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TOPIC: Cloth panel sizing and collision – or why there is a gap in my stitched cloth?

Cloth panel sizing and collision – or why there is a gap in my stitched cloth? 2 months 3 weeks ago #240

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Cloth panel sizing and collision – or why there is a gap in my stitched cloth?

First it’s important to recognize that the mesh you provide Carbon for simulation is only a proxy of the cloth that actually gets simulated, because cloth has thickness !

Cloths are automatically treated as double sided collision objects and the thickness of the cloth is 2 x the fatness of the outer collision. Also, that fatness is added to the open edges of the cloths as well. Here is a diagram from the Carbon documentation.




So, any open edges of a panel will be extended by the outer fatness.
If you want to maintain an exact collision size for a panel you should reduce the panel size on its open edges by the amount of the Outer Fatness (50% of the thickness).

WARNING: When you fuse panels in Tailor Mode, it is ONLY the edges that are left open in the final garment that have fatness. All the panel edges that are fused are not impacted by this. So, it is only the open edges in the final garment that should be trimmed. Edges that you stitch together with Carbon Binding and Carbon Stitching/ Seaming are also open edges in this context.

So, if you visualize the mesh of two panels stitched together you see this gap between the panels.



But if you turn on the Carbon physics draw, which includes the thickness (fatness) in the rendering, then you see this – no gap.



Note that this visualisation is much clearer in Carbon Cloth for Maya than in Carbon for Houdini.
(we will be trying to improve the Carbon visualisation in Houdini in the next major release).

As you can see in the diagram above, the fatness is rounded at the ends of the cloth. If you have a very thick cloth this roundness will be large and can cause an inaccurate representation of the cloth surface for collision. This is not a problem for visualization because you can use the simulation output to deform another mesh that does not have rounded ends. It’s just an issue if you are colliding those fat ends in a way that is impacted by the roundness.

If you need to accurately collide the edges /corners of a fat material then you should consider using a layer of tetrahedrons to represent the material. Tetrahedrons benefit from supporting single sided collision. This means that fatness and thickness are not directly related. There is Outer and Inner Fatness and most of the fatness can be Inner Fatness. This means you can have a very small Outer Fatness and hence a much more representative collision surface, which much smaller rounded corners. Here is a diagram of a single sided object.



You can in fact have no outer fatness if you wish, though a small amount is recommended, or under tough simulation conditions you might get collision issues.

Below are four tet panels 5mm thick (10cm x 10cm each) that have been tightly bound together with zero outer fatness. The red lines are the boundary between the panels and the orange dots are the binding points. (Note that tets are only supported in Carbon for Houdini as of July 2019.


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