What is "Offset" in the fluid simulation "Bake" settings?

Question

I've only just recently started using blender (this is 2.78a) and am amazed at how much can be accomplished with it. I have been playing around with the fluid simulation, but all the different settings are a bit of a mystery, so I have a few questions, but I will start here with just one, and post others separately.

In trying to get multiple fluids to interact, I found that unfortunately, it is not possible, and the best one can get is: How to bake multiple fluid simulations in one scene?. However, in that answer, William Hopper mentions that to delay the start of the sim, one must change the "offset", but isn't that what "Time: Start" is for? What does each of these settings do?

My natural guess form the tooltip "Offset when reading baked cache" is so that one can have multiple baked domains in the same path, without having to do that workaround mentioned in the answer linked above.

Answer 1

This is pulled from another link on baking.

Simulate objects falling into water

original answer by gandalf3♦

The Offset just "offsets" the frames that are being read from the cache.

Time: Note that baking always starts on frame 1, these settings control the behavior of the fluid (e.g. for slow motion effects)

Start: The number of seconds into the simulation to start caching data.

End: The simulation time of the scene End frame in seconds.

Offset: Number of frames to offset reading of the cache. This is the only setting which you can still change after baking.

Answer 2

Answer valid for version 3.1

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There are 3 parameters for caching the fluid domain object in version 3.1:

• Cache parameters:

  

  These parameters are additional to Start and End defining the frame range for the animation, found in the Output tab of the properties (and duplicated on the timeline).

• For information, the frame range:

  

Two of the cache parameters, Start and End, are used when precomputing the result and storing it into the cache for accelerated previewing and rendering (bake is the meaningless wording in Blender's parlance for more meaningful precomputing in common language). These parameters indicate which section of the animation is to be be precomputed and cached.

The third parameter, Offset, is used when the cache is recalled for previewing or rendering the animation. When zero, the animation is played/rendered with the frames cached without alteration. When non zero, frame n in the cache is rendered as frame n+offset. Said otherwise when Blender wants to render frame n, it looks for frame n-offset in the cache.

With default parameters Start=1, End=250, Offset=0, the operations take place like this:

With customized parameters Start=100, End=150, Offset=100, the operations take place like this:

Caching is done on the selected animation frame segment 100-150. 50+1 frames are cached with their original frame number. Note that the selected range must exist, else it is adjusted by removing inexistent frames (as defined in Output tab).

Rendering is done by using the cached range 100-150 for rendered frames 200-250, using the offset. There is no framerate adjustment, neither when caching, neither when rendering.

As for what is the use of Offset in practical? Well I don't know. I don't see any case where you would want to render frames at another time than the original one. If you need time shifting, you can use the video editor which has dedicated options to time shift clips.