The VDB layout which makes it possible to render huge sparse-grid volumes makes rendering everyday volumes slower than the dense-grid AUR format.
If you can choose between AUR and VDB, go with AUR when possible. The rendering speed-up with AUR files can go up to 2x depending on the complexity of the setup.
Furthermore, resimulation results when using VDBs as a base usually differ from the results when using AUR files due to differences in the default compression methods.
However, sparse VDB caches would help reduce the RAM usage, so they are an alternative if you don't have enough memory for rendering an AUR dense cache.
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Resimulation stored as AUR , Render TIme: 3m 45 seconds.
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Resimulation stored as VDB, Render TIme: 6m 28 seconds.
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Rendering roll-out → 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 %.
The Linear sampler will always render 20-30% faster than the Spherical. You should use Spherical only in case you are rendering a low-resolution simulation that shows some grid artifacts and steps, especially when looking along one of the grid axes.
In general, for lower-resolution simulations, the Phoenix FD Light Cache will significantly speed up the rendering. As the resolution increases, the benefit from using the Phoenix FD Light Cache diminishes. If Volume Light Cache is disabled, try enabling it to check if that will speed-up the rendering. You can reduce the render times significantly by increasing the Light Cache Speedup setting. Increasing it from 0.9 to 0.99 to 0.999 should yield a big difference in render speed.
The Light Cache Speedup should be increased until artifacts start appearing in the rendered image. Grid Light Cache artifacts will look like voxel-sized dark and bright spots. They would also flicker in animation. Grid artifacts start appearing when a voxel does not get to cast enough shadow rays – this is usually related to the lighting setup – e.g. the illumination is too dim, there are too many grids in the scene, etc. If you see these artifacts, then your Light Cache Speedup is too high and you have to decrease it. If decreasing it to zero still does not help, disable the Light Cache.
Disable the Volume Light Cache option if:
Reducing the Light Cache Speedup option to zero does not help resolve artifacts in the smoke. If you see bright and dark voxels, this means the light cache speedup is too high. If you see pixel sized noise, then it's the sampling - you should adjust the V-Ray settings accordingly (e.g. increasing the Max Subdivs for the Bucket Sampler).
You are rendering an animation and you notice flickering which will not go away after reducing the Light Cache Speedup.
You are using Progressive rendering.
There are multiple grids in the scene - for a large number of grids in the same scene, disabling the Phoenix FD Light Cache might speed things up, as opposed to a single grid where it's best to keep it enabled.
Enabling Create Fire Lights simulates Global Illumination by placing light sources in the bright parts of the fire, which produces similar results and renders much faster. It can be used in with both GI on and off as well, because it would help in cases where large environments need to be illuminated by very small, very bright Fire Lights. Note however, that if there are many glossy surfaces in the scene to reflect the light from the fire, Create Fire Lightscould slow down rendering, so in this case try turning it off.
Disable Create Fire Lights - if this speeds up the rendering significantly then you can experiment with the options in the Fire Lights section.
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When rendering with V-Ray GPU, the Fire Lights options are ignored entirely. Global Illumination should be enabled from the V-Ray Settings if you need the Smoke and/or the scene to be illuminated by the fire.
