Active Body Objects Slide As Soon As Sim Starts

[stelees]

Hi,
I have 4 small gengons, sitting on top of where I have a water flow ready to go. I have added the base object as static to the active body, I have added the 4 other objects (not static). I have set them to be 8kg in weight (they are small rocks). As soon as I press start on the sim, the 4 objects start to move forward. I thought it was the motion velocity effect in the FD Properties for the object so I set that to 0.0, no change they still move. It is like they are being hit with the fluids before they are actually hit with the fluids. I have wiped out the cache to make sure there is no legacy anything impacting this, re-added the active body also just in incase, The active body is connected to the solver correctly, just not sure why there is initial motion in these objects.

[georgi.zhekov]

Hey,

Does it help if you increase the interaction steps in the Active Body solver and if you increase the Friction with Bodies property in the Phoenix properties of the Active Bodies?

If you can't see the Friction with Bodies option, make sure to get a latest nightly build.

[stelees]

ok I replicated it. The attached scene has three gengons with their vertex snapped together, ActiveBody steps 20 and the friction with bodies set to 1. When you start the sim, the slide. If the gengons are moved apart from each other they don't slide and interact with the fluids correctly. There is an issue when the active body objects are aligned with each other.

[kaloyan.darmonev]

Hello
I ran the attached scene with latest nightly (Feb 24 2021), and got this result, which is different from what you have:
https://drive.google.com/file/d/1_ZJ...ew?usp=sharing

The gengons move very slightly apart in the first few frames (not noticeable in the preview), then they stop and are carried off by the water
Which version of Phoenix are you currently running? If you update to the latest nightly, do you still have this issue?

[stelees]

I have the latest build each night when they are released. When the gengons are vertex snapped together and the sim starts they do sort of jump apart from each other.. but when these are meant to be in a stack with other static objects and the water will just push them apart, any sort of movement ruins the illusion of the water busting through an 'aligned element'.
I have tried friction up and down, the steps up to 50, increased the steps in the sim to 12 and the objects moving is still the same. I was expecting that when the sim starts, they are static. The below images are from 0 and frame 1, I was hoping the objects stay as they are in frame 0

[kaloyan.darmonev]

Hello
This sliding of the gengons happens because each collision shape has a small margin around it to account for situations with fast moving bodies. We're adding a "collision margin" parameter to the Acitve Body Solver to help in a situation like this - by setting it to very a low value, the gengons won't move at all. It should be out in the nightlies in 1-2 days.

[kaloyan.darmonev]

Hello
The "collision margin" parameter of the Active Body Solver is in today's nightlies. Setting it to around the minimum value in the attached scene stops the bodies from sliding away from each other.