The Step % parameter is the main sampling control when rendering volumetric effects. The higher you can set the Step %, the faster the rendering will go. Eventually artifacts will start to appear for very high values, so that is where you must stop increasing the Step %.
The Step % option specifies the ray marching step of the camera rays as a percentage of the cell size. As the renderer traces rays through the simulator, this value controls how often to get information from the grid. If the Step % is more than 100, some cells will be skipped and artifacts may appear. A lower percentage might be necessary so that fine details are not lost when custom changes are made to the Opacity curves for Fire / Smoke.
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Step %: 90, Render Time: 4m 58seconds.
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Step %: 200, Render Time: 2m 01 seconds.
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Step %: 300 , Render Time: 1m 49 seconds.
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Rendering rollout → Shadow Step %
The Shadow Step % is similar to the Step % in a sense that it is responsible for the sampling quality of the volume but for the shadows cast by the volume over itself and the surrounding objects only. The higher you can set the Shadow Step %, the faster the rendering will go. Eventually the shadows will become washed out for very high values, so that is where you must stop increasing the Shadow Step %.
For very simple scenes, setting the V-Ray Render Settings → Global Switches Overrides → Advanced Lighting → Adaptive lightsLight evaluationdrop-down to Full Lights Evaluationevaluationmay speed up the rendering of volumetric effects.
All options under the Rendering rollout are supported by V-Ray Adv (CPU).
The following options are not supported by V-Ray GPU:
Smoke Color → Scattering: Approximate & Approximate+Shadows - V-Ray GPU is significantly faster when using the Ray-Traced (GI Only) option. V-Ray Global Illumination should be enabled for this option to have any effect.
Smoke Color → Volume Light Cache: . This option refers to the internalPhoenix Light Cache, which is not related to the V-Ray Light Cache. It's a CPU-only feature that helps speed up Bucket rendering.
Fire Lights: All Fire Lights options are ignored when rendering with V-Ray GPU. Enable Global Illumination from the V-Ray Settings if you need the Smoke and/or the scene to be illuminated by the fire.
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V-Ray Next Adv. CPU with Brute Force / Brute Force GI
