Retiming OpenVDB Sequence

[justin.marimon]

This may end up a feature request, but I figured I'd ask it as a question first.

Setup: I'm using X-Particles and its built-in Explosia FX to simulate/cache smoke as OpenVDB sequences. At this point, C4DtoA is unable to directly render Explosia generators as a volume. In order to render them, you need to save out an X-Particles cache and specifically set ExplosiaFX to cache as an OpenVDB sequence. You're then able to load the sequence using an Arnold Volume and render it using the channels that were cached.

Question: One newer feature in X-Particles is the ability to retime the playback of X-Particles caches. You can use this to easily speed up/slow down your cache for slow-motion or ramping effects. Believe it or not, this feature also works to retime the oVDB sequence and it seamlessly interpolates and displays the additional frames.

Using C4DtoA, is there any way to retime and render my oVDB sequence to match the functionality of Explosia? I'm creating a scene with both volumes and particles I'd like to speed-ramp.

Attempt: Using the existing Arnold Volume attributes, I'm able to partially get what I want, without the interpolated frames.

I created a smoke sim and cached 72 frames. In Arnold Volume, I'm able to keyframe the "End Frame" attribute from 0 to 72 and the "Loop" setting to freeze. This allows me to control the playback of existing frames, even slowing them down. Unfortunately, this does not match Explosia's ability to create the in-between frames ("End Frame" is locked as an integer input).

I've uploaded my test scene here: (may need to relink Arnold Volume) https://drive.google.com/file/d/1aXYaBx4VgVrowfxqDYcw2S0rG1o39tmz/view?usp=sharing

Feature Request (?): I would love it if the Arnold Volume object included the ability to use float values to advance frames or subframes of a sequence. I'm assuming the ability to interpolate between two VDBs is non-trivial, but perhaps the ability is built into the oVDB spec?

Alternatively, I'd love it if I could attach an Arnold Tag to an Explosia object to directly render as a volume. Might be asking for too much, but the ability to render Explosia directly without creating the VDB cache would be amazing.

[peter.horvath6V6K3]

Right, currently you can only change the playback of existing frames as you described, but no interpolated subframes are supported.

I'd like to have direct support for Explosia, but currently it has no API as far as I know.