Tim Nikias v2.0 wrote:

> But you surely have to calculate where the particle hits the object, don't
> you?  

Yes, of course. I do the regular time stepping as everybody. First there 
is the "main loop" for each particle that divides each frame according 
to the speed and size of the particle. This loop moves the particles and 
checks if there was a collision. If there was it moves to a "sub loop" 
which has smaller time steps. So I do find the moment when the particle 
collides, but it is not mathematically 100% accurate.

> Either I'm missing something here, or you're missing some really big 
> issue. Just take the following: A particle flying with 20mph. It'll
> hit a wall in 50 seconds, but you jump and calculate 2 minutes. So,
> instead of rebouncing off the surface, you just take the exisiting
> position and reverse the direction of velocity, so it'll fly another
> direction. Maybe it'll hit another surface there to, so you reverse
> the velocity once again...

Basically yes, but the timescale will be different because of adaptive 
stepping. During a collision a particle is moved allways only a fraction 
of its radius every step. So the accuracy should be adequate.

> That it should have actually dropped a few meters until it hit the
> first wall is left out?

No, that is why there is the point 2.b (and 1.) which is done at every 
collision.

>>By "lucky" I mean that there seems to be no bugs and everything seems to
>>work as expected after I implemented 2.b and 2.c. BTW, have you tried
>>how Cristophs particle system/MegaPov handles rolling/sliding? Just
>>curious. At least it is a lot faster as it is hard coded.
> 
> 
> Never looked at it, but just ask Christoph, he'll surely know. :-)

Well, maybe I'll try it myself if Christoph doesn't show up automagically :)

> How does accuracy decreas when using a ray per object or one ray total per
> union? Internally, I think POV-Ray just shoots the rays at the different
> union-components, but returns only the closest hit. It's faster than doing
> that yourself, and the comparison in SDL might be less accurate than what
> POV-Ray is capable of internally, but surely not that much?

I meant that the accuracy decreases as you can't shoot the ray allways 
directly to the nearest point in the environment. You have to "guess". 
If you knew the shape of each object it might be possible to shoot the 
ray to the nearest point (as I was doing with the planes).

> Since you're now shooting a ray at every plane individually, I expect you're
> shooting them parallel to the plane's surface-normals, and thus maintain
> that a particle won't intersect a plane because you know it's radius and can
> then easily move it along the normal so that it won't intersect the plane.
> This technique will run into problems with other objects, where the closest
> point to the object isn't found with some arbitrary normal that easily.

I will change to using a few well chosen ray directions (like speed and 
gravity or something between them) and test the system with different 
kinds of environment objects. So, more animations to follow!

Severi

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