| Colors | Fractal | Falloff | Proximity | Animation | Marching | Bounding Box | Scattering | Lights | Render Tree Usage
A ray marcher coupled with a fractal density function. Ray marching determines the look of the smoke by "marching," or stepping, along each ray cast during a render. As it marches along the ray, it captures information about the density of the smoke, which is defined by the light's volume, and returns the information to the mental ray renderer.
The ray marcher simulates a cloud of small particles. Each ray that passes through the volume is broken down into steps, and at each step point the density function is evaluated. The density function returns separate R, G, and B components, which represent the individual densities of particles scattering only the respective color components.
The renderer will slow down considerably if you don't take the following into consideration:
Step size and recursion: Keep the step size as large as possible. It is usually sufficient to have it about half the feature size of the Density parameter. Unless you get spatial aliasing (strange plywood-type effects), don't lower this value. You can also decrease aliasing using the Jitter parameter. For testing, keep the Maximum Subdivisions at 0 and increase it only as necessary.
Density: The fractal is calculated fairly quickly, but it slows down significantly if you have too many iterations. For test purposes, keep the SS Max parameter value low. If you want the shader to be uniform, deactivate fractal altogether by setting SS Max to 0.
Falloff is when the density function is made to return an increasing amount of black according to some distance. A density function of black means that the density in the ray marcher is 0. This is useful for creating layered fog when you want the density to grow toward 0 as you approach some height over ground.
Ambience |
Simulates an ambient light source illuminating the cloud. |
Absorption |
Controls the amount of light that falls on the particles is absorbed. For air or steam, keep this at 0; for dark smoke or soot, make it higher (about 1). |
Asymmetry |
Controls the scattering behavior of the shader. At 0, the same amount of light is scattered back as well as forward; this is the case with small particles, such as air. Higher values make the scattering ever more forward biased, as with larger particles such as smoke or dust. |
Uniform |
Makes the light scattering uniform in all directions, much as with the ambient light that follows. However, light lookup is still performed, and self shadowing may occur (see Volume Effects). If you select Scattering Lights (uniform_scatt), the ray marcher uses them to determine light scattering at that point. Optical depth is accumulated and used to calculate the occlusion of both scattered and transmitted light. |
Density Scale |
Scales the density of the cloud. It is an overall scaling factor: if the cloud/hypertexture appears too dense, you can lower this value instead of having to meddle with the density function. |
Volumic Density |
When the volume shader is applied to an object, this parameter makes the density automatically change according to the scaling of the object. For example, if you scale the object down ten times in the XYZ axes, the density of the texture will increase a thousandfold. |
Transparent |
Makes the texture invisible; i.e., it doesn't block light at all, and the shader becomes a glowing transparent gas, much like neon. |
If you have scattered lights, it slows down the rendering somewhat because each light lookup can be quite time consuming, particularly if you've selected an area light. If you select the same light as both scatter light and shadow light, each light call will result in a ray marching from the current position to the light -- so rendering time may be almost squared! Do this only if you have either a large step size or you have a very dense material (hypertexture).
Scatter Lights |
Lets you define a scatter light in your scene that will be used to compute the fast lens effect. For more information, see Using Light Lists [Direct Illumination]. |
Shadow Lights |
Lets you define a shadow light in your scene that will be used to compute the fast lens effect. For more information, see Using Light Lists [Direct Illumination]. |