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From: Paul Bourke
Subject: Colour from single trace
Date: 27 Oct 2018 11:20:00
Message: <web.5bd4490aa2d1b6e0b912d9b40@news.povray.org>
I am aware of the trace macro that returns the intersection point and normal at
that point on a object given a ray (point and direction). But is there a way to
get the colour?


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From: Bald Eagle
Subject: Re: Colour from single trace
Date: 27 Oct 2018 14:05:01
Message: <web.5bd46f8f306629fe458c7afe0@news.povray.org>
"Paul Bourke" <pau### [at] gmailcom> wrote:
> I am aware of the trace macro that returns the intersection point and normal at
> that point on a object given a ray (point and direction). But is there a way to
> get the colour?

Hi Paul!
Good to see you on here.

In functions.inc, there is eval_pigment which returns the pigment at any given
vector location.

Pre defined functions
eval_pigment(Pigm, Vect): This macro evaluates the color of a pigment at a
specific point. Some pigments require more information than simply a point,
slope pattern based pigments for example, and will not work with this macro.
However, most pigments will work fine.

Parameters:

Vect = The point at which to evaluate the pigment.
Pigm = The pigment to evaluate.


Thanks for all of the information you've posted over the years - it's been a
great help as well as an inspiration.


Bill Walker


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From: clipka
Subject: Re: Colour from single trace
Date: 28 Oct 2018 11:25:14
Message: <5bd59c9a$1@news.povray.org>
Am 27.10.2018 um 13:16 schrieb Paul Bourke:
> I am aware of the trace macro that returns the intersection point and normal at
> that point on a object given a ray (point and direction). But is there a way to
> get the colour?

If by "colour" you mean apparent colour (as it would appear in the 
output image), the answer is a clear "nay".

If by "colour" you mean pigment, the answer is "well, sort of"; you can 
use the `eval_pigment` function Bald Eagle mentioned already, but you'll 
have to separately keep track of transformations and stuff, POV-Ray 
doesn't do that for you. Also, `eval_pigment` doesn't work with pigments 
that depend on properties of the object surface, the ray, or interaction 
betweent the two, such as pigments using the `slope` or `aoi` patterns.


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From: Kenneth
Subject: Re: Colour from single trace
Date: 28 Oct 2018 16:55:05
Message: <web.5bd5e8c7306629fe43b397d0@news.povray.org>
eval_pigment returns the 'linear' color of an object, not a gamma-bent color
(i.e., not at gamma 2.2.) I.e., it returns an rgb color, not an srgb one. If the
*original* color of your object was chosen to be srgb <...> rather than rgb, and
you want to use the returned srgb equivalent in your scene or for some other
purpose, the result should be used as  srgb <0.2,0.5,0.75>, for example, not rgb
<...>.

But if your original color was  rgb <...>, then eval_pigment will return that
same color.

As Clipka noted, eval_pigment doesn't take into account the *illumination* of
the object-- lighting, shadow, phong, etc--, just the basic strict color found
there.


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From: Alain
Subject: Re: Colour from single trace
Date: 29 Oct 2018 01:18:01
Message: <5bd65fc9$1@news.povray.org>
Le 18-10-28 à 12:50, Kenneth a écrit :
> eval_pigment returns the 'linear' color of an object, not a gamma-bent color
> (i.e., not at gamma 2.2.) I.e., it returns an rgb color, not an srgb one. If the
> *original* color of your object was chosen to be srgb <...> rather than rgb, and
> you want to use the returned srgb equivalent in your scene or for some other
> purpose, the result should be used as  srgb <0.2,0.5,0.75>, for example, not rgb
> <...>.
> 
> But if your original color was  rgb <...>, then eval_pigment will return that
> same color.
> 
> As Clipka noted, eval_pigment doesn't take into account the *illumination* of
> the object-- lighting, shadow, phong, etc--, just the basic strict color found
> there.
> 
> 
> 
> 

Also, it probably won't work for UV mapped textures.


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From: Norbert Kern
Subject: Re: Colour from single trace
Date: 29 Oct 2018 20:20:00
Message: <web.5bd76b15306629fe9487b4210@news.povray.org>
"Paul Bourke" <pau### [at] gmailcom> wrote:
> I am aware of the trace macro that returns the intersection point and normal at
> that point on a object given a ray (point and direction). But is there a way to
> get the colour?


What do you have in hands to determine the intersection point?

Personally I love the „Illusion“ code of Rune and its possibilities. Perhaps
there lies a solution for some of the mentioned limitations - in combination
with a mesh camera.


Norbert


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From: And
Subject: Re: Colour from single trace
Date: 10 Nov 2018 04:25:01
Message: <web.5be65c9a306629fefd114ee40@news.povray.org>
"Paul Bourke" <pau### [at] gmailcom> wrote:
> I am aware of the trace macro that returns the intersection point and normal at
> that point on a object given a ray (point and direction). But is there a way to
> get the colour?

I want this function, too. But I want it for UV map...


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From: Le Forgeron
Subject: Re: Colour from single trace
Date: 11 Nov 2018 08:43:24
Message: <5be7ebac$1@news.povray.org>
Le 10/11/2018 à 05:20, And a écrit :
> "Paul Bourke" <pau### [at] gmailcom> wrote:
>> I am aware of the trace macro that returns the intersection point and normal at
>> that point on a object given a ray (point and direction). But is there a way to
>> get the colour?
> 
> I want this function, too. But I want it for UV map...
> 
> 
UV mapping of object does make sense (in term of trace), but I'm still
perplex about getting the colour...

There could be more than a single texture (layered), or a complex
texture, and there is finish too which can interfere (a white pigment
with a mirror finish reflecting a complex environment).

So, when asking for "colour" at intersection point, what is expected ?
#1: the colour (<r,g,b>) of the point as it would appear on a rendered
image (e.g. a white sphere in the dark would return black)
#2: the intrinsect colour(<r,g,b>) of the object at that point (e.g. a
white sphere in the dark would return white)

(Yes, I'm repeating C.Lipka's question, because it went unanswered)


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From: Stephen
Subject: Re: Colour from single trace
Date: 11 Nov 2018 10:02:57
Message: <5be7fe51$1@news.povray.org>
On 10/11/2018 04:20, And wrote:

On 11/11/2018 08:43, Le_Forgeron wrote:
>> I want this function, too. But I want it for UV map...
>>
>>
> UV mapping of object does make sense (in term of trace), but I'm still
> perplex about getting the colour...
> 
> There could be more than a single texture (layered), or a complex
> texture, and there is finish too which can interfere (a white pigment
> with a mirror finish reflecting a complex environment).
> 
> So, when asking for "colour" at intersection point, what is expected ?
> #1: the colour (<r,g,b>) of the point as it would appear on a rendered
> image (e.g. a white sphere in the dark would return black)
> #2: the intrinsect colour(<r,g,b>) of the object at that point (e.g. a
> white sphere in the dark would return white)
> 

I’ve thought about this in the past.
I think that your option #1 is too complicated. It seems to me that it 
would involve a secondary raytracing operation with maybe a limit to the 
depth of reflective and/or refractive bounces. (Sorry, I don’t have the 
right vocabulary.)
Option #2 is closer to what I would find useful. If “trace” also 
returned the RGB* value of the uv image map at the point of intersection 
of the object by trace. That would be usable.
* Also filter and transmit values, if possible.


> (Yes, I'm repeating C.Lipka's question, because it went unanswered)

Your option #2 opens up the discussion. Christoph's answer on first 
reading is too similar to what every other reply to this question has 
been. Which is: No, nay, never.


-- 

Regards
     Stephen


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From: Thomas de Groot
Subject: Re: Colour from single trace
Date: 11 Nov 2018 12:20:30
Message: <5be81e8e$1@news.povray.org>
On 27-10-2018 13:16, Paul Bourke wrote:
> I am aware of the trace macro that returns the intersection point and normal at
> that point on a object given a ray (point and direction). But is there a way to
> get the colour?
> 
> 

That would be nice to have. IT can of course be done now with a 
combination of (1) the trace macro, and (2) the eval_pigment() function.

-- 
Thomas


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