How come your lens effects are attenuating my background scattering
media into oblivion ?





I have two very small objects say x*.2 y*.05 in the xy plane which are
opposing each other translated -x +x a wee bit. In between them is a
light source and lens effect centered at the origin. Behind this is a
spot light about 50 units in the +z direction pointing back towards
the camera's location. Because of the scale of the objects the camera
is necessarily close i.e. z*-.2. I'm presuming that it is your effects
and the scale of my scene that is causing my beautiful scattering media
to look like randomly placed white spots.

Solution or live with it ?

-- 
Ken Tyler -  1200+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html http://www.povray.org/links/

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Ken <tyl### [at] pacbellnet> wrote:
:  How come your lens effects are attenuating my background scattering
: media into oblivion ?

  Hard to say anything without seeing the code. Perhaps it has something to
do with max_trace_level? (Even an advanced user can fall in that sometimes...
I do :) ).

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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On Thu, 02 Dec 1999 21:49:45 -0800, Ken wrote:

>Solution or live with it ?

First, make sure the discs the lens effects plugin puts in front of
the camera are specified as hollow.  That does make a difference,
even for patch objects.  Unfortunately, it means you'll have to edit
Chris's masterwork if they're not.

Second, you might need to add some transparent bounds to force the
interval that passes through your scattering media to be shorter, 
or you may need to increase the number of intervals (as a last 
resort.)  I usually see this effect when the interval passing
through the media is much larger than the media effect itself.  It 
is especially prominent when you specify atmospheric media.

If your media is forward-scattering only, you can get good effects
by adding a plane parallel to the camera plane just beyond the light
source.  If it's backward-scattering only, add such a plane just 
before the light source and another one several units beyond the 
light source (depending on scale.)  If it's both, add a plane at
the light source and another one beyond.  The idea is to force an
interval to evaluate over the part of the media that has the most
contribution to the scene.

-- 
These are my opinions.  I do NOT speak for the POV-Team.
The superpatch: http://www2.fwi.com/~parkerr/superpatch/
My other stuff: http://www2.fwi.com/~parkerr/traces.html

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Are you sure the lens effects are to blame? I sometimes get the white spots when
the light source is in line with the camera vector (i.e. the camera is looking
straight into the light)
You could try bounding the media and increasing intervals/samples. And if all
else fails and the LF objects cause the problem, you could use <shudder> post
processing. Let's be honest, it's a fake lens flare either way.

Margus

Ken wrote:
> 
>  How come your lens effects are attenuating my background scattering
> media into oblivion ?
> 
> I have two very small objects say x*.2 y*.05 in the xy plane which are
> opposing each other translated -x +x a wee bit. In between them is a
> light source and lens effect centered at the origin. Behind this is a
> spot light about 50 units in the +z direction pointing back towards
> the camera's location. Because of the scale of the objects the camera
> is necessarily close i.e. z*-.2. I'm presuming that it is your effects
> and the scale of my scene that is causing my beautiful scattering media
> to look like randomly placed white spots.
> 
> Solution or live with it ?
> 
> --
> Ken Tyler -  1200+ Povray, Graphics, 3D Rendering, and Raytracing Links:
> http://home.pacbell.net/tylereng/index.html http://www.povray.org/links/

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Margus Ramst wrote:
> 
> Are you sure the lens effects are to blame? I sometimes get the white spots when
> the light source is in line with the camera vector (i.e. the camera is looking
> straight into the light)

The media works perfectly with the absence of the lens effects.

> You could try bounding the media and increasing intervals/samples. And if all
> else fails and the LF objects cause the problem, you could use <shudder> post
> processing. Let's be honest, it's a fake lens flare either way.

I will NOT cheat. Besides I don't have a lens effects plug in handy.

-- 
Ken Tyler

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Nieminen Juha wrote:
> 
> Ken <tyl### [at] pacbellnet> wrote:
> :  How come your lens effects are attenuating my background scattering
> : media into oblivion ?
> 
>   Hard to say anything without seeing the code. Perhaps it has something to
> do with max_trace_level? (Even an advanced user can fall in that sometimes...
> I do :) ).

It had no effect except to raise the render time.

-- 
Ken Tyler

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Ron Parker wrote:
> 
> On Thu, 02 Dec 1999 21:49:45 -0800, Ken wrote:
> 
> >Solution or live with it ?
> 
> First,
> 
> Second,

I will ponder your words of wisdom.

-- 
Ken Tyler

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Ken <tyl### [at] pacbellnet> (yes, that Ken!) wrote in a rather
confrontational tone:
>  How come your lens effects are attenuating my background scattering
> media into oblivion ?
>
> I have two very small objects say x*.2 y*.05 in the xy plane which are
> opposing each other translated -x +x a wee bit. In between them is a
> light source and lens effect centered at the origin. Behind this is a
> spot light about 50 units in the +z direction pointing back towards
> the camera's location. Because of the scale of the objects the camera
> is necessarily close i.e. z*-.2. I'm presuming that it is your effects
> and the scale of my scene that is causing my beautiful scattering
media
> to look like randomly placed white spots.
>
> Solution or live with it ?

To be honest I don't have that much experience with scattering media.
Still, I would guess that the problem is not so much with the scale of
the scene as the scale of the lens effects, which appear to be altering
the way the media intervals are evaluated (to make the effects overlay
the rest of the scene the objects involved are scaled down to be 1e-2
units from the camera).

My first suggestion would be to set the effect_always_on_top option to
false: this will create the lens effect as part of the scene, with the
objects actually centred at the effect location.  If this seems to help,
but you want the effects overlaying the other objects you can either
leave the always on top option at false and change the effect location,
or set the always on top option to true and adjust the lens_scale option
(start at, say, 10 and work your way up...).

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Ron Parker <ron### [at] povrayorg> wrote:
> First, make sure the discs the lens effects plugin puts in front of
> the camera are specified as hollow.  That does make a difference,
> even for patch objects.  Unfortunately, it means you'll have to edit
> Chris's masterwork if they're not.

All lens effects are indeed set to be hollow and not to cast shadows,
precisely so the lens effects can be used with POV-Ray's various
atmospheric features.

> Second, you might need to add some transparent bounds to force the
> interval that passes through your scattering media to be shorter,
> or you may need to increase the number of intervals (as a last
> resort.)  I usually see this effect when the interval passing
> through the media is much larger than the media effect itself.  It
> is especially prominent when you specify atmospheric media.
>
> If your media is forward-scattering only, you can get good effects
> by adding a plane parallel to the camera plane just beyond the light
> source.  If it's backward-scattering only, add such a plane just
> before the light source and another one several units beyond the
> light source (depending on scale.)  If it's both, add a plane at
> the light source and another one beyond.  The idea is to force an
> interval to evaluate over the part of the media that has the most
> contribution to the scene.

As I replied to Ken, I suspect it may have something to do with the
distances involved in the creation of the lens effects...bounding the
media may well be part of the solution.

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Ken <tyl### [at] pacbellnet> demanded to know:
> How come your lens effects are attenuating my background scattering
> media into oblivion ?

To which Nieminen Juha <war### [at] punarastascstutfi> complacently replied:
>   Hard to say anything without seeing the code. Perhaps it has
something to
> do with max_trace_level? (Even an advanced user can fall in that
sometimes...
> I do :) ).

If you are using the pre-defined lens effects (Lens.inc) then this
should happen automatically - by default the file increases the level to
20.  With many layered effects or other transparent objects in the scene
this may not be enough, so the trick is to increase the level *after*
you include Lens.inc (otherwise the include file will set it back to
20).

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