[Bf-committers] questions about particles and collisions

[David Jeske]

Quick update.

I have particle / emitter self-collision working, at least in my quick
test. Here is a test showing an emitter surface with both a collision
modifier and a particle system.

  https://www.youtube.com/watch?v=2WUeFGY4Rlg&feature=youtu.be

What I did was introduce a new particle state during the beginning of a
particle lifetime, PARS_BIRTHING, during which it does not collide. They
switch to PARS_ALIVE as soon as they are not colliding with anything. This
prevents particles from colliding with their own emitter while they are
spawning. Kill particles now also works correctly.


https://github.com/jeske/blender_hacking/commit/d9a96cbcf3915eda1e9b4b49437539fd52adf689

At this point, I'm doing more testing, and trying to figure out if this
unintentionally breaks anything. It seemed a rather easy change to make, so
I'm suspicious that there is a deeper reason particle/emitter
self-collision is disabled.

Can anyone think of a reason this is not a good thing to do?

Here is a little more background...

My new BIRTHING state ignores collisions with any object, not just the
emitter. I struggled to think of a scenario where this is bad, since
birthing a particle into a collision state seems both highly unlikely, and
might cause unwanted instability. Can anyone can think of a reason this is
generally bad, and why BIRTHING should be restricted to only ignoring
collisions with the emitter? If so it could be changed to do that instead
(the code for this will just be a more complicated).

My patch appears to work whether the collision modifier is before or after
the particle system, though I didn't yet test for subtlety. As a user, I
think I would expect the collision modifier before the particle system to
mean particles only collide with the emitter, and after the particle system
to mean the particles collide with the emitter and other particle system
particles. I don't know how feasible this is.

Interested in any feedback.




On Mon, May 30, 2016 at 11:31 AM, David Jeske <davidj at gmail.com> wrote:

> I am trying to understand why particles (emitter and hair) ignore
> collisions with their emitter objects, even if the emitter has a collision
> modifier. (because this is very inconvenient)
>
> I'm starting with emitter particles, because the situation there is
> simpler and uncovers some of the challenges.
>
> It seems like at least at some point someone thought particle-to-emitter
> collisions should work, as evidenced by this code in particle_system.c,
> collision_detect(). It is trying to prevent particles from colliding with
> their emitter too early in their lifetime. (afaik this code currently does
> nothing, because the emitter will never appear in the collision list, see
> next)
>
>    for (coll = colliders->first; coll; coll=coll->next) {
>        ...
>        /* particles should not collide with emitter at birth */
>        if (coll->ob == col->emitter && pa->time < col->cfra && pa->time >=
> col->old_cfra)
>             continue;
>
> I can see that normal emitter particles ignore collisions with their own
> emitter object because of this code-line in
> particle_system.c:dynamics_step(). The second parameter (sim->ob) causes it
> to skip itself when constructing the collider list.
>
>    sim->colliders = get_collider_cache(sim->scene, sim->ob, NULL);
>
> If I change the second parameter to NULL, it (may) include itself in the
> collision list (if it has a collision modifier)
>
>    sim->colliders = get_collider_cache(sim->scene, NULL, NULL);
>
> Then placing a collision modifier on a particle emitter seems to cause the
> particles to collide with their own emitter -- although there are some
> issues.
>
> One issue is that some particles appear to collide with the emitter at
> birth (they emit opposite the normal), suggesting the first code-snippet is
> insufficient to prevent this problem.
>
> Another problem, is that if the emitter-collision-modifier is set to kill
> particles, the first particle it kills seems to kill the entire particle
> system. (see [1] below)
>
>
> Are these the only two issues, or are there other problems?
>
>
> I'm trying to figure out if this is reasonably fixable, or if it is just a
> "bad idea" to pull on this thread. For example, is there something about
> particles colliding with their own emitter that violates some expectation
> of the modifier stack? Or is this simply a matter of coming up with
> appropriate solutions to the odd cases and data interactions so it works
> properly?
>
> ---
>
> [1] On a related note, at ...
> https://wiki.blender.org/index.php/Dev:Source/Physics/Collision_Modifier
>
> It says:
>
>      The collision modifier safes the position of the vertices
>     of the object at the corresponding position on the modifier
>     stack. It does this for the current and the last position
>     using GLOBAL coordinates. The data is only valid as long as
>
>       * you don't skip ahead/back several frames
>             [TODO: should be enhanced later]
>       * the number of vertices don't change on the
>         modifier stack.
>
>
> Is this related? Does the particle death kill the collision 'safe' (seems
> like it would, though i don't understand why this would kill the particle
> system)? Does a vertex-count-change invalidate the collision modifer 'safe'
> even when verticies are added "after" the collision modifier in the
> modifier stack?
>
>