> > Just because they're intense doesn't mean they should become a
> > full pixel in size.
>
> Sure it does.  If a sun-sized star is a million times farther from
> Earth than the sun, but a hundred million times brighter, it would
> be visible during the day, even though it is otherwise too small to
> be seen.


Yes, but I think this is caused by different reasons, isn't it? Like, the
atmosphere, the workings of the human eye, stuff like that causing the light
to seem as though it's coming to you from a larger area than it really is.
Again, just because it creates a nice effect in some situations doesn't mean
that it's correct.

 - Slime
 [ http://www.slimeland.com/ ]

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Tek wrote:
>>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?

That's something completely different and does not help to determine the 
best way to antialias a raytracing scene.  Note the only reasons why i 
brought up the photography comparison was to illustrate the interaction 
of averaging processes and nonlinearities (both occurs both in digital 
and conventional photography).

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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Slime wrote:
>> Unfortunately, POV clips the colors before it combines them, rather than
>> afterward...to antialiasing, rgb 20 is the same as rgb 1.
>
>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, they would not in general: Especially if you use adapitive antialiasing,
it will just take more samples until the difference is small enough. Due to
the number of samples taken, there usually will not be a visible aliasing.

    Thorsten

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Tek <tek### [at] evilsuperbraincom> wrote in message 40043c50@news.povray.org:
>> 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?
>

There seems to be many arguments on what's right in this thread but I (who
normaly just reads this group) want to give you all my view on the topic.
I'm not saying that this is more "right" or something like that, all is
IMHO.
I have for several years planned making my own raytracer and have though
long about the antialias problem (having had the clipping before/after aa
problem with pov). I have come to the conclusion that the super sampling
should be done on unclipped values on the argument that it is really an
approximisation to an intergral over the pixel. This gives the expected
result for small bright objects such as stars. The problem is that edges of
very bright object get alias problems, and that's not the expected result.
With a digital camera such an edge will look soft (at least partly) because
imperfections in the lenses and the air will make the bright light "bleed".
My plan is to simulate this bleeding in a post process step before the
clipping/gamma/whatever (the plan is to output the image with raw floating
point color values and do any transforms when saving to ex. png). That
should solve that problem too.

My planned raytracer is probably never going to be implemented, so don't ask
for it. I just enjoy thinking about features for it :)

-- 
Daniel Nilsson

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Daniel Nilsson wrote:
> 
> There seems to be many arguments on what's right in this thread but I (who
> normaly just reads this group) want to give you all my view on the topic.
> I'm not saying that this is more "right" or something like that, all is
> IMHO.
> I have for several years planned making my own raytracer and have though
> long about the antialias problem (having had the clipping before/after aa
> problem with pov). I have come to the conclusion that the super sampling
> should be done on unclipped values on the argument that it is really an
> approximisation to an intergral over the pixel. This gives the expected
> result for small bright objects such as stars. The problem is that edges of
> very bright object get alias problems, and that's not the expected result.
> With a digital camera such an edge will look soft (at least partly) because
> imperfections in the lenses and the air will make the bright light "bleed".
> My plan is to simulate this bleeding in a post process step before the
> [...]

No. While you are perfectly right that a digital camera image contains 
blur because of the imperfections in the optical system and elsewhere 
that is not the reason why there are no aliasing effects.  These won't 
occur even with a perfect optical system because - as Tek correctly 
describes - a digital camera sensor collects the incoming energy and 
therefore does a true integration over the pixel area.

Using post processing to make a render look more like a photograph is 
certainly useful but you should not make the mistake to think this has 
anything to do with antialiasing.

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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"Christoph Hormann" <chr### [at] gmxde> wrote in message
news:ff9### [at] tritonimagicode...

[deletions]

: Using post processing to make a render look more like a photograph is
: certainly useful but you should not make the mistake to think this has
: anything to do with antialiasing.
: Christoph

Are there programs that can antialias specific objects rather than just the
entire frame? Please keep in mind that reality is not my pursuit...
=Bob=

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In article <web.400469f9570536d75800b48e0@news.povray.org>,
 "Thorsten" <nomail@nomail> wrote:

> No, they would not in general: Especially if you use adapitive antialiasing,
> it will just take more samples until the difference is small enough. Due to
> the number of samples taken, there usually will not be a visible aliasing.

Well, if the antialiasing algorithm simply computes the integral of the 
unclipped values, it is possible for a pixel only partially covering a 
bright area of the scene to be pushed over 100%. If the area of the 
scene has an intensity of 10, it need only cover 10% of the pixel to 
reach maximum intensity. Digital cameras have imperfect optics, blooming 
effects, a different detector element geometry...each pixel is usually 4 
squares: a red, two green, and a blue...and various software filters. I 
think this is something that should be taken care of with a post-process 
filter...it wouldn't be a problem if display devices could reproduce the 
true intensities.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

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> But what about if you take a photo of the night sky? The stars are much
smaller
> than 1 pixel, however they show up because of their intensity.

Heh, when I take a picture of the night sky with my digital camera, even with
all night settings on and sensitivity/exposure settings maxed out, I get a black
picture with only a couple of the brighter stars shown. Most stars simply ain't
bright enough. They really are quite faint, but the human eye is sensitive
enough to pick them up. You've gotta love the human eye...

Personally, I've been through the phase of simulating stars with thousands of
small spheres. I gave up and started using a textured sky sphere instead. It's
faster to parse/render and you can achieve greater star 'densities'.

Ryan

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What this really means is that PovRay would benefit from having more 
Intelligent  handeling of >rgb 1 samples.  I've seen several people 
working on just such issues and I hope to see the results in... [Author 
was shot before he could mention a future version of POV in the news groups]

Slime wrote:

>>You could avoid
>>over-bright objects getting jagged edges by using more samples (the
>>anti-aliasing threshold would mean these extra samples only got used on
> 
> those
> 
>>high-contrast areas).
> 
> 
> If 1/9 of a pixel has the color rgb 10000, and the rest of the pixel has
> other colors between 0 and 1, and clipping happens after anti-aliasing, then
> the pixel is going to be 1/9 * 10000 + 8/9 * (something between 0 and 1),
> which works out to be well over 1, causing the entire pixel to be
> oversaturated. More samples just causes this calculation to be more
> accurate, but still well over 1. If this happens at every pixel along the
> edge of an object, it will look pixellated.
> 
>  - Slime
>  [ http://www.slimeland.com/ ]
> 
>

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