> 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.

Well I was thinking of a digital camera, which works by having a grid of colour
sensors that effectively just add together the brightness due to all photons
falling upon that pixel. Surely with that a very small point would always be 1
pixel in size and have a brightness proportional to it's brightness multiplied
by how much of the area of the pixel it covers.

I'm not sure how all that corresponds to gamma ramps and such, but digital
cameras tend to get sharp images without aliasing, so surely that can be used as
a model for an anti-aliasing technique?

-- 
Tek
www.evilsuperbrain.com

"Christoph Hormann" <chr### [at] gmxde> wrote in message
news:mgp### [at] tritonimagicode...
> 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
>
> -- 
> POV-Ray tutorials, include files, Sim-POV,
> HCR-Edit and more: http://www.tu-bs.de/~y0013390/
> Last updated 11 Jan. 2004 _____./\/^>_*_<^\/\.______
>

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