In article <412fef84$1@news.povray.org> , "Eli" <eli### [at] jehoelnet> wrote:

> Sounds weird to me that the atmosphere scattering photons causes us to see
> stars that would be invisible otherwise. The atmosphere merely blurs
> providing it reaches our retinas.

The ability of the human eye or a real camera to see infinitely small but
very bright objects has nothing to do the atmosphere.  It has to do with
sampling an area compared to sampling points.

    Thorsten

____________________________________________________
Thorsten Froehlich, Duisburg, Germany
e-mail: tho### [at] trfde

Visit POV-Ray on the web: http://mac.povray.org

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Eli wrote:

> Sounds weird to me that the atmosphere scattering photons causes us to see 
> stars that would be invisible otherwise. The atmosphere merely blurs 
> providing it reaches our retinas. 
> 
Correct, astronauts do see stars.

The human eye is able to detect a single photons (if adapted
to darkness). Starlight of the faintest stars is single photons just
often enough to see them.

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> The ability of the human eye or a real camera to see infinitely small but
> very bright objects has nothing to do the atmosphere.  It has to do with
> sampling an area compared to sampling points.

Fair enough. But when the human eye does this, it is able to accurately
depict the shape of the object.

In the case of a star, the difference may not be noticable between a
pixel-sized square on your computer screen and the shape of a star in the
sky. As I've said, the 3.6 behavior is useful for those who want to make
stars visible, which is why I'm OK with having the behavior as an option.

But in the case of large bright objects, the image that POV-Ray is providing
looks nothing like what the human eye would see. The human eye would not see
a jagged edge where there is a smooth one, regardless of the brightness of
the object. One's brain would use their eyes to seek out the edge of the
object and portray it as it appears: smooth. POV-Ray 3.6 is not emulating
this (more useful) behavior, in favor of creating an arguably aesthetic
effect in the single case of starfields.

You're saying that the eye has a high enough "resolution" that, if it saw a
star unaffected by atmospheric scattering or anything, despite the fact that
the star would be incredibly tiny, the eye would be able to see it. However,
the eye would display it at the size that it was. POV-Ray 3.6 is taking the
tiny object and blowing it up to the size of a pixel, *by letting it affect
the samples taken around it*. It is also doing this for the edges of larger
objects, allowing shapes that should be smaller than a pixel (like a tiny
triangle cut off of the edge of a sphere) to be blown up to the size of a
full pixel, causing aliasing.

Returning to your statement "It has to do with sampling an area compared to
sampling points," the primary difference is this: the human eye does not
take the samples of that area and average them together and then display the
average value as the value for the whole area. It takes them individually
and displays all objects at the correct size and shape. Obviously POV-Ray
can't do this in its output, since it is limited to pixels. Therefore it
must make a compromise: it can't produce exactly what the human eye would
see, so it must do the best it can to display something that *appears*
correct to the human eye. The compromise in 3.5 appeared correct. The
compromise in 3.6 does not.

The primary usage, purpose, and original intent of anti-aliasing is to
remove aliasing. 3.6's anti-aliasing is failing at this.

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

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run### [at] runevisioncom news:412f1e37$1@news.povray.org

> The disadvantage is that all objects or textures with very bright (or
> brightly lit) colors will now have jagged edges even though AA is
> turned on. 

Imho this *is* the correct behaviour in most cases - if only *10 times 
brighter object occupies 1/10 (or more) of pixel - enitre pixel should be 
white. It should be gray only if object occupies less then 0.1 of pixel 
area IMHO.

-- 
http://www.raf256.com/3d/
Rafal Maj 'Raf256', home page - http://www.raf256.com/me/
Computer Graphics

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"Slime" <fak### [at] emailaddress> wrote in message
news:4130e456@news.povray.org...
> Returning to your statement "It has to do with sampling an area compared
to
> sampling points," the primary difference is this: the human eye does not
> take the samples of that area and average them together and then display
the
> average value as the value for the whole area. It takes them individually
> and displays all objects at the correct size and shape. Obviously POV-Ray
> can't do this in its output, since it is limited to pixels. Therefore it
> must make a compromise: it can't produce exactly what the human eye would
> see, so it must do the best it can to display something that *appears*
> correct to the human eye. The compromise in 3.5 appeared correct. The
> compromise in 3.6 does not.
>

Jumping into the middle of this discussion:

One point to remember is that human eyes are a set of sensing instruments
in the service of the brain.
Brains process sensory input and do a lot of "anti-aliasing".
The image you perceive is a brain-processed image that is heavily influenced
by memory, interpolation, pattern recognition, imagination plus a little
'objective
data'.

"The map is not the territory." -   Korzybski

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> I *am* dreaming - of a POV-Ray where certain developers wouldn't have so
> much of a "we know better, so mind your own business" kind of attitude.

I apologise for the tone of Thorsten's reply. Please understand that we all
get a bit frustrated sometimes. Working on a raytracer as complicated as ours
is a bit of a balancing act - fixing one problem sometimes introduces others
and no matter what we do folks will be unhappy ;-(

We do what we can and sometimes it just doesn't work out the way folks want.
As for this problem we will discuss it internally to see if there is a way
around it (no promises).

-- Chris

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Chris Cason wrote:

>>I *am* dreaming - of a POV-Ray where certain developers wouldn't have so
>>much of a "we know better, so mind your own business" kind of attitude.
> 
> 
> I apologise for the tone of Thorsten's reply. 
You do not have to, (nearly) everbody here has seen how Thorsten
replies in general. I (and again most of us here I guess) assume
that he is just a bit unlucky in the way he responds and is probably
a nice guy when you meet him IRL.
> Please understand that we all get a bit frustrated sometimes. 
we do.
> Working on a raytracer as complicated as ours
> is a bit of a balancing act - fixing one problem sometimes introduces others
> and no matter what we do folks will be unhappy ;-(
I think it is a beautiful example of how fixing one bug introduces
unwanted side effects elsewhere. Do you mind if I use it as an example?
> 
> We do what we can and sometimes it just doesn't work out the way folks want.
> As for this problem we will discuss it internally to see if there is a way
> around it (no promises).
Thanx in advance.

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Chris Cason wrote:
> I apologise for the tone of Thorsten's reply. Please understand that
> we all get a bit frustrated sometimes. Working on a raytracer as
> complicated as ours is a bit of a balancing act - fixing one problem
> sometimes introduces others and no matter what we do folks will be
> unhappy ;-(

Yeah, this conversation should be seen in the light that developers and
users alike can both get frustrated, and we're all human.

I truly appreciate the Team's effort in continuously improving POV-Ray, and
even my reply to Thorsten should not be seen as a personal attack, but just
as a comment to the tone of the specific reply he posted here.

> We do what we can and sometimes it just doesn't work out the way
> folks want. As for this problem we will discuss it internally to see
> if there is a way around it (no promises).

That's very nice to hear! :)

Best regards,

Rune
-- 
3D images and anims, include files, tutorials and more:
rune|vision:  http://runevision.com
POV-Ray Ring: http://webring.povray.co.uk

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"Slime" <fak### [at] emailaddress> wrote:
> > The ability of the human eye or a real camera to see infinitely small but
> > very bright objects has nothing to do the atmosphere.  It has to do with
> > sampling an area compared to sampling points.
>
> Fair enough. But when the human eye does this, it is able to accurately
> depict the shape of the object.

This is a complicated issue, but the end result is that the eye can't do
this. Badly worded and simplified explanation follows.

> You're saying that the eye has a high enough "resolution" that, if it saw a
> star unaffected by atmospheric scattering or anything, despite the fact that
> the star would be incredibly tiny, the eye would be able to see it.

Imagine the surface of the retina divided into a regular grid of pixels.
Although the eye's grid of "pixels" is far from regular, each pixel has
area - each pixel is sensitive to light over a *non-zero* region of the
retina. A photon landing *anywhere* within a "pixel" will be detected. A
source such as a star whose photons all end up within one pixel will light
up only that pixel. No other information will be received about that
source. The star's true image could be circular, square, shaped like a hoop
or anything you like, but as far as the eye is concerned it is shaped like
a pixel, and the pixel is 200 times bigger than the star's true image. The
eye cannot preserve detail it can't resolve. This pixel is then the
smallest unit that the eye deals in. So,

> However the eye would display it at the size that it was. POV-Ray 3.6 is
> taking the tiny object and blowing it up to the size of a pixel, *by letting
> it affect the samples taken around it*.

POV in this case is doing exactly what the eye would do. However, POV must
simulate a finite pixel area by taking additional samples, because a ray
has no area (ideally - I imagine there is a numerical precision issue
here). Both eye and POV are doing the same thing, except that POV is
impersonating a finite pixel area by taking extra samples. In the extreme
case, a raytracer would divide the camera field of view angles by the
number of pixels in each direction (horizontal and vertical) to get a grid
of rectangles and shoot a few hundred rays through each rectangle. That
would imitate as near as possible a finite pixel size, although it would be
closer to a digital camera's response than to that of the eye or film.

> Returning to your statement "It has to do with sampling an area compared to
> sampling points," the primary difference is this: the human eye does not
> take the samples of that area and average them together and then display the
> average value as the value for the whole area. It takes them individually
> and displays all objects at the correct size and shape. Obviously POV-Ray
> can't do this in its output, since it is limited to pixels.

Both eye and raytracer deal in pixels. In the case of the eye the pixels are
irregular in shape and distribution and are somewhat squidgey and
disgusting, but there is still no way for the eye to capture and record
detail smaller than the size of its own active regions/pixels/buckets.

This simplified explanation ignores several things, the most important of
which is the point-spread function of the eye, but the relatively low
resolution of most CG images makes this irrelevant.

> Therefore it must make a compromise: it can't produce exactly what the human
> eye would see, so it must do the best it can to display something that
> *appears* correct to the human eye. The compromise in 3.5 appeared correct.
> The compromise in 3.6 does not.

> The primary usage, purpose, and original intent of anti-aliasing is to
> remove aliasing. 3.6's anti-aliasing is failing at this.

For what it's worth I agree with the general point that effectively limiting
use of bright objects with antialiasing seems a bit harsh. Neither POV-Ray
nor any other CG package outside the most specialist are faithful
imitations of the human visual system, absent super-sampled images and
post-processing. The question then could be about what looks better. Does
anti-aliasing remove jaggies, or not? That point about work-arounds is also
a strong one in my view.

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> As for this problem we will discuss it internally to see if there is a way
> around it (no promises).

Glad to hear that! Thanks for taking the time.

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

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