Awesome! Nice job on simplifying the code. I really like what you have done here.

The only thing I have except that you should add versioning code (as you stated), is that we should perhaps convert dampening and friction to percentage variables.
(So PROP_PERCENTAGE instead of PROP_NONE)

So then in the UI dampening and friction will go from 0% to 100%.

It was very weird before that it was capped at 80 without any mention in unit.

If it is percentages I think it would be more clear.

Or hmm... Maybe just convert the friction variable to be PROP_FACTOR and go from 0 to 1.
Then you don't have to do the 0.01 multiplication later (and it is consistent with the other collision properties like this.

Do you have any performance numbers?

To me it doesn't really matter that much, I'm interested to see how it compares to the old code.

This patch should be tested on more complicated files, has that been done?

A regular grid on a plane is not representative of difficult collision cases.

In D8577#209984, @Brecht Van Lommel (brecht) wrote:

This patch should be tested on more complicated files, has that been done?

A regular grid on a plane is not representative of difficult collision cases.

These are the names of the files I used for testing:
colision_hair.blend: (plane with hair and 2 colliders)
collision_damp_test.blend: (collision between 4 unique triangles)
collision_damp_test_2.blend: (shown in the GIF)
collision_friction.blend: (single triangle sliding into another triangle)
collision_friction_test_2.blend: (shown in the GIF)
collision_intersect.blend: (single triangle intersecting another triangle)
collision_self.blend: (triangle with array modifier and self colision)
collision_tests.blend: (Different tests between 2 triangles)
Flags_test2.blend: (Complex file that tests collision and self collision in high poly geometry)
Flags_test.blend: (A simulation of a flag in the wind. Used for benckmark)

It would be good to put any cloth test files in lib/tests then when this gets committed.

These kinds of tests are useful, but the existing cloth collision algorithm was tested with more complex scenes as well:
https://vimeo.com/200206460
https://vimeo.com/193881479

@Luca Rood (LucaRood), do you still have .blend files used for these videos?

I tried to redo one of the test files shown in the video:

It is very difficult to find a friction value that holds the knot and does not destroy the simulation.
But it seems that this is a limitation in both systems.

I realized that when I tighten the knot, the whole cloth vibrates a lot in the new system.
I know what is causing the vibration, but I'm not sure how to resolve it.
I need to investigate further.

I did the tests and I believe that the new system is working well.
The only problem I see now is versioning.
Changing the range from [0 - 100] to [0 - 1] only works if the property is not animated.

I don't know if there is any utility that updates the F-curves.
Other parts of versioning do not update F-curves.
I wonder if it's really important.

@Brecht Van Lommel (brecht) I talked to Sergey and it seems like updating animated fcurvers will be very cumbersome and involved.
So perhaps we can just skip that case?

I don't think that there will be many files that had these animated as the friction variable didn't really do much even when maxed out before this rework.

It's fine to not update animated values in versioning, we don't do that generally.

Great to see someone working on this, as the currently released code is indeed some kind of borderline incomprehensible mess of patches, hacks, and workarounds. A rewrite is well overdue, so, thanks @Germano Cavalcante (mano-wii)!

But what I can say is that such a change should be extensively tested. It would be really good to see this in some real world use-cases.
Collision response is extremely fiddly, and when I was working on it, despite being actively tested with the Agent 327 project, and later Next Gen, some serious issues still slipped through unnoticed.

I could not find those specific files, @Brecht Van Lommel (brecht), but I can contribute some files that test for specific (corner)cases. But keep in mind that this is by no means an exhaustive test set, so serious real world testing is still absolutely necessary.

A different bow simulation (note that this one should actually properly hold the knot prior to being untied).

Feel free to add these files to Blender's test set.
I hope this helps a bit, and really look forward to seeing more results with this patch!

Thanks for the files @Luca Rood (LucaRood). They will definitely help a lot.

Locally I have already made other changes to the patch.
A change for example is in compute_collision_point_tri_tri that when the triangles intersect it returns now the shortest distance to "repel" the triangle (and not the points of intersection).
One idea that can be useful is to return the repulsion weight for each vertex in compute_collision_point_tri_tri, so that all vertices can be repelled at the same time.

This patch messed up the culling system a bit and needs to be rethought.

@Germano Cavalcante (mano-wii) This new paper about cloth collision might be interesting: https://mmacklin.com/sdfcontact.pdf.

Thanks for the @Tobias Fuchsberger (Kronk) paper.
I've seen it, but I'm not sure if it's applicable in Blender.
I'll take a look with more details later ;)