"Bill Pragnell" <bil### [at] hotmailcom> schreef in bericht
news:web.4360929f30852916731f01d10@news.povray.org...
> Just in case any of you were getting bored of ringworlds...
>
> This is a substantial improvement on my previous effort. The ring is
pretty
> much the dimensions stated in the book (and elsewhere here) - 90 million
> miles in radius (although it should be 95 I believe), with 20 shadow
> squares orbiting 12 million miles from the sun. The sun is an area_light 1
> million miles in diameter.
>
> The atmospheric effects are of course the most difficult, and in this
regard
> I cheated somewhat - the haze is merely a ground fog, and the blue sky is
a
> semitransparent sky sphere about a million miles wide, i.e. between the
> camera and the arch.
>
> I think the landscape is of realistic dimensions, but sadly the clouds are
> slightly exaggerated - they are 200 miles in the air, with the camera at
an
> altitude of 100 miles. The reason for this is that I couldn't place the
> camera any lower without some kind of precision artifact creeping into the
> lower half of the image (I'll post an example in a bit).
>
Excellent images, both of them! The atmosphere is much better than in my own
experiments. I am afraid that we have to cheat a bit to get it really right.
In truth, because of the dimensions, the whole *foreground* is a landscape
in itself, almost separate from the background arch.

Thomas

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Both of your WIPs are great.
I've been thinking a bit about having a go
at Rama.. But, I need to read it again
before that.
Both these posts inspired me to
get in gear :)

"Thomas de Groot" <t.d### [at] internlnet> skrev i meddelandet 
news:4360cd4a@news.povray.org...
>
> "Bill Pragnell" <bil### [at] hotmailcom> schreef in bericht
> news:web.4360929f30852916731f01d10@news.povray.org...
>> Just in case any of you were getting bored of ringworlds...
>>
>> This is a substantial improvement on my previous effort. The ring is
> pretty
>> much the dimensions stated in the book (and elsewhere here) - 90 million
>> miles in radius (although it should be 95 I believe), with 20 shadow
>> squares orbiting 12 million miles from the sun. The sun is an area_light 
>> 1
>> million miles in diameter.
>>
>> The atmospheric effects are of course the most difficult, and in this
> regard
>> I cheated somewhat - the haze is merely a ground fog, and the blue sky is
> a
>> semitransparent sky sphere about a million miles wide, i.e. between the
>> camera and the arch.
>>
>> I think the landscape is of realistic dimensions, but sadly the clouds 
>> are
>> slightly exaggerated - they are 200 miles in the air, with the camera at
> an
>> altitude of 100 miles. The reason for this is that I couldn't place the
>> camera any lower without some kind of precision artifact creeping into 
>> the
>> lower half of the image (I'll post an example in a bit).
>>
> Excellent images, both of them! The atmosphere is much better than in my 
> own
> experiments. I am afraid that we have to cheat a bit to get it really 
> right.
> In truth, because of the dimensions, the whole *foreground* is a landscape
> in itself, almost separate from the background arch.
>
> Thomas
>
>
>

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"Stefan Persson" <azy### [at] hotmailcom> schreef in bericht
news:43617930$1@news.povray.org...
> Both of your WIPs are great.
> I've been thinking a bit about having a go
> at Rama.. But, I need to read it again
> before that.
> Both these posts inspired me to
> get in gear :)
>

Excellent! That is what these forum's (fora?) are for!!

Thomas

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Thomas de Groot wrote:
> 
> Excellent! That is what these forum's (fora?) are for!!
> 
forums!

SCNR   ;-)

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"Stefan Persson" <azy### [at] hotmailcom> wrote:
> Both of your WIPs are great.
> I've been thinking a bit about having a go
> at Rama.. But, I need to read it again
> before that.
It's useful having such technically-minded authors - they provide complete
blueprints in prose, with dimensions and everything. It's almost as if they
knew that one day ppl like us would sit down in front of our difference
engines and build their creations as virtual realty...

> Both these posts inspired me to
> get in gear :)
Why did I come back to using POV after 4+ years of inactivity? Inspiration
gained from lurking here, of course... :)

Bill

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Right, just to illustrate this suspected precision glitch, here's 7 images
of a simplified Ringworld (please excuse the crudity of the composite).

The Ring is a hollow open cylinder 1 million units tall and 90 million units
in radius, with a light at the centre. I've translated it by 90 million y
so that the origin lies on the Ring surface with the x axis at a tangent.

The first three images are, left to right, a camera y = 100, 10, 1.
The middle image is camera y = 0.
The final three images are, left to right, a camera y = -1, -10, -100.

Firstly, Thomas, do you see the same thing with your Ring?
Secondly, can anyone clarify what exactly is happening here? Is it a
precision problem, or something else?

code:

camera { location <0, 100, 0>
         up <0, 1, 0>
         right <4/3, 0, 0>
         direction <0, 0, FoV_75>
         look_at <-1000, 0, 0> }

#declare Ring = union {
  light_source { <0, 0, 0> color White }
  cylinder { <0, 0, -500000>, <0, 0, 500000>, 90000000 open hollow
             pigment { color SkyBlue }
             finish { ambient 0.5 } } }

object { Ring translate <0, 90000000, 0> }


Bill, confused.

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Attachments:
Download 'ringtest.png' (9 KB)

Preview of image 'ringtest.png'

> It's useful having such technically-minded authors - they provide complete
> blueprints in prose, with dimensions and everything. It's almost as if
they
> knew that one day ppl like us would sit down in front of our difference
> engines and build their creations as virtual realty...

Yep, and I need to get the images back in my head.
All I remember is that it's a 5x2 km cylinder .. hehe

But, yes, Clarke usually desbribes everything in a most
detailed technical way. On the other hand authors like Asimov
leaves more to the imagination.. I don't know which is better
from the artists point of view. I mean, the first, like you said,
provide almost blueprints.. but that also means that you have
less freedom.

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"Bill Pragnell" <bil### [at] hotmailcom> schreef in bericht
news:web.4362127eca18a3ea731f01d10@news.povray.org...
> Right, just to illustrate this suspected precision glitch, here's 7 images
> of a simplified Ringworld (please excuse the crudity of the composite).
>
> The Ring is a hollow open cylinder 1 million units tall and 90 million
units
> in radius, with a light at the centre. I've translated it by 90 million y
> so that the origin lies on the Ring surface with the x axis at a tangent.
>
> The first three images are, left to right, a camera y = 100, 10, 1.
> The middle image is camera y = 0.
> The final three images are, left to right, a camera y = -1, -10, -100.
>
> Firstly, Thomas, do you see the same thing with your Ring?
> Secondly, can anyone clarify what exactly is happening here? Is it a
> precision problem, or something else?
>

Yes, Bill. The same happens with my Ring. If I lower my camera towards the
surface I get exactly the same artifacts.

Thomas

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Thomas de Groot wrote:
> "Bill Pragnell" <bil### [at] hotmailcom> schreef in bericht
> news:web.4362127eca18a3ea731f01d10@news.povray.org...
>>Firstly, Thomas, do you see the same thing with your Ring?
>>Secondly, can anyone clarify what exactly is happening here? Is it a
>>precision problem, or something else?
>>
> 
> 
> Yes, Bill. The same happens with my Ring. If I lower my camera towards the
> surface I get exactly the same artifacts.
> 
> Thomas

I've run into problems like this before.  POVRay is designed for 
rendering scenes of "normal" sizes where "normal" is in the range of 
arount 100,000 units from the origin.

When using floating point numbers, you can't test for exactly 0 because 
floats are always slightly inaccurate.  Instead, you have to test for 
numbers smaller than some epsilon value.

There's a value called EPSILON (see frame.h) which defines a small value 
and anything smaller would be considered 0 in some of the calculations. 
  This value is set to 1e-10 which is dangerously close in magnitude to 
1 / 90,000,000 (the scale of Ringworld).

To solve this problem, you may have to try decreasing the epsilon value 
and recompiling POVRay.  You may still run into problems, though, 
because doubles may not have enough precision to handle the 
calculations.  You may come up with calculations like

	90,000,000.00000012 - 90,000,000.0000001

The correct answer should be 0.00000002, but the numbers just can't 
represent 90,000,000.00000012 accurately.  That's the nature of limited 
precision floating point calculations. It's bad news for people trying 
to render Ringworld, though.

Hope this helps
David Buck

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"David Buck" <dav### [at] simberoncom> schreef in bericht
news:43635ed8$1@news.povray.org...

>
> I've run into problems like this before.  POVRay is designed for
> rendering scenes of "normal" sizes where "normal" is in the range of
> arount 100,000 units from the origin.
>

Would more scaling down help? My model has currently a radius of 95,000 pov
units, but also translated 95,000 units to keep the camera near the origin.
It is possible to scale everything down more of course, without changing the
aspect of the scene...

Thomas

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