Illumination

Illuminating smoke is the most time consuming part of a rendering gaseous fluids. TurbulenceFD offers several illumination modes with different speed and quality to choose from. In the list, the speed decreases (first mode is fastest, last mode is slowest) and quality increases (first mode has lowest quality, last mode has best quality).

Note that the default Fast mode may be all you need in many scenes even at high quality requirements. However, the thicker the smoke gets, the more likely you're going to see stairstep artifacts. In these situations, choose the fastest mode that delivers the quality you need.

Both Corrected modes use more memory the more light sources illuminate the fluid container.

When light hits a gas it either gets absorbed or bounces off into a different direction. Different types of gases have different preferences of directions into which they scatter light. Water vapor for example tends to scatter light in forward direction. That means that most of the light that hits the vapor does not change it's direction too much and continues to travel forward. As a result, the vapor will be brighter when the camera is on the other side of the vapor cloud as the light source.

The Scattering Anisotropy parameter specifies the preferred direction of light scattering. A positive value means forward scattering, a negative value means backward scattering.

For example, a value of 1.0 means that the light does not change it's direction at all. You will only see lit smoke if the camera is looking at the light source at the exact angle of the light.

Vice versa, a value of -1.0 means that the camera has to look into the same direction as the light to catch any light scattered off the smoke. All light is scattered in exactly the opposite of it's incoming direction.

The neutral value of 0.0 means that light gets scattered into all directions equally.

In practice you would usually only use small values like 0.2 to get brighter backlit smoke for example.

Illuminating smoke takes most of the render time. You might want to reduce the resolution at which the illumination is computed to save render time.

On the other hand, if you have extremely thick smoke you may see grid artifacts unless you increase the illumination resolution.

Enabling Multiple Scattering allows for two effects.

• It works like Global Illumination for smoke. That is, it computes additional bounces of light inside the smoke, effectively making it brighter. This is the preferred way of creating ambient light. Using the ambient light parameter on the light source will create a low quality approximation of this effect.

• It allows for the Fire Shader to illuminate the smoke

The higher the Dpeth value, the more bounces will be computed. More bounces will make the smoke brighter but also cost more render time.

You can create a cheap approximation of a higher Depth value by lowering the Falloff (see below) instead.

Increase this value if you see ray artefacts in your smoke. A higher Directional Resolution will increase the render time.

The higher this value, the faster the light will fall off as it travels through smoke. You can get brighter smoke by using a lower value here.

This allows you to separately adjust how much external lights contribute to Mutiple Scattering.

This allows you to separately adjust how much brightness the Fire Shader contributes to Mutiple Scattering.