Re: [pygame] Physics module status note, more things...

[Peter Gebauer <peter.gebauer@xxxxxxxxxxxxxxxxxxxxx>]

Hi again, Greg!

I agree that it needs a new attribute, but it's gravity that
makes mass continue down, not the sudden loss of drag.
In our simple simulation a constant drag force applied in opposite to 
gravity should be sufficient, we don't have to take in account all the 
other details of non-solid friction. Naturally, just like in real life
the drag force cannot exceed the force pulling it down, i.e gravity.
A drag force equal to gravity would be considered solid, anything below
make it sink very slow or very fast depending on viscosity.

I'm experimenting with something now that simulates viscosity using the
extra attribute "viscosity" where 0.0 is vacuum and 1.0 is a solid.
What happens in zero gravity is that the drag actually will stop the
bodies at some point given enough time in the netwonian fluid unless other
forces are applied.

I haven't started experimenting with bouyancy yet, that will be done
in a separate step and involves only already existing attributes (mass 
and, normally volume, but for us area), but I'm thinking of adding "density" 
as an extra attribute, I'll get into detail on this once I have viscosity 
working.

The one thing I have not been able to do yet is properly reduce rotation 
velocity, if an edge hits the "water" with enough speed it should roll
on the surface until the drag has reduced it to zero.

There's all sorts of odd circumstances that doesn't have to work. The normal
case is having a huge area with water with a large mass. A smaller
solid object falls into and penetrates the water, but consider the other way 
around;
Small fluid objects hitting a large solid, in particular statics, will seem 
to sink into the solid, this behavior does not work or look like rain 
hitting pavement, for example. I think this is a fair trade-off for the 99% 
of the time scenario where solids and non-solids hit large areas of static 
non-solids.

I could even go as far as to decide that all non-solids are considered 
static in our little pretend universe, just because that is how they will
mostly be used. Other simulations will require enhancement outside the scope 
of simple game creation.

/Peter

On 2008-08-17 (Sun) 11:40, Greg Ewing wrote:
> Peter Gebauer wrote:
>
>> Great point, I was thinking about using the friction attribute to 
>> achieve this, but maybe something like viscosity is better to calculate 
>> drag?
>
> A separate parameter would probably be better, since unlike
> friction between solid surfaces, the drag force has to be
> velocity-dependent -- otherwise the object would slow down
> and stop somewhere in the fluid instead of reaching a
> terminal velocity.
>
> -- 
> Greg
>
>