What are the most effective techniques for creating volumetric fog in game development?

The term "ray marching" describes the process of repeatedly moving a point along some direction, in incremental steps. Given a starting point and direction, you step forward in that direction by some offset. You have therefore "marched" along that direction (ray). This is repeated a number of times up to an end condition, with some kind of function performed at each step. Raymarching is often used for rendering volumetric effects, such as fog, explosions, and even medical scans. Keep in mind that raymarching for visual effects is typically implemented as a complete loop per-pixel of however much the target surface is visible on the screen. It can be computationally expensive even for GPUs, thus low-resolution buffers are typical.

You can also use procedural noise if you're not too haphazard. GPUs across all hardware are typically more than powerful enough in the 2020's to be able to handle even 3D noise functions. The key, of course, is to not get carried away. And, as usual with anything transparent, watch the overdraw.

Mastering lighting and shadows is pivotal for elevating fog effects in a design, adding depth, contrast, and mood to the scene. Techniques like shadow mapping cast shadows from light sources onto fog, while light scattering mimics rays passing through the fog. Incorporating ambient lighting imparts a global illumination effect, and fog lights, as specialized sources, uniquely impact the fog. These nuances contribute to a more realistic and immersive fog experience, enhancing the overall visual appeal.

Some tips to quickly improve lighting: - Apply some anisotropic phase functions to directional lights. Cornette-Shanks, Heyney-Greenstein... - Use an inverse-square falloff for point lights. - Use Beer-Lambert (literally just exp()) for calculating absorption from point lights. - Apply self-shading from point lights by doing a secondary raymarch loop from the current position to the light if you're within it's range. You can then accumulate the density between the current point and light source and use that to attenuate the light.

Blending methods such as alpha blending, additive blending, or screen blending play a crucial role in determining how the fog harmoniously interacts with the background or foreground elements. These steps add a layer of polish, elevating the realism and visual appeal of the overall composition.

A really cheap way to simulate multi-scattering blur is to use the existing density (aka transmittance, aka 'alpha') of the volumetric fog to first blend in some blurry version of the scene before compositing the fog itself.

Raymarching is expensive. A relatively simple solution is to execute the shader on a low-res target, some fraction of the total screen resolution (half, quarter, 1/8...), then upscale with filtering. You can reduce the total ray steps, add some animated noise (blue noise, interleaved gradient noise, etc...), perform depth-aware, separable blurs, then finally upscale using a nearest-depth filter. With simplified lighting calculations, this can get you some decent volumetric fog with light and shadow working at 60fps+ even on mobile.