Christopher James Huff wrote:
>
>>You mean fortunately. POV-Ray does this in the mathematically correct way.
>>Antialiasing is a method of making an image with finite resolution appear as
>>though it has infinite resolution. If POV-Ray did not do it this way, then
>>extremly bright objects which are *large* (such as a sphere with color rgb
>>20) would look very aliased.
> 
> 
> No, do I mean unfortunately. Clipping before antialiasing is the wrong 
> way to fix this problem. Small, intense sources get completely wiped 
> out. Antialiasing supersamples the scene, not some virtual model of an 
> image file in a format that's limited to a tiny dynamic range.
> 

I tend to agree with you but the whole thing is not that easy.  What 
antialiasing does is to try to approximate the integration over the 
pixel area by taking several samples.

But the problem is if it is correct to do the averaging of the samples 
(which is the approximation of the integral with a sum) with or without 
the non-linearities of the tone-mapping used (in case of standard POV 
this is gamma correction and clipping).

You can't really answer this from looking how it works in real life - 
when you take a photograph of a star the ideal camera outside the earth 
atmosphere in ideal empty space will only show an infinitely small 
point.  Of course nothing is ideal so the result will be a spot of 
finite size (with the intensity diminishing from the center to the rim). 
but how exactly the non-linearities of the film exposure work here is 
hard to tell (meaning it would be difficult to predict how bright two 
stars very close to each other will look from the appearance of them a 
distance apart).

Coming back to the actual antialiasing problem - in cases like this it 
would be quite nice to antialias before clipping, in relation with the 
high dynamic range output in next megapov this will be possible as well. 
  But there are also situations where this causes problems, see for 
example the wall edge on the right in:

http://www-public.tu-bs.de:8080/%7Ey0013390/pov/pool/chpool.jpg

where despite antialiasing the stairstep effects are quite strong which 
is to some extent caused by the tone mappping being applied after 
antialiasing.

An idea i had thought of to solve this problem (without the necessity to 
accept clipped output) was to make the tone mapping function (or an 
approximation of it) available to POV (for example as a user defined 
function) and to apply this function before the averaging of samples. 
To get back to the linear result afterwards you would need the inverse 
of this function of course (it needs to be invertible so you should not 
use hard clipping).

I don't know how useful such a feature would be but it would allow to 
adjust continuously between the two extemes.

Christoph

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