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Emissive volumes may be very slow to converge, especially if they are the only light source in the scene - it may require many hundreds or even thousands of camera samples to render a noise free image. Until volumes are supported as full light sources that can be used for direct lighting, we We suggest that emissive volumes be used very sparingly, instead use the Light Source parameter.
Multiple Scattering is a very costly effect, both because it means the light is bouncing that many more times in a volume, but also because PxrVolume cannot take more samples itself (the Samples parameter is ignored), and is at the mercy of the Integrator settings. Moreover, multiple scattering may slow down the convergence of objects behind the volume. Therefore we suggest that whenever possible, multiple scattering should be avoided. For media with low albedo, multiple scattering adds very little to the final render. For some media like clouds, whose light transport involves many tens or hundreds of light bounces, unfortunately multiple scattering may be unavoidable.
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The above parameter for "Light Source" is useful for many applications when using a volume with emission properties. But there may be times you need to control the influence of the light from the volume. This requires the use of a light filter. The PxrVolume material doesn't have a way to use a light filter so you can apply the PxrMeshlight material to the volume object along with the PxrVolume for shading. In doing so this will light the scene but allow you to connect and use a light filter through the meshlight controls. But there is a caveat to this workflow that requires extra consideration in your setup. When you apply the meshlight to the volume you may notice an increase in illumination and/or additional noise. This is because of double contribution of the volume emission and the meshlight itself. Here are the current steps to reduce this issue.
Examples:
Using Light Source
Adding the meshlight increases light in the scene and noise, this is the incorrect workflow
This image is using the above workflow to create a well converged render while allowing use of a light filter |
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