POV-Ray : Newsgroups : povray.general : Focal Blur banding? Server Time
26 Nov 2022 03:23:44 EST (-0500)
  Focal Blur banding? (Message 1 to 4 of 4)  
From: Shinzo
Subject: Focal Blur banding?
Date: 27 Oct 2022 10:35:00
Message: <web.635a963e10de3b3fdcd06a839b63754b@news.povray.org>
Hello everybody. I just rendered the "chess2.pov" from the
"advanced"-example-directory and noticed a very strong banding effect on the
chess pieces in front of the camera. Since I know a little bit about modern
raytracing (I have written some pathtracers for personal use) I asked myself
what's wrong here since raytraced DOF should NEVER lead to a banding-effect.

My guess is that POVRay calculates the lense sample positions just once for the
whole image instead of resampling the lens positions for every pixel.

If that is the case you might want to change that as it is really sad to see
those artefacts from Scanline renderers of the 80ties in a real raytracer.

BTW.: Have you thought about changing the RandomNumberGenerator? It seems that
sometimes in special cases there appear sampling artefacts (Hyperplane
artefacts) that seem to come from an Linear Congruential Generator (LCG). If
POVRay uses an LCG-Generator you should think about exchanging it to an
LCG/XORSHIFT combination or at least to a generator that uses a combination of
linear-generation and non-linear-generation.


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From: Alain Martel
Subject: Re: Focal Blur banding?
Date: 27 Oct 2022 12:35:08
Message: <635ab33c$1@news.povray.org>
Le 2022-10-27 à 10:31, Shinzo a écrit :
> Hello everybody. I just rendered the "chess2.pov" from the
> "advanced"-example-directory and noticed a very strong banding effect on the
> chess pieces in front of the camera. Since I know a little bit about modern
> raytracing (I have written some pathtracers for personal use) I asked myself
> what's wrong here since raytraced DOF should NEVER lead to a banding-effect.
> 
> My guess is that POVRay calculates the lense sample positions just once for the
> whole image instead of resampling the lens positions for every pixel.
> 
> If that is the case you might want to change that as it is really sad to see
> those artefacts from Scanline renderers of the 80ties in a real raytracer.
> 
> BTW.: Have you thought about changing the RandomNumberGenerator? It seems that
> sometimes in special cases there appear sampling artefacts (Hyperplane
> artefacts) that seem to come from an Linear Congruential Generator (LCG). If
> POVRay uses an LCG-Generator you should think about exchanging it to an
> LCG/XORSHIFT combination or at least to a generator that uses a combination of
> linear-generation and non-linear-generation.
> 
> 

The sampling is done at the pixel level. And, if antialiasing is in 
effect, individually for each sub-pixel when antialiasing get triggered.
You may :
Try slightly increase blur_samples. Increase gradually, no benefit in 
taking 50 samples when 20 would be enough.
Try adding a minimum value for the blur_samples : blur_samples 9, 20
Try increasing the confidence value (default to 0.9) Max value =1.
Try decreasing the variance value (default 1/128) MUST be >0.


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From: tTh
Subject: Re: Focal Blur banding?
Date: 27 Oct 2022 14:40:45
Message: <635ad0ad$1@news.povray.org>
On 10/27/22 16:31, Shinzo wrote:

> BTW.: Have you thought about changing the RandomNumberGenerator? It seems that
> sometimes in special cases there appear sampling artefacts (Hyperplane
> artefacts) that seem to come from an Linear Congruential Generator (LCG). If
> POVRay uses an LCG-Generator you should think about exchanging it to an
> LCG/XORSHIFT combination or at least to a generator that uses a combination of
> linear-generation and non-linear-generation.

    Please, don't change the actual random generator, just add a
    new one if needed.

tTh

-- 
-- -- -- -- -- http://la.buvette.org/POV/Inside/making-off.html


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From: jr
Subject: Re: Focal Blur banding?
Date: 3 Nov 2022 08:20:00
Message: <web.6363b1305fe6b371c0ce110b6cde94f1@news.povray.org>
hi,

"Shinzo" <zza### [at] gmxde> wrote:
> ...
> My guess is that POVRay calculates the lense sample positions just once for the
> whole image instead of resampling the lens positions for every pixel.

it's open source, so no guessing needed.  ;-)


> If that is the case you might want to change that as it is really sad to see
> those artefacts from Scanline renderers of the 80ties in a real raytracer.
>
> BTW.: Have you thought about changing the RandomNumberGenerator? It seems that
> sometimes in special cases there appear sampling artefacts (Hyperplane
> artefacts) that seem to come from an Linear Congruential Generator (LCG). If
> POVRay uses an LCG-Generator you should think about exchanging it to an
> LCG/XORSHIFT combination or at least to a generator that uses a combination of
> linear-generation and non-linear-generation.

more (unnecessary :-)) guess-work.  I've no idea of POV-Ray's "innards" but am
sure that if you were to post one or more patches providing (measurable)
improvement(s), they'd be looked at + considered.


regards, jr.


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