Help Objects placed on top of Obi Cloth keep rotating

[LuckyPretzel]

Hi!

Depending on the mass ratio between the cloth and the objects resting on top of it, it will sag more (heavier objects) or less (lighter objects). Note that the only important thing is the mass ratio, that means behavior will be identical for a 1 kg cloth and 2 kg objects and for 10 kg cloth and 20 kg objects, since 1/2 == 10/20 == 0.5.

However, in the absence of any objects on top, the cloth it will behave the same regardless of its mass. Mass doesn't have any impact on the internal dynamics of the cloth, neither in a simulation or the real world. This is because mass has zero effect on accelerations: since gravity is an acceleration objects fall at the exact same speed regardless of their mass. Mass only has an effect on forces.


This should be the default behavior of cloth, what are you getting in Obi instead?


This would make no difference, since using seams basically joins two different cloth actors into one.


Stretchiness is limited as much as possible - depends on how many substeps you're using- by default, creating slack in the model itself doesn't have any advantage.


Internally all 3D engines use triangles for both rendering and physics. A square (quad) is just 2 triangles, so it makes no difference at all. The only advantage of using triangles is that you can control the direction of the diagonal in quads when modeling, which is sometimes useful when doing low-poly models.


My advice would be to always use real-world units for distances, forces, mass ratios and accelerations. This is specially important for distances, since larger objects appear to move slower (since the distance they travel per second at the same speed is smaller compared to their size). There's a huge difference in simulating the exact same scene at a 10 cm scale and a 10 m scale.

Also use as many substeps as you can get away with given your performance budget (will find them in your ObiFixedUpdater component), and adjust distance and bending constraints depending on the results you're after. For instance, you may want to allow distance constraints to compress a bit by setting max compression > 0, and allow for more bending.

kind regards,

Thanks for the quick response. I'll spend some time making the adjustments you suggested and update my results here (good or bad). I figured sizing might be an issue and would impact the overall physics. This is why I tried tackling this small subset before building out the entirety of the game and had to scale up on everything, which could be a bit troublesome. Thanks again!