+AD4-We know nothing about the size of the whole image, the
object
+AD4-count, are you using atmosphere, I'm sure that has some
random
+AD4-factor, media too.
+AD4-
+AD4-I'd be tempted to look at the code.


I could post the code, but I don't think it would make much
sense. There was no media or atmosphere. At the first render
there were lots of objects in the scene, but I commented all
but one out when I started to look at the tiny details (to
spare some time).

I still think it's just the anti-aliasing. Does any of the
programmers around here know if POV uses a random function
for AA?
I'm going to have another look at the no-AA renders. Maybe
there was no difference after all.. (that would mean no
gremlins either).

It's really an 'academic' problem, for the differences
between the images are invisible... but I'm just curious...

Greetings all

Julius

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Hmm, I've had this problem too... I was trying to get my RLE encoder to work
properly but it wouldn't because the images changed slightly...  The way
around it I found was to go into POV-Ray, render... exit POV-Ray, go back in
and render again and you'll get the same image (providing you don't use any
AA jitter or crand).

Then again, it's already @725 here in Australia so I'm tired... I'm going to
sleep... Maybe I'll get in a Lucid Dream if I'm lucky too...

--
Lance.


---
For the latest 3D Studio MAX plug-ins, images and much more, go to:
The Zone - http://come.to/the.zone

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Crand and jitter are the only really random functions... Well, probably
media, too.

Margus

Julius Klatte wrote in message <3710a905.0@news.povray.org>...
>
>I could post the code, but I don't think it would make much
>sense. There was no media or atmosphere. At the first render
>there were lots of objects in the scene, but I commented all
>but one out when I started to look at the tiny details (to
>spare some time).
>
>I still think it's just the anti-aliasing. Does any of the
>programmers around here know if POV uses a random function
>for AA?
>I'm going to have another look at the no-AA renders. Maybe
>there was no difference after all.. (that would mean no
>gremlins either).
>
>It's really an 'academic' problem, for the differences
>between the images are invisible... but I'm just curious...
>
>Greetings all
>
>Julius
>
>

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I think it's AA jitter. Set jitter to 0.0 (+j0.0) regardless of your
AA setting (turn it off as well). Maybe jitter randomises the ray
position within the bounds of the current pixel even though
anti-aliasing is turned on. Is the opposite explicitly stated
anywhere?

---------
Peter Popov
ICQ: 15002700

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Yip

In you go out of Windoze and go back in probably you've cleared
the registers. 

Lance's post made me think of something.  The images we saw in
your post were grey, I don't know how many decimal places the RGB
settings were using, maybe depending on certain things (such as
memory usage), it only rounds up or down to a certain distance. 
You or I couldn't tell the difference between 0.989877754142 and
0.989877754141 could we, so maybe in certain circumstances it
rounds down or up closer to the decimal point.  

I'm not sure about any of this, havn't tested any of it, it's
just an idea that someone with more time might want to look into.

Cheers
Steve








Lance Birch wrote:
> 
> Hmm, I've had this problem too... I was trying to get my RLE encoder to work
> properly but it wouldn't because the images changed slightly...  The way
> around it I found was to go into POV-Ray, render... exit POV-Ray, go back in
> and render again and you'll get the same image (providing you don't use any
> AA jitter or crand).
> 
> Then again, it's already @725 here in Australia so I'm tired... I'm going to
> sleep... Maybe I'll get in a Lucid Dream if I'm lucky too...
> 
> --
> Lance.
> 
> ---
> For the latest 3D Studio MAX plug-ins, images and much more, go to:
> The Zone - http://come.to/the.zone

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OK, I've woken up now... :-)  Nice dreams but not Lucid... :-(

Anyway, back to the subject, ahem, no, I don't think this would happen
because then it would basically undermine the whole idea of computers being
accurate+ACEAIQAh-  So I still think that it must be either a pseudo random number
thing (that is affected by previous renders) or it is something to do with
jittering in AA, even though it's set to 0 maybe it's not really off or
something...  Just a thought...

Then again, it might just be a memory thing... Who knows?

--
Lance.


---
For the latest 3D Studio MAX plug-ins, images and much more, go to:
The Zone - http://come.to/the.zone

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Think so too.
Did you mean to say "anti-aliasing turned off" instead of "on"?
Anyhow, arealights can use jitter, though not on until the keyword is
used. Focal blur mentions jitter however possibly not the same type of
thing at all since only animation and arealight is said to cause
randomization of pixels.
Would like to hear about the further results of tests just in case
though. I sound lazy don't I, anyone should be able to do a quick check.


Peter Popov wrote:
> 
> I think it's AA jitter. Set jitter to 0.0 (+j0.0) regardless of your
> AA setting (turn it off as well). Maybe jitter randomises the ray
> position within the bounds of the current pixel even though
> anti-aliasing is turned on. Is the opposite explicitly stated
> anywhere?
> 
> ---------
> Peter Popov
> ICQ: 15002700

-- 
 omniVERSE: beyond the universe
  http://members.aol.com/inversez/homepage.htm
 mailto:inv### [at] aolcom?Subject=PoV-News

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Post the code here and let a few of us try rendering it and
report our results back to the group.

Steve

Julius Klatte wrote:
> 
> Strange...
> I always thought that POV-ray's rendered images were always
> exactly the same for the same code, like with 'normal'(?)
> instructions.
> 
> Example: if you calculate tan 1 with a calculator, you
> always
> get the same result: 1.55740772465490223050697480745836(...)
> 
> However, it appears this doesn't work this way for POV-ray
> image output (?).
> I've rendered the same POV code more than once on the same
> computer without using any jittering or crand or any other
> random
> function that I know of. The results were slightly
> different, as
> you can see in the attached image (that's why I posted it in
> this group)
> (close-ups of the rendered images).
> Can anybody tell me why the output is different?
> 
> Just wondering...
> 
> Julius
> 
>  [Image]

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I think Peter just pointed out the answer: AA jitter.

Here come the docs (subject: "jitter", actually about NOT
using jitter for animations):

"Jitter is a very small amount of random ray perturbation
designed to diffuse tiny !!!aliasing!!! errors that might
not otherwise totally disappear, even with intense
anti-aliasing. By randomizing the placement of erroneous
pixels, the error becomes less noticeable to the human eye,
because the eye and mind are naturally inclined to look for
regular patterns rather than random distortions."

Conclusion: there is a random function in AA.
the docs continue:
"For this reason, we should always set jitter to off in area
lights and !!!anti-aliasing!!! options when preparing a
scene for an animation."

Conclusion: beside area lights, jitter is also used in AA.
Problem solved?

Thanks for the many replies

Julius Klatte

>I think it's AA jitter. Set jitter to 0.0 (+j0.0)
regardless of your
>AA setting (turn it off as well). Maybe jitter randomises
the ray
>position within the bounds of the current pixel even though
>anti-aliasing is turned on. Is the opposite explicitly
stated
>anywhere?

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>I don't think this would happen
>because then it would basically undermine the whole idea of
computers being
>accurate

That's why the thing surprised me so much in the first
place...

Julius Klatte

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