of the skybox, fully in screen space. So the effect is more volatile, visible only when looking against the light direction, etc., but also cheaper. Use LightShafts.cs when you need a more grounded effect, in world space, visible from the side - and can afford it.

What's next?

I'll probably add the effect to Unity's image effects standard package, when it's done.
1D min/max mipmap optimization: not sure if I'll implement it. It's usefullness is limited to dx11 and non-colored lights without cookies, mostly. Pull requests welcome, though :)
Some dithering would be nice to avoid banding in dark scenes.
Re-using the internal shadowmaps instead of rendering new ones - hmm...
Cookies: for directional lights too (need an offset setting) and premultiply with color filter, if both are enabled.
Make sample visualisation work on dx9 and opengl as well.

License

Public domain.