What is Participating Media?
Frequently within archaeological visualisation, there is an important need to represent mediums that refract light, such as smoke and dust. This provides both depth and immersion to reconstructions by providing a sense of how they could have been influenced by cultural and geographical factors. In models like those found at Çatalhöyük, where there are clear examples of hearths and fires that would have been used to heat and cook, this can be very important, “In archaeological sites in particular, the materials used to provide interior light, for example, candles and wood fires, would have generated smoke, perhaps significantly affecting visibility in these environments” [Rushmeier 1995]. Participating media is the label for this wide range of ways that light can be scattered by various particles and a rendered example of the heavy use of the shader to create silt and dust can be seen in the image below.
A very visible use of the Mental Ray Parti Volume shader
The Mental Ray Parti-Volume shader
Within 3DS Max and Mental Ray there is a very robust shader that can be used to create these effects in a ray-traced fashion, “Fog, clouds, and saltwater scatter the light that passes through them; in other words, they contribute to the light transport. To simulate this effect, you need a Parti Volume shader. (Parti is an abbreviation for participating.)” (Van Der Steen 2007: 81). Although it is not the only way to achieve a volume light effect (Another can be found in the environment/effects rollout), one of the real bonuses of this method is that it works with Final Gather, Photons and Mental Ray lighting in an energy efficient way, alongside ray-traced shadows. It also provides a number of different ways it can be applied (Either onto a surface, or straight into the camera shader), and a variety of effects that can be achieved using it.
The most common way of applying this shader is to put it directly into the camera shaders option in the rendering rollout. As shown below, the main components for the shader are mode, scatter colour and minimum and maximum step. Mode changes whether the particles affect the entire scene or simply elements of it. Its base setting, 0 will fill the entire scene, whilst 1 will relegate the phenomena to certain areas and ask for a height modifier to determine where in the scene it occurs. One of the most vital settings is scatter colour, as it determines how bright the image is and often the default setting can lead to some very bright, blown out images which can confuses a lot of new users. Although at default it is set to a mid grey, usually to provide a pleasing image, this setting needs to be reduced to around at least 0.08% white (Very near black). Minimum and Maximum step effect the sample size and it is useful to start big and then slowly decrease until you get a smooth image. The shader can often be quite a time consuming addition to an image, so it is advised to test the waters first using lower settings.
An example of the Parti Volume shader
r, g1 and g2 control the scattering of the volume and changing them effects how the final pass will look. Certain thresholds have been provided by Mental Ray to help achieve a specific look, these can be seen in the graph below:
|Rayleigh (Cigarette smoke/dust)||0.50||-0.46||0.46|
|Hazy Mie (Sparse water droplets/fog)||0.12||-0.50||0.70|
|Murky Mie (Dense water droplets/fog)||0.19||-0.65||0.91|
What to do for Maximum control with this shader?
Like the Ambient Occlusion technique shown in an earlier post, this shader can also be used as a pass to provide maximum efficiency and control in post production. This becomes especially beneficial in an animation workflow, as there is no need to re-render parts of the sequence and many decisions can be left until the compositing stage. An example of such a pass can be seen below and to achieve this result, a black non reflective material should be placed onto every object in the scene, either manually or using the override option in the render settings dialogue. This is to allow the pass to be ‘added’ in the compositing program of your choice to your existing passes. Furthermore to ensure this method will work, when there is open air in the model, a containing object (Such as an invisible sphere surrounding the scene) is necessary to allow the shader to consider every part of the geometry as a volume. Also, unless you require the use of photons to create volume caustics you can also turn off the indirect illumination settings.
An example of a volumetric pass using Parti Volume shader
Rushmeier, H. E. 1995. Rendering participating media: Problems and solutions from application areas. In Proceedings of the 5th Eurographics Workshop on Rendering. Springer-Verlag.
Van der Steen 2007 Rendering with Mental Ray