POV-Ray : Newsgroups : povray.general : Bright Spot Server Time: 28 May 2020 22:47:37 GMT
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From: Bald Eagle
Subject: Re: Bright Spot
Date: 22 Dec 2019 17:30:01
Message: <web.5dffa8005bbce4274eec112d0@news.povray.org>
> Op 22/12/2019 om 02:14 schreef AndreyG:

> > So you are saying in real life if I place glass perpendicularly to Sun rays and
> > camera axis will be perpendicular to glass surface there always be reflection?

Andrey,


Of course. The closer you are to a perfect 180-degree reflection, the brighter
it will be.  at exactly 90-degrees, we would expect there to be zero reflection.

Take a look at the basic Phong reflection model, and other to get an idea about
how it all works.

https://infogalactic.com/info/Specular_reflection
https://infogalactic.com/info/Phong_reflection_model

http://news.povray.org/povray.binaries.images/attachment/%3Cweb.57cb6721f854a3db5e7df57c0%40news.povray.org%3E/phong_mo
del.png




> > I make simplest geometry: camera and spotlight.
> > Spotlight points at camera, camera looks at spotlight.
> > For whatever reason camera does not see the light.

There is no light.   There's also no camera and no objects - there is only math.

https://www.youtube.com/watch?v=uAXtO5dMqEI

Pov-Ray will ultimately interpret and render the "light" as a color closer to
whatever color the light source is defined as, but I do believe that Thomas
correctly surmised that POV-Ray needs that light to be reflected from an object
first.  Always try to have at least one object in your scene.

I see you're using version 3.7
I'd recommend using version 3.8, as there have been many bug fixes, and you will
be off to a better and more reliable start if you begin your learning with the
latest version.

The best thing to do would be to construct a standard scene for you to begin
experimenting with.  Then there will be far fewer unanticipated problems for you
to deal with when trying to render a real scene.

Learning how to use some of the conditional statements and features such as
sky_sphere will help you better grasp the effect that certain things have on how
things look.

Learn to define constants that can be used for things like colors and locations,
etc.  This will make writing scenes and debugging, and making changes later on
much much easier.


Welcome, and we hope you have lots of fun and enjoyment.  :)

Bill


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From: Alain Martel
Subject: Re: Bright Spot
Date: 22 Dec 2019 18:13:44
Message: <5dffb258$1@news.povray.org>
Le 2019-12-20 à 20:31, AndreyG a écrit :
> I image wood textured surface covered with glass.
> Camera positioned perpendicular to imaged surface
> Illumination done using Parallel light source.
> Light falls perpendicular to imaged surface.
> When there is no glass camera shows image with uniformal brightness across the
> field â what I would expect.
> When image overlaid with glass camera depicts bright spot right in the centre of
> the image.
> As it is shown here:
> https://imgur.com/a/xzflV2T
> 
> What causes bright spot? Is it reflection of light source? But there is no
> source, all light rays a parallel and fall perpendicularly to the surface. What
> causes bright spot?
> 
> 

The finish of your glass have specular of phong highlight enabled.
The light is still a point, it's just that the light it emit is assumed 
to always have the same direction.

You can shift the light so that the highlight is outside the visible 
area, or remove the phong or specular components from the finish.



Alain


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From: Alain Martel
Subject: Re: Bright Spot
Date: 22 Dec 2019 18:21:07
Message: <5dffb413$1@news.povray.org>
Le 2019-12-21 à 20:14, AndreyG a écrit :
> Thank you for your explanation Thomas,
> 
> So you are saying in real life if I place glass perpendicularly to Sun rays and
> camera axis will be perpendicular to glass surface there always be reflection?
> (Can not do experiment it is raining for a week in Vancouver! :).

In the case where the Sun reflect in the glass, you will have a very 
bright spot, even with the light coming from the Sun is essentially 
parallel.
The result will be similar to your render, just a much brighter spot.


> 
> I am total newbe with Povray trying to make sense of it.
> I have another question can not grasp.
> I make simplest geometry: camera and spotlight.
> Spotlight points at camera, camera looks at spotlight.
> For whatever reason camera does not see the light.
> https://imgur.com/a/xeJ3Ykm
> Can't find what i am doing wrong.
> 
> 

You did not do anything wrong. The lights are not actual objects, just 
locations from where light come from. You can NEVER see any light directly.

If you want to have something visible at the light's location, you need 
to place an actual object there. The «looks_like» feature does exactly that.
The origin of a looks_like object is the associated light's location, 
not the scene's origin.
That object have an automatic no_shadow attribute.



Alain


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From: AndreyG
Subject: Re: Bright Spot
Date: 23 Dec 2019 00:05:01
Message: <web.5e0003ee5bbce4278dfadf040@news.povray.org>
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
??


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From: Bald Eagle
Subject: Re: Bright Spot
Date: 23 Dec 2019 02:05:01
Message: <web.5e0020045bbce4274eec112d0@news.povray.org>
"AndreyG" <nomail@nomail> wrote:
> 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).

Excellent.  Good experiment.

I haven't had a chance to look it over in detail, and I may not be able to for a
while.

But what i would suggest is to model what you think it ought to be with
cylinders, cones, tori, whatever lets you "draw" the result without interfering
with the scene - and then see how it compares.

If you put all of those objects into a union with "no_shadow", that will help a
lot.

http://wiki.povray.org/content/Documentation:Tutorial_Section_2.1#The_Spotlight_Source


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From: AndreyG
Subject: Re: Bright Spot
Date: 23 Dec 2019 03:40:01
Message: <web.5e0035f55bbce4278dfadf040@news.povray.org>
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?
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.


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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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