Thomas de Groot wrote:

> 4) 800k raindrops and slight down-scaling. The problem with down-scaling 
> is that the drops become invisible, so increasing the number of drops 
> does not change the final view much. 

I think this issue is similar to rendering star fields. As with
stars, individual raindrops that are far away (> 10m(*) or so assuming
a raindrop is 3mm in diameter) may still be visible to the unaided eye
but as unresolved points (or unresolved lines for falling drops).

In POV if they become smaller than a pixel you can entirely miss
them unless strong non-adaptive aliasing is enforced. But you can fake
it by compensating the size of the object to keep its angular size
at the 1 pixel level (but this then depends on render resolution).

Also if you do increase the size but use the same texture or
interior you will exaggerate the effect, so you should try to
compensate e.g. by reducing reflection / filter / absorption
values or similar by scale factor.

At some point even the distance between two falling drops
will be smaller than the resolution of the eye (> 300m(*) assuming
raindrops lines are spaced 10 cm apart). Then the effect will be
indistinguishable from continuous fog / media.

(*) Assuming 1 arcmin resolution. In practise this will likely not
be reached for fuzzy moving raindrops so you probably don't need to
render individual raindrops at extreme distances.

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