View Full Version : Questions about Bullete Dynamics

12-07-2015, 08:21 AM
LW 11.6 please,

1)Are those simulations reversible? can I have a piece of cloth to go back to its original flat form?

2)Is it possible to bake motions to keyframes?

More questions are yet to come later.

12-07-2015, 08:33 AM
You should be able to save out the simulation as an MDD, then reverse the baked animation once it's re-applied to the object

12-07-2015, 09:46 AM
But there is no reverse button under mmd loader (again in 11.6 ).

12-07-2015, 10:03 AM
Add your MDD via Node displacement. You can set the speed to -100 percent or ping pong! Should work like a charm.

12-07-2015, 10:13 AM
Just set your Frame Offset to the number of frames in your sim/MDD and use -100% as your Replay Speed. For example, if your MDD is 120 frames long, enter 120 as your Offset--otherwise, the animation will play backwards from frame zero into negative frames.


12-07-2015, 10:21 AM
Thanks G... I was working from a foggy memory. :)

12-07-2015, 10:38 AM
Oh, I was just mentioning an alt.

Either way works fine of course. Naturally, nodal can offer more options. :)

12-07-2015, 12:48 PM
I got it, should I later adjust the position manually? The object gets translated (along y axis in my scene).

I also thought after baking I should see the keyframes in timeline, nothing appears there. I am just curious to find out, it is not critically important for now.

12-07-2015, 01:20 PM
I you want to change the position (orientation or scaling), you need to switch the MDD player tool you're using from World to Local space. (Some MDD players use different terminology but they all have this feature.) When you switch to Local, you can also add animation to it and parent it to other objects. Leave it on World when you absolutely don't want anything to affect the position.

Also, when you're retiming the MDD motion, note that some players use frames and some use seconds so you may need to do some conversion. Not a big deal, just an annoyance sometimes.

Hope this helps.

12-08-2015, 05:58 AM
Yes, now I totally got it, I was going to ask about possibility of parenting but you already answered that as setting to local makes it possible.

Just out of curiosity, is reversing mdd (specifically for bullete) sth rare in your pipeline or you use it pretty regularly? I saw a c4d behind the scene stuff and there were lots of things in there which could be used by this technique, I am not sure if the c4d user was deploying this approach but I can replicate by reversing simulation.

12-08-2015, 10:08 AM
I've used backward running MDD in some fx work the same way I might reverse live footage. Sometimes it's easier pull something away from a target and play it backwards than it is to hit a specific target in forward motion.

Of course, then you want to minimize the physics looking weird--unless of course weirdness is what you want. Not a CG example but I'm reminded of the scene in Coppola's Dracula when Harker enters the castle. As he steps over the threshold, something looks really odd, like he's entered a world where the laws of nature have changed. Most people don't realize it but the footage is actually running backwards for no other reason than to make it look unnatural.

In my own dopey little short film 'Hello Frankenstein', when platform is lowered onto the stand, that's backwards running footage. I did that because it was easier to raise the platform off the stand than it was to lower and rest it perfectly onto the stand.

Trying to think of a cg example. I know I've used backwards running Bullet or Flocking in a film a couple of years ago but I've forgotten exactly what that was. Ugh...I should cut a new reel because I can't remember anything I've worked on anymore. :question:


12-10-2015, 02:09 PM
1.18 of the following link,


this might be very difficult by other techniques than just playing simulation backward.

One more question please,
It seems Bullete does not recognize morphing when the object is set to deforming, am I right? perhaps there is a trick for doing it.

12-12-2015, 12:55 PM
Did you take a look at the video? I would like to get your opinion on this please.

12-12-2015, 01:20 PM

Is this the area of the video you were talking about? I think I'm lost on the question. If you build the shape, then run a sim having it "blow" away, then reverse that as a cache you can get something very similar. As far as using morphs... you'd have to apply the morph to the cached animation. I don't think bullet will see the morph. Hope this helps! Good luck

12-15-2015, 08:19 AM
Thanks mummyman, Thanks G, I learned a lot during this discussion but I could not do morph. maybe it is fixed in 2015 that mummyman mentioned procedure for it.

Would you please help me a little more on this (if you find free time of course).

12-15-2015, 02:55 PM
I fiddled around with this a little bit during lunch today. I found that Bullet does recognize morphing during collision but you have to set the morphing object to Deforming, stiffen up all the settings and give the object a lot of mass so it can push the other objects.

That said, I got some funny results. My setup was an expanding object that was supposed to push a ball off a table, and the expansion was done using a morph. The expanding object did push the ball off the table but it also exploded shortly after doing so. I'll have to look at this scene again tonight to see what's going wrong. (I think my expanding object might have started off penetrating the table object.)

Even though the actual morphing collision was working, I think I'd still want to bake the morph to MDD before running the sim. Just a theory but I think the object may hold it's shape better this way. I'll run a comparison tonight to see if this makes any sense.

Of course, if this particular test was an actual job, I would just match move a hidden rigid Kinematic object to follow the movement of the morphing object and be done with it. This 'invisible hand of God' approach is generally a lot easier, faster than deforming collision, and the results can be more predictable.


12-15-2015, 05:58 PM
Can this help? https://www.youtube.com/watch?v=gLtuK99R4Bo Not sure if I'm still understanding what you're trying to do for the effect you're doing with bullet. But if you have a low rez object... then run the simulation on that low rez object. You can save out the .MDD file. Reapply it back to the low rez object in a fresh scene. Parent the high rez object to the low rez object. The high rez object has a morph target on it. You should be able to add Metalink and metalink morph and have it do both. Or... are you trying to use the morph target inside the physics calculation? Can you outline your process Farhad?

12-18-2015, 09:01 AM
Sorry mummyman for late answer, I missed your post,

Actually I asked this out of curiosity, my main concern was reversing simulation which I am totally happy now after your help (and Greenlaw's) and I just wanted to ask other questions about bullete in very post instead of making entirely new thread later.

Please give me some time so that I can experiment a little more having your replies in my mind and then maybe we can make more progress.

I say it again that reversing baked information is a great thing and I am totally satisfied. Thank you.

12-18-2015, 10:15 AM
Excellent! Good luck