POV-Ray : Newsgroups : povray.general : Bright Spot Server Time: 28 May 2020 22:14:41 GMT
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From: jr
Subject: Re: Bright Spot
Date: 23 Dec 2019 08:15:01
Message: <web.5e00771d5bbce4278c662f470@news.povray.org>
hi,

"AndreyG" <nomail@nomail> wrote:
> Hello Bald Eagle,
> Thank you for your comments, this might be of interest only if I manage to model
> physically realistic optical scenes. According to laws on physics, geometrical
> optics in particular.
> So far it does not look like it maybe because I am not using it correctly?

see this discussion concerned with .. optically realistic, includes code:

<http://news.povray.org/595f7b16%241%40news.povray.org>

hth.

> If I specify beamwidth of certain angle it should result in spot of light of
> certain size at certain distance. If angle is changes size of the light spot
> should change.
>
> So far I do not see it in my models.


regards, jr.


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From: Bald Eagle
Subject: Re: Bright Spot
Date: 23 Dec 2019 19:00:00
Message: <web.5e010da95bbce4274eec112d0@news.povray.org>
"AndreyG" <nomail@nomail> wrote:

> Thank you for your comments, this might be of interest only if I manage to model
> physically realistic optical scenes. According to laws on physics, geometrical
> optics in particular.

All I'm saying is that the documentation states certain things about the
behaviour of a spotlight.
Whenever I use a feature and it doesn't do what I expect it to do, I:

Re-read the docs
Model the thing on my own to compare my interpretation with the POV-Ray
implementation
and then try to describe in detail what the problem is

Sometimes, even now, things in the POV-Ray source code still aren't correct.
http://news.povray.org/povray.beta-test.binaries/thread/%3C5daec47c%241%40news.povray.org%3E/

I usually wind up answering my own questions 75% of the time when I start
writing a detailed request for help.


> So far it does not look like it maybe because I am not using it correctly?

I'm not sure - the other thing to do is run a search and see if there are any
other scenes with spotlights that might give a clue.  Sometimes "3rd party"
websites have interesting scenes / blogs / tutorials that are clearer or more
complete.

> If I specify beamwidth of certain angle it should result in spot of light of
> certain size at certain distance. If angle is changes size of the light spot
> should change.
>
> So far I do not see it in my models.

The other thing to do in order to visualize the light source is just add a
vertical "wall" very close to the plane of the spotlight for the cone of light
to shine down along.

As a last resort, sometimes I try to hunt down the section where the feature is
implemented in source code so I can see _exactly_ how it's implemented - I'd
probably look at 3.6 or earlier to start with, as 3.7/3.8 can be really hard to
follow.


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From: Alain Martel
Subject: Re: Bright Spot
Date: 23 Dec 2019 22:50:08
Message: <5e0144a0$1@news.povray.org>
Le 2019-12-22 à 19:01, AndreyG a écrit :
> Thank you everybody for the comments and encouragement. Ok. I accept that: I can
> not see the light only it's reflection I can see.
> I create a mirror (ambient 0 diffuse 0 reflection 1 -perfect mirror?) point
> spotlight to the mirror at 45deg and look at the place where it reflected with
> camera (at -45deg).
> 
> I do indeed see bright circular spot but ...
> I defined spot light with sharp cut-off: radius=falloff= 30(deg)
> With light source being 283(mm) from mirror I expect circle to be 327mm dia
> spot. And spot I see is much narrower.
> What's more, when I change beamwidth of spotlight from 30 to 15 deg, diameter of
> spot depicted by camera does not change.
> https://imgur.com/2ofeH7W
> ?
> I create 2d mirror to reflect the beam of light one more time. I expect to see
> spot twice as wide with intensity diminished 4 times
> Instead i see spot of smaller diameter.
> https://imgur.com/hnjxNTU
> ??
> 
> 

If you want to see the intensity of the light hitting a surface, that 
surface can't be a mirror, or at least, not a good mirror.

What you see in a shiny surface or a mirror is the specular highlight.
The extent of the specular highlight is controlled by the roughness 
parameter. It is never sharp even with ridiculously small values for the 
roughness.

What you want to see is the diffuse illumination of the surface.
Those are two totally different things controlled by different settings.

Use the following texture and you'll see the area illuminated by your 
spotlight clearly (totally replace your current texture with this one) :

texture{
  pigment{rgb 1} // A plain white pigment

  finish{
   diffuse 0.5 // smaller than standard diffuse
   ambient 0   // no ambient
   reflection { 1 }
  // Braces not need here but take the habit of using them.
   specular 1  roughness 0.000001
  // SHARP specular highlight
  }
}
This show that the highlight is different from the illumination.

Note that, as it is, the light coming from the light will not get 
reflected to illuminate something else in your scene.

For that, you need to use photons :
global_settings{
  photons{spacing 0.1} // OR photons{count 1e6} Don't mix.
}

And add this to your mirror :
photons{target reflection on}


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From: Bald Eagle
Subject: Re: Bright Spot
Date: 24 Dec 2019 01:05:00
Message: <web.5e01634c5bbce4274eec112d0@news.povray.org>
Andrey,

I gave it a quick look and I have something simple that looks like it works.

Try this:

#version 3.8;

global_settings {assumed_gamma 1.0}
#include "colors.inc"
//#include "Metals.inc"
//#include "textures.inc"

#declare Height = 10;
#declare Radius = 10;
#declare Falloff = 40;

#if (Falloff < Radius)
 #error "Falloff value must always be larger than the radius value!"
#end

camera {
   location  <0.0, Height, 0.0>
   look_at   <0.0, 0.0,  0.0>
   up y
   right     x*image_width/image_height
}

//#declare Mirror =  box {0, 1 translate -0.5 scale <10, 10, 0.1> texture
{Polished_Chrome} translate z*10}



// create a point "spotlight" (conical directed) light source
light_source {
   <0, 10, 0>
   color rgb <1,1,1>
   spotlight
   point_at <0, 0, 0>
   radius Radius   // angle of the fully illuminated cone.
   tightness 60   // determines the rate of the light falloff
   falloff Falloff   // angle of the umbra cone where the light falls off to
darkness
}

plane {y, 0 pigment {White} finish {diffuse 1}}

// tan theta = WIDTH/adjacent
#declare HalfWidth1 = tan(radians(Radius))*Height/2;
#declare HalfWidth2 = tan(radians(Falloff))*Height/2;
#declare Line = 0.015;

torus {HalfWidth1, Line pigment {Red} finish {emission 1}}
torus {HalfWidth2, Line pigment {Green} finish {emission 1}}


Bill


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