Hi all,

I'm still pretty new to Houdini, but I offered to do my friend a favor by attempting to sim a spaceship crash for his short sci film. It was more of an excuse to learn RBDs in Houdini than anything else, but now that I've invested this much time I really want to make it work.

Anyhow, I started with Steven Knipping's Applied Houdini Rigids III Vehicle Destruction course and once I finished that, I tried replacing the supplied helicopter model with a spaceship that I modeled in C4D using a Kitbash kit, so the geometry shouldn't be an issue. It would appear that I'm very close to getting a workable sim because I got it crashing and denting with the original animation that I set up for the ship. However, I've been running into a bit of a snag if I change the crash animation.

For whatever reason, if I move the ship at all to match any of the tracked alembic cameras I was sent, either by changing the keyframes of the crash itself, or even by simply attaching the entire animation to another null and moving it in the scene, the ship totally blows up. You can see what I mean in the last 3 previews found in VFTV\render of the attached project folder. Maybe I'm missing something very basic, but as far as I can tell this shouldn't be happening, as my understanding is that the hard constraints I'm using on most of the ship aren't supposed to break unless I tell them to in the art directed cuts section of the frac SOP.

Anyway, the first preview in the VFTV\render folder shows that the sim more or less works with the camera matching but without denting, and the second shows the crash with denting but with no camera matching. But, for whatever reason I couldn't get the source tracked cameras working directly in Houdini, so I took them into C4D first, changed the resolution and alembic camera scale, and exported them at .01cm from C4D, and after that they seemed to line up in Houdini once I changed the camera view resolution to match. However, my troubleshooting brain is telling me that I might've messed something up during this process and perhaps the explosion is due to mismatching scene/camera scale issues with the alembic cameras that the tracking artist sent causing the sim to be calculated differently.

I'm not even sure if the C4D camera scale option affects the sim in Houdini and/or C4D, but scene scale certainly does, and if the imported camera scale is messing with scene scale, then maybe that's the problem. That said, I don't think that's the reason, because if I leave the original animation alone it seems to sim fine even with the new cameras. And, I believe I'm matching the scale correctly and using the right conversion which is supposed to be .01cm from C4D to Houdini.

I've also messed around with exporting different camera scales and scene scales out of C4D and importing the alembics back into Houdini using several different methods, all with the same result. But, I realize it might help me to know more about which software each shot was tracked in, so I'm double checking with the guy at Framestore, who did the tracks, regarding the scene scale conversions. I was thinking Syntheyes might be different from Nuke for example.

That said, I would be eternally grateful if anyone was willing to look at the project and let me know what they think the problem could be. My hope is it's something stupid on my part, and not some overly complex issue.

Here's a Dropbox link to a reduced version of the project with all the assets
[www.dropbox.com]

Regards and thanks a lot,

BenjaminMarkus
eternally grateful

I have yet to check out the Applied Houdini courses, and only had a quick glance at your scene file, but could it be you are messing with transformations at the wrong level and the constraints don't follow?

Also that camera workflow sounds.. messy.

Whoa! Thanks for looking at this one too. Didn't think anyone was gonna get back to me regarding this. Well the set up is pretty much the same as Steven's so I'm almost positive it doesn't have anything to do with the order of operations. I haven't changed any of that. I have however made some progress with a different camera and backplate, so it's no longer exploding, but I'm still not getting the kind of denting I want, and the ship still seems to be moving too fast despite it being much bigger in scale than the helicopter. I'll upload some flipbooks when I get a chance.