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Mode | animmode – Chooses between different options for animation control:

Linear – This is the default mode. The cache sequence is played with a constant speed and can be offset forward or backward on the timeline, as well as sped up or slowed down.
Cache Index – The Direct Cache Index specifies which cache file will be loaded for the current timeline frame. Can be used to either show a static simulation, or the Direct Cache Index can be animated in case you want varying play speed, including playing the simulation backwards.
Loop – A specified piece of the simulated sequence is looped. Can be used for flowing and repeated effects such as fireplaces, campfires or torch fires, water in fountains, waterfalls or boiling liquid. In this mode, the Cache Origin parameter specifies the beginning of the looped sequence, the Length parameter specifies the length of the loop, and Loop Overlap specifies the number of overlapped frames that ensure smooth transition between the end and the start of the loop. Note that you need to have simulated at least Cache Origin + Length + Loop Overlap cached frames for this mode to work correctly. When looping particles, make sure to export the particle ID channel in the Output rollout.

Direct Cache Index | t2f – Used in Cache Index mode and specifies the cache file index for the current timeline frame. This can be animated in order to achieve more interesting time-bend effects.

Timeline Origin | playat – An offset specifying which timeline frame the starting cache will be placed on. You can think of it this way: "Put cache #(Cache Origin) on timeline frame #(Timeline Origin)".

Cache Origin | inpoffset – An offset specifying which cache file from the sequence will be placed on the timeline at frame Timeline Origin. You can think of it this way: "Put cache #(Cache Origin) on timeline frame #(Timeline Origin)".

Grid Blend | frmblend  – Used for grids when the Play Speed parameter is not exactly 1.0, or the Direct Cache Index for the current timeline frame is fractional, or you have specified a Loop Overlap in Loop mode. In these cases, a single timeline frame must be constructed from two cache files by blending between them. Each time the timeline is scrolled to a new frame, the caches for this frame will be blended again. You can choose between three different methods for blending between grids from cache files. During Resimulation the Grid Blend option will be ignored and the blending will be handled by the resimulation process. Note that particles are blended using a much simpler method and Grid Blend does not affect them:

Interpolation – Simple linear interpolation suitable for slow simulations. This is the fastest method but it does not capture movement well and may produce flickering.

Velocity – Velocity-based interpolation. Produces better results, but works more slowly. Captures well the movement of the fronts of the plumes, but does not work well for smoke moving backwards, and also may produce flickering. It  requires an exported grid velocity channel from the Output rollout.

Precise Tracing - Improved Velocity based interpolation for Fire and Smoke simulations. Captures plume movement very well and can handle very low Play Speeds. Requires an exported grid Velocity channel, as well as Advection Origin channel from the Output rollout.

Frame Blending is better suited for simulations without much variety in velocity. For more chaotic simulations, it is better to run a Resimulation using Time Bend controls. Time Bend Resimulation will calculate a better intermediate result for each frame and store it in new cache files that can later be loaded faster, as opposed to Frame Blending which re-launches every time the timeline frame changes. However, for very slow moving simulation the Precise Tracing method produces better looking results than Time Bend Resimulation. For more information, see How to slow down a simulation, animate the time-scale, etc.