From: Robert Templeman <mbcaprt@mphhpd.ph.man.ac.uk>
Newsgroups: comp.graphics.algorithms
Subject: Re: liquid motions
Date: 8 Sep 1995 14:21:23 GMT
Organization: home watching TV
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janne.sinivirta@triplex.fipnet.fi (JANNE SINIVIRTA) wrote:
>        hi!
>
>    i'd need some help with liquids like water. I'm trying to
>create an algo to animate water or something like it to be poured
>from a bottle to sink, so the water spreads all over the sink and
>then goes to the hole.
>
>thanks in advance,
>        vertex /TDC

I've written a nice water 'simulator' (2D, but easily extended to 3D) essentially i use about 10000
particles, each is convieniently 1 pixel square!, and i process them as follows;

(1) position all the liquid particles at the same point (crucial this bit).  For your bottle you
could put all the particles in the top inside of the bottle, let them flow to fill the rest of the
bottle & then open the top (bottom, hehe) to allow them to flow out.
  This 'relaxation' stage is cruicial because it correctly orders the paricles so that the lowest
particles are the 1st updated.  This has the effect of bubbles of air bubbling upwards when the
fluid displaces empty screen space, which looks totally realistic (to the untrained eye!)

(2) To move the fluid do the following;

    Loop over all the fluid particles;
      If there is a free space below a particle move it down one pixel
      If there isnt a free space below it randomly check to the left or
      the right of the particle and move it correspondingly.

      If the fluid particle has been in free fall for more than 2 iterations
      randomly move it from left to right with a low probability (this is for
      a nice spreading effect)
    endloop

And thats it.  This gives the impression of a fluid with a high Reynolds no. (obviously its not a
very good physical simulation, no pressure actually exists)

Ways to make it look awesome are (for water this is) when the fluid moves sideways randomly change
its colour to lighter shades of blue for 1 iteration & when its in free fall randomly change to even
lighter shades of blue.  When the fluid skits sideways in freefall change its colour to white for an
iteration, also do this when the fluid moves from freefall to stationary.
  I've tried this with 20000 water 'molecules' on a 1000x500 ish display and it looks absolutely
awesome, especially when it cascades from one object to another.
  Other things to try are moving the liquid further in one iteration, this alters the fluids
'viscosity' quite nicely.
  This is easily extended to 3D but you would need a fearsomly large no. of particles for a voxel
like image, better would be to have the particles finite size cubiods and do the comparisons in a
vector like fashion (obviously your plughole would need to be big enough for a rectangle to flow
down it!)

 Robert Templeman