Low-Tech Low-Fog

We've recently resolved to do weekly seminars in the office with a view to pooling and sharing knowledge that the various super talented people here have.  Some are more blog worthy than others but it seems logical to post some of them here which may be of interest. 

Low fog in Corona was this morning's seminar.  We use the Corona volume material in almost every scene to some degree even if almost imperceptibly to give a slight background haze as always appears in reality on even the clearest of days. 

For a general volume material tutorial check out Corona's volumetric fog tutorial which is really clear.

This post is more to do with how you can use volume materials in different ways to generate very quick and simple atmospheric effects without resorting to more technical simulation techniques such as FumeFX. (although this dancing amphibious hominid is pretty cool) 

Here are a few basic examples which were generated using PG Skies test scene. One with a Corona Sun / Sky system and the other with PG Skies 1714.  A great sky incidentally as it is one which although having been shot in Moroccan Desert works equally well for warm summer scenes and cold winter scenes.

Here are a few more examples of the fog with different lighting conditions and with a lower more zoomed in camera. 

Basic volume material set up below:  I typically use an absorption colour value of around 220 and a scattering colour of roughly the same.  Single bounce only is much faster and makes very little difference to the result. 

All you need to do then is apply the volume material to a piece of geometry which will act as your fog or cloud.  Below is an example of a basic object with multiple noise modifiers applied of varying scales and strengths.  Basically play around with the noise parameters to achieve different effects.  Rough steamy fog or smooth low fog. 

It may look nothing like fog with a default material at this stage by the way

One important thing to remember is not to overlap the fog objects as this can result in dark patches at the intersections where the fog has not been correctly calculated rather like co-planar surfaces with transparent materials. 

It is also important for the geometry to be complete, i.e. no missing polys as the volume material is not distributed throughout the geometry but gives this effect when viewed from the outside.  With this in mind the camera must always be outside of the fog object.

Below is a view port screen grab of the fog object in the PG Skies scene. Note that the camera is just outside the fog object. 

The density of the fog can be adjusted by the absorption distance.  Higher values for a less dense fog, lower values for greater density.  See examples below. 

The final thing to mention is that the apparent density of the fog is dictated by the angle of the sun striking it relative to the position of the camera.  If the sun is coming straight into the camera the fog will appear more dense as in reality.  Sometimes if there is no sun the fog may appear less visible.  This is where Emission can be useful.  Rather than increasing the density of the fog by reducing the absorption distance it is possible to add a sort of self illumination with Emission Distance.  This is set to 255 (white) by default.  The distance usually has to be quite high to avoid a glowing cloud!   See examples below.

So, might seem a little old school and a bit of a hack but it's proved really effective and super quick in many situations recently.  Hope it helps.