In article <3CE2C221.AD4ABAFA@gmx.de>, chr### [at] gmxde says...

> You have to take into account that due to buoyancy the influence of
> gravity is more than compensated under water.

Thats why they don't sink permanently... but nevertheless they should 
slow down much faster, since the (parmanent) friction of a moving ball 
under water should be much higher then the one of an elastic ball 
bouncing on the ground - unless it is not water but some exotic liquid 
with extremely low friction (like liquid helium).

I looked at the animation again, and it seems the balls are suddently 
accelerating when they go through the water surface from inside the water 
to the outside. A bug perhaps?

Lutz-Peter

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"Christoph Hormann" <chr### [at] gmxde> wrote in message
news:3CE2B6C2.16B9C494@gmx.de...
>>     1) hits the bottom: suggesting heavy density
> No, just high kinetic energy.
>

Acquired from a fall from such a small height??

What I meant before is if you're using textbook values for forces, but then
applying the forces frame by frame instead of some kind of integration over
time, then you will not mirror reality, and you'll have to change your
values from those in the textbook.

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"Greg M. Johnson" wrote:
> 
> [...]
> 
> What I meant before is if you're using textbook values for forces, but then
> applying the forces frame by frame instead of some kind of integration over
> time, then you will not mirror reality, and you'll have to change your
> values from those in the textbook.

I am fairly sure the calculations are quite correct mathematically.  Just
to clarify, i'm using 33 integration steps per frame with 0.00111 seconds
per step wich should give real time results with 30 fps.

I made another version showing better the lack of surface tension and how
drag in water works, this time without the water movement.  The balls are
only slightly heavier than water.

http://www-public.tu-bs.de:8080/~y0013390/files/ball_sim4.mpg

Christoph

-- 
POV-Ray tutorials, IsoWood include,                 
TransSkin and more: http://www.tu-bs.de/~y0013390/  
Last updated 05 May. 2002 _____./\/^>_*_<^\/\.______

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Excellent algorithm, cool realistic setup, but I guess I'm still bothered by
how the balls move.

How about a bit deeper water and all ***stationary*** at frame 0?

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I was amazed when i saw this, but the i saw a small mistake. When the balls
hit the water (or are in the water), the surface of the water should raise.
For as far as i can see it doesn't happen. The rest of the animation looks
realy good.


"Christoph Hormann" <chr### [at] gmxde> wrote in message
news:3CE291FC.E1C0909C@gmx.de...
>
> I have improved the model, fixed several errors and tried different
> configurations.
>
> Here are two results, the first is with medium heavy balls (2000 kg/m^3)
> and contains a flaw with the water.  The first ball hitting the water at
> fairly high speed does not leave a trace at all. ;-) This is fixed
> meanwhile.
>
> The second one is with balls lighter than water (800 kg/m^3), the balls
> floating leads to quite some problems as you can see.  The water
> disturbance, although speed dependant meanwhile, is still too strong and
> there is no good representation for water surface tension.
>
> Does anyone know a good method for modeling water surface tension? I right
> now only use a simple model that slows down the balls according to the
> energy required for increasing the (flat) surface area, but there probably
> are other important effects that are not covered by this.
>
> Apart from that i added realistic calculation of drag in water and air, i
> also modeled friction during the environment collisions, but without
> rotation this does not really improve realism much.  And the animations
> are in real time now.
>
> Christoph
>
> --
> POV-Ray tutorials, IsoWood include,
> TransSkin and more: http://www.tu-bs.de/~y0013390/
> Last updated 05 May. 2002 _____./\/^>_*_<^\/\.______

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