# Physics-based modelling and reality

I am not very good at English, so I hope I can tell what I mean clearly based on how I find this problem.
I build up a square terrain with sides of 1024 unit. I treat the unit as meter, so the terrain is about 1 square kilometer. By using gluPerspective and gluLookat, I make the whole terrain almost displayed in the window. After this, I began to simulate raining. I let raindrops begin to fall at 500 meters height with constant velocity, 200 meters per second (I believe that raindrop falling velocity could never reach 200 mps.) But, with this velocity, raindrops only fall across the window very slowly compared to what we really see in the natural environment. Within the nearly 2.5 seconds (500/200), I clearly see raindrops crawling across the sky, especially raindrops far away from the camera. I think this is caused by the fact opengl compressed a big world into a small window, say in 2.5 seconds, we see raindrops fall through maybe 200 meters in natural world, but only maybe 20 centimeters on the screen. So, I believe there might be a magic ¡°factor¡± between the real world and opengl. I can physically model raindrops except for multiplying their speed by this factor.
But the point is: Does this factor have some relationship to opengl settings such as FOV, etc.? Does this factor can be calculated? Does this factor vary at different place? Or does this factor not exist at all?
Does anybody experience this and how do you solve this? Please give me your opinion.

A raindrop falling from 500 meters will take at least 10 seconds to hit the ground (and probably much longer due to air resistance).
The reason this doesn’t look right is this: you are rendering individual raindrops that are up to a kilometer away. If you want it to look better, only render individual drops that are within a few meters and render the rest as fog.

Your visual cues are probably incorrect.

Are your raindrops the correct size?

Where is your near clip plane? If it’s too far out you won’t see the nearest drops which would move across the screen fastest.

The other reply is spot on too. I can’t see a rain drop 1km away it just becomes a fog like effect which brings me to my next point. You should turn on a grey fog if you have heavy rain to simulate the rain in the distance, for snow it might be a white fog. Then draw a few particles in the foreground.

If you see rain falling in the distance you tend to see sheets of rain, not individual particles and any structure in there appears to falls slowly when it’s far away.

[This message has been edited by dorbie (edited 06-15-2002).]