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From: AndreyG
Subject: Bright Spot
Date: 21 Dec 2019 01:35:00
Message: <web.5dfd75fdec9eb23a8dfadf040@news.povray.org>
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  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?


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From: Thomas de Groot
Subject: Re: Bright Spot
Date: 21 Dec 2019 12:08:07
Message: <5dfe0b27$1@news.povray.org>
Op 21/12/2019 om 02:31 schreef AndreyG:
> 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?
> 
> 

It is the reflection of the light source. In your case, I assume you have:

light_source {
   <0, 0, 0>
   color rgb YourColor
   translate YourLightPosition
   parallel
}

The term parallel does not mean that the light source becomes a wide, 
flat, surface. It remains a point light and is reflected as such. 
Parallel only means that the incident light rays on the surface from a 
very distant source are parallel, like for instance Sun light. See:

http://wiki.povray.org/content/Reference:Light_Source#Parallel_Lights

-- 
Thomas


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From: AndreyG
Subject: Re: Bright Spot
Date: 22 Dec 2019 01:15:01
Message: <web.5dfec36c5bbce4278dfadf040@news.povray.org>
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! :).

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.


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From: Thomas de Groot
Subject: Re: Bright Spot
Date: 22 Dec 2019 07:59:22
Message: <5dff225a$1@news.povray.org>
Op 22/12/2019 om 02:14 schreef AndreyG:
> 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! :).

Real life is not raytracing! and I cannot answer you for the real life 
situation (I leave that to my betters). But I guess that you better not 
look into the glass in such a set-up if you want to keep your vision 
intact. ;-)

> 
> 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.
> 

Again, raytracing does not work exactly like real life. A spotlight or 
any light in POV-Ray, is not physically "shinning" like a lamp, it needs 
an object to shine upon, and a camera, by itself, is not an "object" 
either. Well, I hope I tell this right; if not, someone will correct me 
I am sure.

Try this:

//--------------------------------------------------------
#version 3.7;

global_settings {
   assumed_gamma 1.0
}

// perspective (default) camera
camera {
   location  <0.0, 0.0, -10.0>
   look_at   <0.0, 0.0,  0.0>
   right     x*image_width/image_height
   angle 70
}

// create a point "spotlight" (conical directed) light source
light_source {
   <200, 100, 100>
   color rgb <1,1,1>
   spotlight
   point_at <0, 0, 0>
   radius 50
   tightness 5
   falloff 8
}

// linear prism in z-direction: from ,to ,number of points (first = last)
prism {
   -1.00 ,1.00 , 6
   <-1.00, 0.00>,  // first point
   < 1.00, 0.00>,
   < 1.00, 1.00>,
   < 0.00, 2.00>,
   <-1.00, 1.00>,
   <-1.00, 0.00>   // last point = first point!!!!
   rotate<-90,0,0> scale<1,1,-1> //turns prism in z direction! Don't 
change this line!

   texture {pigment{ color rgb 1 }}

   scale <1.00,1.00,1.00>
   rotate <0,30,0>
   translate <0.00,0.00,0.00>
} // end of prism //--------------------------------------------------------

If you are new to POV-Ray, I suggest you read carefully the 
documentation (I know! it is huge but it is worthwhile!) and play with 
the many example scenes given. You will discover that a new world and 
new insights will be revealed to you :-)

http://wiki.povray.org/content/Main_Page

And of course, do not hesitate to call here. We are a helpful bunch of 
guys/girls (not heard much of the latter unfortunately...)

-- 
Thomas


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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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