EagleSun nous apporta ses lumieres ainsi en ce 2004-12-31 17:12... :

>Hi yall,
>
>I'm trying to render the sun and earth... the problem is, if I set the sun
>far away properly to 149 million kilometers (scale = 1000), or even 149
>billion meters (scale = 1), the sun won't render at all, but the object
>details on earth look nice.
>
>If I try to change the scale, like setting the sun to 149 thousand
>megameters (scale = 1,000,000), the sun renders fine, but the earth details
>are bombed.
>
>I made every effort to set proper scaling on earth surface, water turbulance
>(water seems to look nice on any scale), clouds (looks nice on scale 1 thru
>100), atmosphere (looks nice on scales 1 thru 100), distance of sun
>(renders if scale is larger than 1000).
>
>Can anyone give me clues?
>
>Ideally, I would like to render the sun at 149 billion meters, and
>eventually trillions of meters for Pluto.
>
>Thanks.
>
>
>
>  
>
You hit a hardware limitation, that of the FPU that is "only" 64 bits 
(double presision). To be able to use such whide ranges, you'd need quad 
(128 bits) or octuple (256 bits) presision math, with a processor 
equiped with a FPU capable of such.
Maybe some super computer have such a beast...

The best way around that problem is to fake the scaling. Make you'r sun 
1000 tomes closer, 1000 times smaller. Have a light_source with the 
"parallel" atribute to fake the actual sun distance. For Pluto, you'd 
need to put it 1 000 000 times closer and smaller, maybe more, than it 
should. In this case, it'll look like a single pixel point at 1600*1200 
resolution.

Alain

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Tim Cook <z99### [at] bellsouthnet> wrote:
> The problem is basically in how computers handle numbers.  They can't 
> easily do very small and very large (or normal and extremely large) 
> numbers at the same time.

  Actually the reason why the object disappears is not floating point
inaccuracy but because POV-Ray drops the object if it's too far away.
  Naturally POV-Ray does this in order to avoid the accuracy problems,
but it's not the accuracy which is dropping out the object but POV-Ray.
There's a reason for this: It's very possible that if POV-Ray didn't
drop the object, it might render with bad artifacts.

  See http://tag.povray.org/povQandT/languageQandT.html#largescaleproblems
for details.

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

Post a reply to this message

"Slime" <fak### [at] emailaddress> wrote:

> > http://tag.povray.org/povQandT/languageQandT.html#largescaleproblems
>
> That VFAQ entry could probably benefit from a suggestion or two about
> workarounds.
>
> To EagleSun: Specifically, the best solution is probably to greatly scale
> down your solar system (aside from the earth) with reference to the camera's
> position. So basically, do this:
>
> object {earth ...}
>
> union {
> object {sun ...}
> object {planet ...}
> ... more objects ...
>
> translate -cameraPosition
> scale smallValue
> translate cameraPosition
> }

Ahh :( Thanks for this tip guys....

I tried 2 views... basically at 1 point, I was 4 meters above water viewing
at a position 20 meters away, the sun was 149 billion meters away with a
diameter approximately 10* the size of the sun, so that you should have
been able to see the sun sphere quite big and bright.  To make it appear, I
brought it closer and made it smaller (both by factors of 10000 and
1/10000, respectively).  This scaling factor can get complicated, because
at 1 point, I wanted to render a scene where the camera was about 4000 km
above the surface (or 4,000,000 meters above the surface).  Setting the
scale to 1000 caused too much fading of color in the atmosphere, and in
certain situations, setting the scale to 1 caused the atmosphere to be
"disconnected" from the planet.  I guess rendering scenes that Astronauts
get to see are extremely difficult......

Changing the scale and distance of various objects is gonna be tricky.......
and I was hoping to reserve this kind of mixed scaling for objects outside
of Solar System....

And after reading the article at
"http://tag.povray.org/povQandT/languageQandT.html#largescaleproblems", I
agree that it's too restrictive... and I was hoping that POV Team could let
the users decide on the restrictions.  Granted faraway objects will not be
accurate.... but... viewing the sun at such a faraway distance, I don't
think anyone will notice.

Post a reply to this message

EagleSun <nomail@nomail> wrote:
> And after reading the article at
> "http://tag.povray.org/povQandT/languageQandT.html#largescaleproblems", I
> agree that it's too restrictive... and I was hoping that POV Team could let
> the users decide on the restrictions.  Granted faraway objects will not be
> accurate.... but... viewing the sun at such a faraway distance, I don't
> think anyone will notice.

  If the sphere is full of dark and white random pixels, they will.

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

Post a reply to this message

EagleSun wrote:

> And after reading the article at
> "http://tag.povray.org/povQandT/languageQandT.html#largescaleproblems", I
> agree that it's too restrictive... and I was hoping that POV Team could let
> the users decide on the restrictions.  Granted faraway objects will not be
> accurate.... but... viewing the sun at such a faraway distance, I don't
> think anyone will notice.

Unfortunantly, the restriction isn't imposed by POV-Ray in and of 
itself. Its a precision problem inherent to how most computers handle 
floating point (decimal) numbers. While the numbers show an astronomical 
range, the *precision* of the numbers has only a relatively very narrow 
range, somewhere in the neighborhood of a few billion truely 
representable numbers before you have to start dropping digits. When the 
digits are dropped, the calculations are no longer accurate enough and 
artifacts do occur.

Post a reply to this message

> > And after reading the article at
> > "http://tag.povray.org/povQandT/languageQandT.html#largescaleproblems",
I
> > agree that it's too restrictive... and I was hoping that POV Team could
let
> > the users decide on the restrictions.  Granted faraway objects will not
be
> > accurate.... but... viewing the sun at such a faraway distance, I don't
> > think anyone will notice.
>
> Unfortunantly, the restriction isn't imposed by POV-Ray in and of
> itself.

Well, POV-Ray does make far-away objects invisible on purpose. But as Warp
pointed out, the problems that it's trying to avoid are much worse than,
say, the sphere being sized wrong or slightly off its intended position. The
artifacts tend to be much more ugly: grainy and inconsistent.

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

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Slime wrote:

> Well, POV-Ray does make far-away objects invisible on purpose. But as Warp
> pointed out, the problems that it's trying to avoid are much worse than,
> say, the sphere being sized wrong or slightly off its intended position. The
> artifacts tend to be much more ugly: grainy and inconsistent.

Maybe I misread..

When did the far away objects being invisible take effect. It used to be 
that POVRay was happy to allow far away objects to render. They were 
difficult (if not impossible) to see, unless you set the camera's angle 
to a rediculously small amount to see said object...

Post a reply to this message

"Alain" <aze### [at] qwertygov> wrote in message
news:41d5efe6@news.povray.org...

> You hit a hardware limitation, that of the FPU that is "only" 64 bits
> (double presision). To be able to use such whide ranges, you'd need quad
> (128 bits) or octuple (256 bits) presision math, with a processor
> equiped with a FPU capable of such.
> Maybe some super computer have such a beast...

If you want it fast, it's a hardware limitation. Isn't it theoretically
possible to emulate it in software at the cost of speed? (no, i can't offer
examples... just thought it is possible)

if someone really cared, it seems they could probably substitute math
subroutines in povray with a "big math" library.

i had a course on this in college. i forget every word about it, except that
I rendered a scene of the intersection of earth with a fictional comet as
part of a word problem for a  homework assignment. bah

Post a reply to this message

Michael Raiford <mra### [at] hotmailcom> wrote:
> Slime wrote:
>
> > Well, POV-Ray does make far-away objects invisible on purpose. But as Warp
> > pointed out, the problems that it's trying to avoid are much worse than,
> > say, the sphere being sized wrong or slightly off its intended position. The
> > artifacts tend to be much more ugly: grainy and inconsistent.
>
> Maybe I misread..
>
> When did the far away objects being invisible take effect. It used to be
> that POVRay was happy to allow far away objects to render. They were
> difficult (if not impossible) to see, unless you set the camera's angle
> to a rediculously small amount to see said object...

The sun disappeared about 14.9 million kilometers away.  In realworld, the
sun is 149 million kilometers away, and it's 1.39 million kilometers in
diameter.  The focus is the earth, and the sun is a small disc in the sky.
In realworld, this size and distance should make the sun look as big as the
moon with normal viewing (zoom = 1).

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Ross wrote:
> "Alain" <aze### [at] qwertygov> wrote in message
> news:41d5efe6@news.povray.org...
> 
> 
>>You hit a hardware limitation, that of the FPU that is "only" 64 bits
>>(double presision). To be able to use such whide ranges, you'd need quad
>>(128 bits) or octuple (256 bits) presision math, with a processor
>>equiped with a FPU capable of such.
>>Maybe some super computer have such a beast...
> 
> 
> If you want it fast, it's a hardware limitation. Isn't it theoretically
> possible to emulate it in software at the cost of speed? (no, i can't offer
> examples... just thought it is possible)
> 
> if someone really cared, it seems they could probably substitute math
> subroutines in povray with a "big math" library.
> 
> i had a course on this in college. i forget every word about it, except that
> I rendered a scene of the intersection of earth with a fictional comet as
> part of a word problem for a  homework assignment. bah
> 
It's absolutely possible to have virtualy infinite presision maths. But it get 
sllooooooooooooooowwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww. How do you think they find new,
bilions digits 
long, prime numbers? And why they find them less than once a year using some super
computers full 
time? You have to simulate pen and paper operations using numerous sub results,
calculating each 
digit individualy.

Alain

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