POV-Ray : Newsgroups : povray.binaries.images : Questions about photons Server Time
16 Oct 2021 13:09:06 EDT (-0400)
  Questions about photons (Message 1 to 4 of 4)  
From: kurtz le pirate
Subject: Questions about photons
Date: 5 Sep 2021 05:04:39
Message: <61348827$1@news.povray.org>
Hello,

Two questions about the test image attached here.

* Why the beam circled in black is "dimmer" than the others ?
They are all in the same media.


In the general settings, the parameters for the photons are :

   photons {
     count 320000
     max_trace_level 10
     media 500, 2
     }
I'm not sure they are all good

Render Statistics give :
   ---------------------------------------
   Number of photons shot:          170798
   Surface photons stored:             173
   Media photons stored:             44288
   Gather function called:         2634991
   ---------------------------------------

* Where does the difference between the number of photons requested 
(320000) and the number of photons shoted (170798). It's about half



Merci
-- 
Kurtz le pirate
Compagnie de la Banquise


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Attachments:
Download 'visiblelaser_red.jpg' (88 KB)

Preview of image 'visiblelaser_red.jpg'
visiblelaser_red.jpg


 

From: Alain Martel
Subject: Re: Questions about photons
Date: 5 Sep 2021 19:38:11
Message: <613554e3$1@news.povray.org>
Le 2021-09-05 à 05:04, kurtz le pirate a écrit :
> Hello,
> 
> Two questions about the test image attached here.
> 
> * Why the beam circled in black is "dimmer" than the others ?
> They are all in the same media.
> 
> 
> In the general settings, the parameters for the photons are :
> 
>    photons {
>      count 320000
>      max_trace_level 10
>      media 500, 2
>      }
> I'm not sure they are all good
> 
> Render Statistics give :
>    ---------------------------------------
>    Number of photons shot:          170798
>    Surface photons stored:             173
>    Media photons stored:             44288
>    Gather function called:         2634991
>    ---------------------------------------
> 
> * Where does the difference between the number of photons requested 
> (320000) and the number of photons shoted (170798). It's about half
> 
> 
> 
> Merci

In that scene, only the first mirror should be set as target. If there 
is a second target object in a location where no photon can reach it, 
then, the photons destined for that extra target are lost.
If you have two target, one behind the other, and even if the first 
target object is fully transparent, then, that first object will 
intercept and block all the photons shot at the second object.
Also, does the projector object restrict the beam ? That's another thing 
that may cut down the number of photons shot.

Also, if you have a second light, each light will attempt to shot it's 
half of the requested photons. Any light_source that you don't want to 
shot photons absolutely need this photon block :
photons{reflection off refraction off}

ALL light_source except the shadowless ones have this default photons 
block :
photons{reflection on refraction on}



For the dimer part, it's probably related to the media used. Looks like 
you used type 5 with a negative eccentricity, probably between -0.25 and 
-0.5. Try using an isotropic scattering media.
Try using type 1 (isotropic) or 4 (Rayleigh).


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From: kurtz le pirate
Subject: Re: Questions about photons
Date: 11 Sep 2021 03:00:12
Message: <613c53fc$1@news.povray.org>
On 06/09/2021 01:38, Alain Martel wrote:


> In that scene, only the first mirror should be set as target. If there 
> is a second target object in a location where no photon can reach it, 
> then, the photons destined for that extra target are lost.
> If you have two target, one behind the other, and even if the first 
> target object is fully transparent, then, that first object will 
> intercept and block all the photons shot at the second object.

Each  mirror as this settings :
photons { target refraction off reflection on collect off }


> Also, does the projector object restrict the beam ? That's another thing 
> that may cut down the number of photons shot.

No, the light source for the beam is out of the "projector".


> Also, if you have a second light, each light will attempt to shot it's 
> half of the requested photons. Any light_source that you don't want to 
> shot photons absolutely need this photon block :
> photons{reflection off refraction off}

I have an other light source. Kind of "global illumination" for the 
scene. for her  setting is :
photons { refraction off reflection off } media_interaction off

> ALL light_source except the shadowless ones have this default photons 
> block :
> photons{reflection on refraction on}
Yes, there is only one light source for the laser with
    photons { refraction on reflection on }


> For the dimer part, it's probably related to the media used. Looks like 
> you used type 5 with a negative eccentricity, probably between -0.25 and 
> -0.5. Try using an isotropic scattering media.
> Try using type 1 (isotropic) or 4 (Rayleigh).
> 
> 
The global media is defined like this :
box {
  <32, 0, -1>, < -1, 6, 32> // 28>
  hollow
  texture { pigment { color rgbf 1 } }
  interior {
   media {
    scattering {
     1,
     color White
     extinction 0
     }
    method 2 // uniform
    intervals 2
    samples 4
    }
   }
   photons { target }
   }



-- 
Kurtz le pirate
Compagnie de la Banquise


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From: Alain Martel
Subject: Re: Questions about photons
Date: 12 Sep 2021 12:43:23
Message: <613e2e2b$1@news.povray.org>
Le 2021-09-11 à 03:00, kurtz le pirate a écrit :
> On 06/09/2021 01:38, Alain Martel wrote:
> 
> 
>> In that scene, only the first mirror should be set as target. If there 
>> is a second target object in a location where no photon can reach it, 
>> then, the photons destined for that extra target are lost.
>> If you have two target, one behind the other, and even if the first 
>> target object is fully transparent, then, that first object will 
>> intercept and block all the photons shot at the second object.
> 
> Each  mirror as this settings :
> photons { target refraction off reflection on collect off }

The only relevant part is «collect off».

> 
> 
>> Also, does the projector object restrict the beam ? That's another 
>> thing that may cut down the number of photons shot.
> 
> No, the light source for the beam is out of the "projector".

Good.

> 
> 
>> Also, if you have a second light, each light will attempt to shot it's 
>> half of the requested photons. Any light_source that you don't want to 
>> shot photons absolutely need this photon block :
>> photons{reflection off refraction off}
> 
> I have an other light source. Kind of "global illumination" for the 
> scene. for her  setting is :
> photons { refraction off reflection off } media_interaction off

OK. This turn photons shooting off for that light.

> 
>> ALL light_source except the shadowless ones have this default photons 
>> block :
>> photons{reflection on refraction on}
> Yes, there is only one light source for the laser with
>     photons { refraction on reflection on }

That photons block is the default photons block. It's not required.

> 
> 
>> For the dimer part, it's probably related to the media used. Looks 
>> like you used type 5 with a negative eccentricity, probably between 
>> -0.25 and -0.5. Try using an isotropic scattering media.
>> Try using type 1 (isotropic) or 4 (Rayleigh).
>>
>>
> The global media is defined like this :
> box {
>   <32, 0, -1>, < -1, 6, 32> // 28>
>   hollow
>   texture { pigment { color rgbf 1 } }
>   interior {
>    media {
>     scattering {
>      1,
>      color White
>      extinction 0
>      }
>     method 2 // uniform
>     intervals 2
>     samples 4
>     }
>    }
>    photons { target }
>    }
> 
> 
> 

This is not correct : «photons { target }». This may be the reason you 
are missing about half your photons. The light_source is attempting to 
shoot half of the photons at the media box.

It should be «photons { pass_through }» if the light is outside the 
media box. No need for a photons block is the light is inside the media box.

Using method 2, you may not have enough intervals, or samples. This may 
cause the sampling to miss half of the beam. Try using intervals 3 or 4. 
OR, you may not have enough samples. Try samples 6, 7 or 8.

You may try method 3 instead, without the intervals statement, and 
increase samples to about 10 (default value for method 3).


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