Le Forgeron <jgr### [at] freefr> wrote:



> It might be different if instead of an <area light> object, you used
> a real collection of point lights...

An interesting idea. I'll do an experiment, to compare this to an area light
(of the same number of point lights), to see just what a visual difference
there might be on a scene's illuminated objects, as well as the shadow
qualities produced.

KW

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:

>   The box casts a shadow, which is calculated from the area light
> parameters.

Ah yes. I had neglected to consider that.

>   Whether the objects "see" the light source is determined by the
> shadow calculations, which takes into account area light paramters.
>   Once povray calculates that the object indeed "sees" the light
> source (even if partially) then it calculates the illumination using
> only the point light source part of the light.

That's quite fascinating. So in essence, POV-Ray is performing a sort of
"backwards" calculation when using an area light--the shadow being the
determing factor.
>
>   Btw, as a curiosity: If you are wondering whether it makes any
> considerable difference whether the direct lighting is calculated
> as a point light source or as an area light, the answer is that in
> some scenes it makes a big difference:
>
> http://warp.povusers.org/images/arealight.jpg
> http://warp.povusers.org/images/lightgrid.jpg
>
Thanks for the images.  It does indeed produce a much bigger difference than
I imagined.

Ken

Post a reply to this message

"Kenneth" <kdw### [at] earthlinknet> wrote:
> Le Forgeron <jgr### [at] freefr> wrote:
>
>
>
> > It might be different if instead of an <area light> object, you used
> > a real collection of point lights...
>
> An interesting idea. I'll do an experiment, to compare this to an area light
> (of the same number of point lights), to see just what a visual difference
> there might be on a scene's illuminated objects, as well as the shadow
> qualities produced.
>

Of course, an area light can use jitter, which would automatically make the
shadows (from the area light's discreet point lights) smoother, compared to
using a similar array of "real" point lights. The latter would simply cast a
number of separate, discreet shadows, all slightly displaced.

It would still be interesting to see the difference in the surface
illumination of an object, comparing both methods.  I.e., the area light's
single "point light" direct illumination vs. an array of real point lights.

Here's a really wacky experiment to try (though I haven't done ot yet):
Start with two copies of the same object. Use the area light to create the
shadows on one copy--but use the no_image tag for that object, so it's
invisible.  Then light the 2nd copy with the array of "real" point
lights--but use the no_shadow tag for that one. So what you end up with is
the object lit by multiple real point lights, with the shadow created by
the area light.  The best(?) of both worlds!

KW

Post a reply to this message

Kenneth <kdw### [at] earthlinknet> wrote:
> It would still be interesting to see the difference in the surface
> illumination of an object, comparing both methods.  I.e., the area light's
> single "point light" direct illumination vs. an array of real point lights.

  Didn't I post links to two images showing this difference?

-- 
                                                          - Warp

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> Kenneth <kdw### [at] earthlinknet> wrote:
> > It would still be interesting to see the difference in the surface
> > illumination of an object, comparing both methods.  I.e., the area light's
> > single "point light" direct illumination vs. an array of real point lights.
>
>   Didn't I post links to two images showing this difference?
>

Hmm. Didn't know that for sure at the time (since your discription of the
images didn't give specific details.)  But I stand corrected. I should have
realized this.

Do the white blocks represent the extent of the light array? I suppose it
should be obvious, but I don't want to assume.

KW

Post a reply to this message

> Warp <war### [at] tagpovrayorg> wrote:
>

> >   Btw, as a curiosity: If you are wondering whether it makes any
> > considerable difference whether the direct lighting is calculated
> > as a point light source or as an area light, the answer is that in
> > some scenes it makes a big difference:
> >
> > http://warp.povusers.org/images/arealight.jpg
> > http://warp.povusers.org/images/lightgrid.jpg
> >
> Thanks for the images.  It does indeed produce a much bigger difference than
> I imagined.
>
> Ken

I should have been more detailed in my follow-up to your images. What
strikes me immediately is that lightgrid.jpg (which I take to be an array
of real point lights) produces *much* smoother illumination on the wall and
sphere than does the area light.  Surprisingly so. And the shadows from the
sphere look smoother than I would have imagined. Do you happen to recall
the difference in rendering times of the two images?

KW

Post a reply to this message

Kenneth <kdw### [at] earthlinknet> wrote:
> Do the white blocks represent the extent of the light array? I suppose it
> should be obvious, but I don't want to assume.

  The white box is the same size as the area light / light grid.

-- 
                                                          - Warp

Post a reply to this message

Kenneth <kdw### [at] earthlinknet> wrote:
> I should have been more detailed in my follow-up to your images. What
> strikes me immediately is that lightgrid.jpg (which I take to be an array
> of real point lights) produces *much* smoother illumination on the wall and
> sphere than does the area light.  Surprisingly so.

  Why surprisingly?

> And the shadows from the
> sphere look smoother than I would have imagined.

  Well, there's the same amount of point lights than in the area light.
The shadows should be pretty similar.

> Do you happen to recall
> the difference in rendering times of the two images?

  Nope. I might try to find the original scene and re-render to measure
the times.

-- 
                                                          - Warp

Post a reply to this message

Warp <war### [at] tagpovrayorg> wrote:
> > Do you happen to recall
> > the difference in rendering times of the two images?

>   Nope. I might try to find the original scene and re-render to measure
> the times.

  Rendering at 800x600 +a +am2, the area_light version took 17 seconds
and the lightgrid version 1 min 11 seconds.

-- 
                                                          - Warp

Post a reply to this message

Kenneth nous apporta ses lumieres en ce 21-01-2007 13:00:
> "Kenneth" <kdw### [at] earthlinknet> wrote:
>> Le Forgeron <jgr### [at] freefr> wrote:



>>> It might be different if instead of an <area light> object, you used
>>> a real collection of point lights...
>> An interesting idea. I'll do an experiment, to compare this to an area light
>> (of the same number of point lights), to see just what a visual difference
>> there might be on a scene's illuminated objects, as well as the shadow
>> qualities produced.


> Of course, an area light can use jitter, which would automatically make the
> shadows (from the area light's discreet point lights) smoother, compared to
> using a similar array of "real" point lights. The latter would simply cast a
> number of separate, discreet shadows, all slightly displaced.

> It would still be interesting to see the difference in the surface
> illumination of an object, comparing both methods.  I.e., the area light's
> single "point light" direct illumination vs. an array of real point lights.

> Here's a really wacky experiment to try (though I haven't done ot yet):
> Start with two copies of the same object. Use the area light to create the
> shadows on one copy--but use the no_image tag for that object, so it's
> invisible.  Then light the 2nd copy with the array of "real" point
> lights--but use the no_shadow tag for that one. So what you end up with is
> the object lit by multiple real point lights, with the shadow created by
> the area light.  The best(?) of both worlds!

> KW




With the no_shadow, the lights will illuminate the shadowed area just as if the 
object was not there at all, making the shadow very faint.
What you can do, is to use an aray of smaller area_light. Something like a 3*3 
aray of smaler area_light. That way, you can have very smooth shadows, save 
rendering time and have an illumination and highlights closer to real life. Set 
the specular or phong to be rather large.

-- 
Alain
-------------------------------------------------
Don't cry because it is over, smile because it happened.

Post a reply to this message