Is my object scale too small?

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kossio
Posts: 10
Joined: Tue Jul 18, 2017 7:48 pm

Is my object scale too small?

Post by kossio » Tue Nov 07, 2017 7:54 pm

Hi everyone ^_^

I'm experiencing some jittering and unsmooth movement, which as far as I can tell nobody else experiences. While trying to suspect something, I suspect that my world scale is too small.

I use all defaults for all settings. I mentally use ~meters with my character height 1.f, almost all objects that I have are in [0.5f;2.f] on their smallest axes (some are bigger only few are smaller)?

So yeah, is this scale too small? If so, how big is enough?

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drleviathan
Posts: 387
Joined: Tue Sep 30, 2014 6:03 pm
Location: San Francisco

Re: Is my object scale too small?

Post by drleviathan » Wed Nov 08, 2017 4:57 pm

Convex shapes in Bullet have a default collision margin of 0.04m, so when you get within a few multiples of that you can start to have problems where the shapes are actually smoother/rounder than they look, or you can have visible gaps between stacked objects. The extra roundness can make some objects more likely to tip over (the resting footprint may be much smaller than it appears).

I've seen objects behave in the way you describe when their inertia tensors are too small for their mass/shape: torques from side collisions can rotate the objects too fast. You may gain stability by artificially increasing inertia tensors, but do this too much and your objects may become too hard to roll or will slide without rolling: the friction force does not provide enough torque to spin the objects to match their linear motion.

You can also gain stability by reducing the restitution (bounciness) of the colliding objects.

You can increase angular damping. This is a non-physical effect, but can sometimes be tuned higher without introducing too many other problems.

Finally, you can reduce jittering by reducing the substep period (taking more simulation steps per second).

kossio
Posts: 10
Joined: Tue Jul 18, 2017 7:48 pm

Re: Is my object scale too small?

Post by kossio » Wed Nov 08, 2017 8:53 pm

Thanks again for the help!
I do not have that small objects. This jitters and non-smooth movement is something that appears rarely, randomly (as far as I can tell), and for small period of time.
Otherwise I do 3 substeps per frame assuming 60 FPS. I kind of feel like this is enough but I really have no arguments to defend it. I'm not sure how I should pick that value.

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