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Hello guys,
i came around the documentation site
http://www.povray.org/documentation/view/3.6.1/348/
and it says: "Adding reflection to a texture makes it take longer to render
because an additional ray must be traced."
I am wondering why it would take longer if i specify "reflection {1}". I always
thought that you are then able to instantly calculate in which direction the ray
must go and then just take the first objects color which is hit by the reflected
ray. If you have some diffuse object then you must send out many rays in
different directions to find out the color at the specific point and even then
it can be inaccurate.
Did i get something wrong?
Thank you very much.
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Am 20.09.2012 15:36, schrieb Dabear:
> Hello guys,
>
> i came around the documentation site
> http://www.povray.org/documentation/view/3.6.1/348/
> and it says: "Adding reflection to a texture makes it take longer to render
> because an additional ray must be traced."
> I am wondering why it would take longer if i specify "reflection {1}". I always
> thought that you are then able to instantly calculate in which direction the ray
> must go and then just take the first objects color which is hit by the reflected
> ray. If you have some diffuse object then you must send out many rays in
> different directions to find out the color at the specific point and even then
> it can be inaccurate.
>
> Did i get something wrong?
Yes, you did: Diffuse surfaces are, by default, not computed by shooting
hundreds or thousands of secondary rays around the scene, but by
shooting rays towards the light sources only in order to determine
shadowing. (A reflective surface just takes a single secondary ray, but
that one will typically hit a diffuse surface, so the diffuse stuff is
virtually always there.)
If you use radiosity, then for diffuse surfaces POV-Ray does indeed
shoot hundreds or thousands of secondary rays around the scene, but only
for some samples on the surface, which are then weighted and averaged
for any points in between.
(And yes, radiosity is still slower than reflections indeed.)
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From: Warp
Subject: Re: Performance and inner workings of reflection
Date: 20 Sep 2012 11:42:24
Message: <505b3960@news.povray.org>
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Dabear <nomail@nomail> wrote:
> i came around the documentation site
> http://www.povray.org/documentation/view/3.6.1/348/
> and it says: "Adding reflection to a texture makes it take longer to render
> because an additional ray must be traced."
> I am wondering why it would take longer if i specify "reflection {1}". I always
> thought that you are then able to instantly calculate in which direction the ray
> must go and then just take the first objects color which is hit by the reflected
> ray.
It's slower because now you have to trace two rays instead of one (which
would be the case when there's no reflection).
--
- Warp
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> > Did i get something wrong?
>
> Yes, you did: Diffuse surfaces are, by default, not computed by shooting
> hundreds or thousands of secondary rays around the scene, but by
> shooting rays towards the light sources only in order to determine
> shadowing. (A reflective surface just takes a single secondary ray, but
> that one will typically hit a diffuse surface, so the diffuse stuff is
> virtually always there.)
>
> If you use radiosity, then for diffuse surfaces POV-Ray does indeed
> shoot hundreds or thousands of secondary rays around the scene, but only
> for some samples on the surface, which are then weighted and averaged
> for any points in between.
>
> (And yes, radiosity is still slower than reflections indeed.)
Very good answer, now i got it! Thank you, both of you.
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