I was just looking around www.digitalblasphemy.com and looking at all the
pictures he has with rays of light shooting through trees.

Now, I'm aware that other programs like lightwave are able to create this
effect much faster than POV-Ray does. But how do they do it? I'd be
interested in reading a bit about the algorithm used. Obviously it's not
just sampling a ray at intervals, because that would be as slow as POV-Ray
is about it. Does anyone know how it's done with other programs?

They must be storing information about the media or something. But still...

And is there a reason why POV-Ray doesn't implement one of these other
techniques?

 - Slime
[ http://www.slimeland.com/ ]

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Slime wrote:
> I was just looking around www.digitalblasphemy.com and looking at all the
> pictures he has with rays of light shooting through trees.
>
> Now, I'm aware that other programs like lightwave are able to create this
> effect much faster than POV-Ray does. But how do they do it? I'd be
> interested in reading a bit about the algorithm used. Obviously it's not
> just sampling a ray at intervals, because that would be as slow as POV-Ray
> is about it. Does anyone know how it's done with other programs?
>

I suppose their media can only have constant density and it only works with
meshes, am I right?
Otherwise I wouldn't see a way of doing it that much faster than POV-Ray's
media.
I saw this at a friend of mine who has Cinema 4D and asked him if he could
assign a pattern to the density of the media and he said no. He could only
turbulate it.

Simon

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In article <3d2f1e0c@news.povray.org>, "Slime" <slm### [at] slimelandcom> 
wrote:

> Now, I'm aware that other programs like lightwave are able to create this
> effect much faster than POV-Ray does. But how do they do it? I'd be
> interested in reading a bit about the algorithm used. Obviously it's not
> just sampling a ray at intervals, because that would be as slow as POV-Ray
> is about it. Does anyone know how it's done with other programs?

As you said, they don't sample a ray. Most likely, they use a scanline 
method, projecting layers of media onto the screen to build up an 
additive media effect.
Also, those programs are able to use assembly level optimizations and 
SIMD instructions, which POV can't take advantage of.


> And is there a reason why POV-Ray doesn't implement one of these other
> techniques?

POV-Ray is a raytracer, not a scanline renderer. (Persistance Of Vision 
*Raytracer*)

-- 
Christopher James Huff <chr### [at] maccom>
POV-Ray TAG e-mail: chr### [at] tagpovrayorg
TAG web site: http://tag.povray.org/

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On Fri, 12 Jul 2002 14:20:58 -0400, "Slime" <slm### [at] slimelandcom> wrote:

>Now, I'm aware that other programs like lightwave are able to create this
>effect much faster than POV-Ray does. But how do they do it?

It's simple - they construct a separate shadow map for media for every
scanline by projecting the meshes onto the plane defined by the
scanline and the camera origin using the light source as a projection
origin (multiple overlayed shadow maps merged into one in the case of
multiple light sources).


Peter Popov ICQ : 15002700
Personal e-mail : pet### [at] vipbg
TAG      e-mail : pet### [at] tagpovrayorg

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> It's simple - they construct a separate shadow map for media for every
> scanline by projecting the meshes onto the plane defined by the
> scanline and the camera origin using the light source as a projection
> origin (multiple overlayed shadow maps merged into one in the case of
> multiple light sources).


Ah! That makes sense! If I'm understanding correctly.

Too bad POV-Ray can't do this.

 - Slime
[ http://www.slimeland.com/ ]

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> I suppose their media can only have constant density and it only works
with
> meshes, am I right?
> Otherwise I wouldn't see a way of doing it that much faster than POV-Ray's
> media.
> I saw this at a friend of mine who has Cinema 4D and asked him if he could
> assign a pattern to the density of the media and he said no. He could only
> turbulate it.


Mmm. Well, that's not really the worst restriction in the world. It still
allows for large areas of fog, which is what you really need the speed for,
usually.

 - Slime
[ http://www.slimeland.com/ ]

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> > And is there a reason why POV-Ray doesn't implement one of these other
> > techniques?
>
> POV-Ray is a raytracer, not a scanline renderer. (Persistance Of Vision
> *Raytracer*)


Yes... I figured it would probably have to do with the scanline method these
programs use, though I was still unsure how they did it using that method.

 - Slime
[ http://www.slimeland.com/ ]

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