On 04/01/2011 08:47 AM, Trevor G Quayle wrote:
> Jim Holsenback<jho### [at] povrayorg>  wrote:
>
>> I've been using:
>> #declare LightDist = vlength(LightPos);
>>
>> as my value for fade_distance, Which is WRONG ... I want a smaller value
>> for that and up the intensity of the light source to compensate ...
>> @Christoph: Did I get that right?
>>
>> Jim
>
> This is something I learned from doing the matinee scene.
>
> What I have ended up doing, is set a relatively small fade distance (say the
> size of the light itself).  I then figure out what general value I want it at a
> certain distance (i.e., distance from light to your object or scene centre).
> Then use the formula given in the help to back calculate the initial intensity.
>
> For example, in my matinee scene for the projector, the distance from my
> projector light to the screen is about 2400, I set the fade distance to about
> 4.5.  Back-calculating to get a value of 1 at the screen (calculate the
> attenutaion value and take the inverse), gives an initial value of ~142,000!
> This seems very high, but it works out well because of the samll fade distance.
>
> -tgq
>

OK ... so here's what I've come up with and it seems to work.

#declare Intensity = function (LD,FD) { 1/(2/((1+(LD/FD))*(1+(LD/FD)))) };

 From your example I used LD=2400 and FD=4.5 and get 142756 and some 
change ;-) ... If I missed something feel free to set me straight as I'd 
like to include this in [3.4.7.9] along with a small narrative.

Jim

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