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TOPIC: What are the main factors that affect the look of the cloth sim?

What are the main factors that affect the look of the cloth sim? 5 months 2 weeks ago #287

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Here are the main factors that affect the look of the cloth sim.

1) Cloth mesh resolution: You are not going to get 10 mm folds with 20mm polygons.
Equally, you should aim to match your garment resolution to the nature of the material, as higher poly garments sim slower. If you have heavy canvas or thick leather, you are not looking for small folds, hence there is no sense in burdening the sim with a high res garment. Note: Changing mesh resolution significantly, will change the cloth behavior.

2) Cloth mesh tessellation / polygon layout: Besides the size of the creases, the tessellation controls where cloth can bend, i.e. if there are long straight lines in the tessellation, this will form a long crease / pleat in the simulated cloth. Hence, you can use the tessellation pattern to influence the look. If you want uniform bending behavior, then use Delaunay triangulated meshes, where possible.

3) Cloth mesh thickness: You will struggle to perceive the right behavior of a heavy canvas with a 0.1mm mesh! However, remember that most garments like jackets are made of layers of cloth and so you need to make a judgement call between a cloth thickness and a garment thickness if you are simulating several layers as one.

4) Physics Draw: Having set up your mesh thickness, you need to enable Physics Draw to see this actual mesh thickness, as its only visible as a Carbon object.

5) Cloth weight: Weight is always in kg/m2 i.e. it’s not affected by scene scale. You should use a real cloth weight in the sim. Maybe you can tweak weight later as part of art direction, if you have aerodynamics enabled and the director wants the cloth to push through the air a little faster / slower.

6) Cloth attributes: It goes without saying that bend / stretch etc. are crucial, but one of the most important attributes is angular plasticity. You will never get the look you want without tweaking angular plasticity. The amount and speed of recovery of a folding cloth, tells our brains a huge amount about the material. Also, in fashion, the anisotropic nature of woven fabric is exploited extensively. Carbon fully supports anisotropic fabric properties and using Carbon Tailor model, you can work just like a tailor.

7) Constraints: For more complex garments, it is important to remember that binding / stitching has a high impact on the look. Think of a suit jacket and about just how many seams and stitches there are to create an equivalent of a skeleton/rigid structure that shapes the material and holds it in place.

8.) Cloth fit: When fitting a character with a garment, try to obey real world rules, i.e. you can't put a size 2 dress on a size 16 character and expect reasonable simulation results. Also, if you are matching a real-world reference garment, double check the actual garment measurements, we find hand stitched garments rarely match the pattern we were given.

9) Static Air: Yes, especially for lighter clothing, air resistance plays a huge part in the look. You won't get silky floating cloth without a Carbon Flow Node and some aerodynamics settings on the cloth.

10) Dynamic Air: If you want that long princess' dress to float across the scene, then you probably want more than static air; you want to have a gentle airflow under the dress to help it float.

11) Simulation frequency: This is FPS x Subdivisions. So, 24 FPS with 10 subdivisions is a 240Hz simulation frequency. If you are trying to capture fast moving cloth behavior, you need at least a simulation frequency 2x the behavior frequency.

Secondly, if you have fast moving cloth / collider, you need a simulation frequency that ensures the movement of the cloth is less than its thickness, during the time of one subdivision. So, if the cloth is 1mm thick it must not move more than 1mm during the time of one subdivision. At 240Hz, the cloth must be moving less than 0.240m/sec.

In practice if you see the cloth passing through the collider / another cloth or through itself (with self-collide enabled) then first fix is to double the number of subdivisions and see if that improves / solves the intersection.

12) Simulation Iterations: Iterations control the convergence of the simulation, so the more iterations the better the convergence towards the solution. In practice, this means that if your cloth contains more triangles / quads, it needs more iterations to converge, for example to enforce the Stretch Extension parameter and prevent the cloth from stretching too much under its own weight in gravity. Also, some parameters such as bend stiffness need more iterations as you crank up the stiffness. So, if changing a Carbon Cloth setting does not seem to have an impact on the simulation result, try increasing the Simulation Iterations. Don’t be afraid to crank up iterations 10x to see if that is the solution and then you can dial them back to what is required for your scene.

13) Scene scale: If your adult character is 1.72 units high then you are using 1 unit = 1 meter and Carbon simulation scale should be set to 1. If its 172 units then you are in cm and Carbon scene scale should be set to 0.01. If you have the wrong scene scale then things are not going to look / sim correctly.

14) Reference mesh: During the sim, the cloth is always trying to get back to the settings of the Reference mesh and so you can use that to direct the cloth look. If the director wants a certain default look to a dress, why not sim that look, so its organic, and then take the desired frame mesh and load it as a Carbon Cloth Reference Mesh.

15) Matching dimensions: If you are trying to replicate a real garment, make sure you have exact measurements. We have been given patterns before and struggled to replicate a real garment, so have asked for the actual garment and found the dimensions are not as per the pattern! Relatively small differences will result in folds in different places etc.

16) Sim resolution vs render resolution: Always remember, you can sim with a lower res mesh and wrap back to a higher res one for rendering.

Final note –

Iterations & Subdivisions Relationship
The number of iterations and the number of subdivisions also combine. For example, imagine a simulation using 10 subdivisions but also needing 50
iterations to converge as desired. The same scene with a setting of 20 subdivisions might only need 20 iterations or less to reach the desired convergence while increasing the accuracy of a problem that struggled to converge.
The computation time in both cases is roughly the same since the physics will execute 500 solver steps with 10 collision detections in the first case and 400 solver steps with 20 collision detections in the second case. It's usually better to set iterations and crank up subdivisions first – say up to 250, before increasing iterations further if required.

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Last edit: by Sebastian.
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