Consider the following setup:

- a camera
- an object in front of the camera
- a solid/non-transparant wall behind the object
- a light_source behind the wall

Now, for some arcane reason, I want the light_source to shine on the 
object while the wall remains visible to the camera.

I first thought of a light_group {} with object and light_source in it, 
but the wall still blocks the light.

Any smart idea? I am sure it should be possible and I have the nagging 
feeling that it has been done before.

-- 
Thomas

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Am 08.11.2017 um 13:36 schrieb Thomas de Groot:
> Consider the following setup:
> 
> - a camera
> - an object in front of the camera
> - a solid/non-transparant wall behind the object
> - a light_source behind the wall
> 
> Now, for some arcane reason, I want the light_source to shine on the
> object while the wall remains visible to the camera.
> 
> I first thought of a light_group {} with object and light_source in it,
> but the wall still blocks the light.
> 
> Any smart idea? I am sure it should be possible and I have the nagging
> feeling that it has been done before.

Did you try no_<https://youtu.be/_euDhMDDRq4>?

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On 11/08/2017 07:36 AM, Thomas de Groot wrote:
> Consider the following setup:
> 
> - a camera
> - an object in front of the camera
> - a solid/non-transparant wall behind the object
> - a light_source behind the wall
> 
> Now, for some arcane reason, I want the light_source to shine on the 
> object while the wall remains visible to the camera.
> 
> I first thought of a light_group {} with object and light_source in it, 
> but the wall still blocks the light.
> 
> Any smart idea? I am sure it should be possible and I have the nagging 
> feeling that it has been done before.
> 
Have you tried the no_shadow object qualifier on your wall object?

If no_shadow cannot be used for some reason, perhaps the light_source's 
'projected_through' object feature might do the trick? There the object 
could be a say thin box of sized how ever you want relative to the wall 
and toward the object(1) from the wall. To this object you'd add the 
usual no_image, no_shadow, no_reflection qualifiers as needed.

Bill P.

(1) - An idea I've had on my to to-try list for a long while, is to 
variously light all the rooms in a large building - or a bunch of street 
lights say - with a single light source using the projected_through 
feature with a multi-shape object providing for 'magic light wormholes' 
into each room/location. Think it would work to the first order. Unsure 
it would result in a faster render - and perhaps the look of the lights 
would be too limited...

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On 11/08/2017 08:21 AM, clipka wrote:
> Am 08.11.2017 um 13:36 schrieb Thomas de Groot:
> 
> Did you try no_<https://youtu.be/_euDhMDDRq4>?
> 

:-)

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Le 17-11-08 à 08:27, William F Pokorny a écrit :
> On 11/08/2017 07:36 AM, Thomas de Groot wrote:
>> Consider the following setup:
>>
>> - a camera
>> - an object in front of the camera
>> - a solid/non-transparant wall behind the object
>> - a light_source behind the wall
>>
>> Now, for some arcane reason, I want the light_source to shine on the 
>> object while the wall remains visible to the camera.
>>
>> I first thought of a light_group {} with object and light_source in 
>> it, but the wall still blocks the light.
>>
>> Any smart idea? I am sure it should be possible and I have the nagging 
>> feeling that it has been done before.
>>
> Have you tried the no_shadow object qualifier on your wall object?
> 
> If no_shadow cannot be used for some reason, perhaps the light_source's 
> 'projected_through' object feature might do the trick? There the object 
> could be a say thin box of sized how ever you want relative to the wall 
> and toward the object(1) from the wall. To this object you'd add the 
> usual no_image, no_shadow, no_reflection qualifiers as needed.
> 
> Bill P.
> 

The object in a projected_through is NEVER visible. It never cast 
shadow, is never seen in reflections nor refractions, and don't affect 
radiosity. Ity's just a kind or portail through the light comes.

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>
> I first thought of a light_group {} with object and light_source in it,
> but the wall still blocks the light.

You almost had it: Light_group of object and light_source, ALONG WITH no_shadow
for the wall.

Here's a small test scene, with two cameras to test that it works, and an extra
front-light to show that the sphere object still casts its own shadow on the
wall.
------------
#version 3.71;
global_settings{assumed_gamma 1.0 max_trace_level 5}

camera {
  perspective
  location  <.5, .5, -5>
  look_at   <.5, .5,  0>
  right     x*image_width/image_height
  angle 67
}

/*
camera {
  perspective
  location  <30, 30, -5>
  look_at   <.5, .5,  0>
  right     x*image_width/image_height
  angle 80
}
*/

// wall behind object
box{0,1
no_shadow
translate <-.5,-.5,0>
scale 30*<1,1,.1>
pigment{rgb .7}
translate 1.5*z
}

light_group{
light_source {
  0*x
  color rgb <1,1,1>
  looks_like{sphere{0,1 pigment{srgb <1,.5,0>}finish{emission 1}}}
  translate <-15, 15, 20>
}

// object
sphere{0,1
pigment{rgb <0,1,0>}finish{ambient .05 emission 0 diffuse .8}
}
}

// extra DIRECTION POINTER between light and object
cylinder{<-15, 15, 20>,<0,0,0> .1
no_shadow
pigment{rgb <0,.5,1>}finish{emission 1}
}

// extra front-light for object and wall
light_source {
  0*x
  color rgb .4
  translate <20, 5, -20>
}

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clipka <ano### [at] anonymousorg> wrote:

>
> Did you try no_<https://youtu.be/_euDhMDDRq4>?

That video is blocked in the U.S. :-(  It probably contains confidential State
Secrets...

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"Kenneth" <kdw### [at] gmailcom> wrote:

>
> You almost had it: Light_group of object and light_source, ALONG WITH
> no_shadow for the wall.
>

Hmm, for your real scene, I think there is still a problem: How to get your wall
to cast its own shadows from other lights in the scene. With my test set-up, it
doesn't do that, as the wall is necessarily made no_shadow in order to work
correctly :-/

I tried adding an identical NO_IMAGE wall into the light_group (without the
no_shadow tag), but that doesn't work-- this extra wall again blocks the light
to the sphere object.

My past use of this set-up never needed the 'wall object' to cast its own shadow
(I was shining a light up through a no_shadow height_field.) I wish I could
suggest a workable solution...

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On 8-11-2017 14:21, clipka wrote:
> Did you try no_<https://youtu.be/_euDhMDDRq4>?
> 

I did. Got a bit bruised... :-)

Good music by the way, but I had expected more from the POV-Ray scene 
render and the use of Poser figures. I missed the chequered plane...

-- 
Thomas

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On 8-11-2017 14:27, William F Pokorny wrote:
> Have you tried the no_shadow object qualifier on your wall object?
> 

Incredible! Just adding no_shadow does the trick indeed and exactly as I 
need it, without the light_group even. I had not thought of this at all 
as the name implies that the object does not cast shadows, not that it 
also becomes transparent to the light. This is not implied in the docs.

Thanks a lot!

> If no_shadow cannot be used for some reason, perhaps the light_source's 
> 'projected_through' object feature might do the trick? There the object 
> could be a say thin box of sized how ever you want relative to the wall 
> and toward the object(1) from the wall. To this object you'd add the 
> usual no_image, no_shadow, no_reflection qualifiers as needed.
> 
> Bill P.
> 
> (1) - An idea I've had on my to to-try list for a long while, is to 
> variously light all the rooms in a large building - or a bunch of street 
> lights say - with a single light source using the projected_through 
> feature with a multi-shape object providing for 'magic light wormholes' 
> into each room/location. Think it would work to the first order. Unsure 
> it would result in a faster render - and perhaps the look of the lights 
> would be too limited...

That would be a very interesting thing to do indeed.

-- 
Thomas

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On 9-11-2017 7:40, Kenneth wrote:
> "Kenneth" <kdw### [at] gmailcom> wrote:
> 
>>
>> You almost had it: Light_group of object and light_source, ALONG WITH
>> no_shadow for the wall.
>>
> 
> Hmm, for your real scene, I think there is still a problem: How to get your wall
> to cast its own shadows from other lights in the scene. With my test set-up, it
> doesn't do that, as the wall is necessarily made no_shadow in order to work
> correctly :-/
> 
> I tried adding an identical NO_IMAGE wall into the light_group (without the
> no_shadow tag), but that doesn't work-- this extra wall again blocks the light
> to the sphere object.
> 
> My past use of this set-up never needed the 'wall object' to cast its own shadow
> (I was shining a light up through a no_shadow height_field.) I wish I could
> suggest a workable solution...
> 
> 

It doesn't matter. I am still going to study your little test scene as 
it provides some interesting possibilities. Thanks indeed for your 
effort though.

-- 
Thomas

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Am 09.11.2017 um 07:08 schrieb Kenneth:
> clipka <ano### [at] anonymousorg> wrote:
> 
>>
>> Did you try no_<https://youtu.be/_euDhMDDRq4>?
> 
> That video is blocked in the U.S. :-(  It probably contains confidential State
> Secrets...

British State secrets, if anything.

Nothing spectacular to see though. Just substitute any audio copy of
Mike Oldfield's "shadow on the wall" ;)

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Am 09.11.2017 um 09:20 schrieb Thomas de Groot:
> On 8-11-2017 14:27, William F Pokorny wrote:
>> Have you tried the no_shadow object qualifier on your wall object?
>>
> 
> Incredible! Just adding no_shadow does the trick indeed and exactly as I
> need it, without the light_group even. I had not thought of this at all
> as the name implies that the object does not cast shadows, not that it
> also becomes transparent to the light. This is not implied in the docs.

Uh... and what exactly would "not cast shadows" do, if not make the
object "transparent to the light [directly from light sources]"?

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On 9-11-2017 13:37, clipka wrote:
> Am 09.11.2017 um 07:08 schrieb Kenneth:
>> clipka <ano### [at] anonymousorg> wrote:
>>
>>>
>>> Did you try no_<https://youtu.be/_euDhMDDRq4>?
>>

To tell the truth, it took me some while to get the pun... ;-)
(I am slow)

>> That video is blocked in the U.S. :-(  It probably contains confidential State
>> Secrets...
> 
> British State secrets, if anything.
> 
> Nothing spectacular to see though. Just substitute any audio copy of
> Mike Oldfield's "shadow on the wall" ;)
> 

Maybe those ladies in black leather are considered shocking for US 
viewers? ;-)

-- 
Thomas

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On 9-11-2017 13:46, clipka wrote:
> Am 09.11.2017 um 09:20 schrieb Thomas de Groot:
>> On 8-11-2017 14:27, William F Pokorny wrote:
>>> Have you tried the no_shadow object qualifier on your wall object?
>>>
>>
>> Incredible! Just adding no_shadow does the trick indeed and exactly as I
>> need it, without the light_group even. I had not thought of this at all
>> as the name implies that the object does not cast shadows, not that it
>> also becomes transparent to the light. This is not implied in the docs.
> 
> Uh... and what exactly would "not cast shadows" do, if not make the
> object "transparent to the light [directly from light sources]"?
> 

Well, my thoughts did not go into that direction to tell the truth. Just 
no shadows, without implications.

-- 
Thomas

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clipka <ano### [at] anonymousorg> wrote:

> Uh... and what exactly would "not cast shadows" do, if not make the
> object "transparent to the light [directly from light sources]"?

I wasn't gonna say it....   :|

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Thomas de Groot <tho### [at] degrootorg> wrote:

>
> Maybe those ladies in black leather are considered shocking for US
> viewers? ;-)
>

Ladies in black leather, eh? Well, maybe *that* explains the actual YouTube
message I'm seeing, "This video is blocked in your area"  My area is the city of
Virginia Beach, in the U.S. state of Virginia-- which just happens to be the
home of televangelist Pat Robertson's sprawling religious/conservative empire.
(BTW, it's HUGE and influential here, like an octopus with 100 tentacles.)

Hmm, time to protest!! "I demand ladies in black leather!".

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On 09/11/2017 16:39, Kenneth wrote:
> Hmm, time to protest!! "I demand ladies in black leather!".

You and me too. ;-)

-- 

Regards
     Stephen

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Le 17-11-09 à 01:40, Kenneth a écrit :
> "Kenneth" <kdw### [at] gmailcom> wrote:
> 
>>
>> You almost had it: Light_group of object and light_source, ALONG WITH
>> no_shadow for the wall.
>>
> 
> Hmm, for your real scene, I think there is still a problem: How to get your wall
> to cast its own shadows from other lights in the scene. With my test set-up, it
> doesn't do that, as the wall is necessarily made no_shadow in order to work
> correctly :-/
> 
> I tried adding an identical NO_IMAGE wall into the light_group (without the
> no_shadow tag), but that doesn't work-- this extra wall again blocks the light
> to the sphere object.
> 
> My past use of this set-up never needed the 'wall object' to cast its own shadow
> (I was shining a light up through a no_shadow height_field.) I wish I could
> suggest a workable solution...
> 
> 

Any object in a light_group will always cast shadows for any light in 
the scene. The light in the light_group can also get shadowed by any 
object outside the light_group.

The light(s) in a light_group can only illuminate objects that are part 
of the same light_group.
The objects of the group don't receive illumination from lights outside 
of the group unless you add "global_lights on" to the group.

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On 11/09/2017 07:51 AM, Thomas de Groot wrote:

> Maybe those ladies in black leather are considered shocking for US 
> viewers? ;-)
> 

If they were being shot, or hacked up, that'd be ok.  But just standing 
there looking good? My sensitive eyes!

-- 
dik

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On 9-11-2017 19:02, Stephen wrote:
> On 09/11/2017 16:39, Kenneth wrote:
>> Hmm, time to protest!! "I demand ladies in black leather!".
> 
> You and me too. ;-)
> 

I agree. This is pure gender discrimination!  ;-)

-- 
Thomas

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On 9-11-2017 15:40, Bald Eagle wrote:
> clipka <ano### [at] anonymousorg> wrote:
> 
>> Uh... and what exactly would "not cast shadows" do, if not make the
>> object "transparent to the light [directly from light sources]"?
> 
> I wasn't gonna say it....   :|
> 
> 
> 

Making my brain cells work a little harder than they normally do, I came 
to the same conclusion ;-)

-- 
Thomas

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dick balaska <dic### [at] buckosoftcom> wrote:

> If they were being shot, or hacked up, that'd be ok.  But just standing
> there looking good? My sensitive eyes!

Maybe it's just me, but I can read that in several ways.

"Maybe if that were a B-rated hack and slash video, I'd be into it - but just
plain old hot chicks in leather?"

:D

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Thomas de Groot <tho### [at] degrootorg> wrote:
> On 9-11-2017 13:46, clipka wrote:

> >
> > Uh... and what exactly would "not cast shadows" do, if not make the
> > object "transparent to the light [directly from light sources]"?
> >
>
> Well, my thoughts did not go into that direction to tell the truth. Just
> no shadows, without implications.
>

I'm actually with Thomas on this one-- and not just to be argumentative ;-)

In its strictest sense, no_shadow implies only what it says (IMO), because the
object (the wall) is still *visually* OPAGUE. The fact that no_shadow also
allows light to pass *through* the wall-- as if it's invisible to the light--
seems to me to be a 'secondary effect', not necessarily implied by the meaning
of no_shadow-- and might well be an unexpected effect for new users.

When I started my make my test code example, *I* actually didn't realize (or
remember) the 'dual nature' of that keyword, and didn't even try adding it to
the wall as the first step(!). Instead, I went off on a tangent, coming up with
the (unnecessary!) light_group idea first, to figure out a way to get the light
to strike my sphere object through the 'opague' wall! :-0

The documentation says "You may specify the no_shadow keyword in an object to
make that object cast no shadow. This is useful for special effects and for
creating the illusion that a light source actually is visible"-- then it refers
to the looks_light object in a light. The latter part of that sentence might
actually makes little sense to a new user, unless he/she is already familiar
with what a looks_like object is. (But no_shadow is no longer required there
anyway, as a looks_like object already has an implied no_shadow keyword.)

Since both Thomas and I both failed to initially grasp no_shadow's full meaning,
I imagine there may be others as well...

My two cents :-)

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Alain <kua### [at] videotronca> wrote:

>
> Any object in a light_group will always cast shadows for any light in
> the scene. The light in the light_group can also get shadowed by any
> object outside the light_group.
>
> The light(s) in a light_group can only illuminate objects that are part
> of the same light_group.
> The objects of the group don't receive illumination from lights outside
> of the group unless you add "global_lights on" to the group.

As it turns out, my light_group wasn't needed anyway!

But I was also 'over-thinking' Thomas's original example-- guessing that his
wall was actually part of a much bigger scene, requiring the wall to *cast*
shadows onto other objects (but not onto his single specific object.)

Years ago, I made a REALLY complex scene like that, requiring no_shadow for some
of the objects, and had to figure out a way to get shadows to fall onto the rest
of the scene. If I recall, I had to use multiple (or even nested) light_groups.

In any case, my test code here probably confused things. :-(

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On 11/08/2017 03:29 PM, Alain wrote:
> Le 17-11-08 à 08:27, William F Pokorny a écrit :
> 
> The object in a projected_through is NEVER visible. It never cast 
> shadow, is never seen in reflections nor refractions, and don't affect 
> radiosity. Ity's just a kind or portail through the light comes.
> 

Good point. Where a projected_through object is not otherwise used in 
the scene, it's only a light portal.

Bill P.

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"Kenneth" <kdw### [at] gmailcom> wrote:
> Thomas de Groot <tho### [at] degrootorg> wrote:
> > On 9-11-2017 13:46, clipka wrote:
> > >
> > > Uh... and what exactly would "not cast shadows" do, if not make the
> > > object "transparent to the light [directly from light sources]"?
> > >
> >
> > Well, my thoughts did not go into that direction to tell the truth. Just
> > no shadows, without implications.
> >
>
> I'm actually with Thomas on this one-- and not just to be argumentative ;-)
>
> In its strictest sense, no_shadow implies only what it says (IMO), [blah,blah]

Well, a lightbulb finally went off in my brain, so please forget my silly
argument.

If the wall object (with no_shadow) casts no shadow from a light, then what is
supposed to fill that no-longer-existing 'shadow area'? The light, of course.
Otherwise, there would be an illogical 'void' in that area, filled with...
what??  ;-)

Duh. It took me 'only' ten days to realize this... :-[

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On 23-11-2017 0:34, Kenneth wrote:
> "Kenneth" <kdw### [at] gmailcom> wrote:
>> Thomas de Groot <tho### [at] degrootorg> wrote:
>>> On 9-11-2017 13:46, clipka wrote:
>>>>
>>>> Uh... and what exactly would "not cast shadows" do, if not make the
>>>> object "transparent to the light [directly from light sources]"?
>>>>
>>>
>>> Well, my thoughts did not go into that direction to tell the truth. Just
>>> no shadows, without implications.
>>>
>>
>> I'm actually with Thomas on this one-- and not just to be argumentative ;-)
>>
>> In its strictest sense, no_shadow implies only what it says (IMO), [blah,blah]
> 
> Well, a lightbulb finally went off in my brain, so please forget my silly
> argument.
> 
> If the wall object (with no_shadow) casts no shadow from a light, then what is
> supposed to fill that no-longer-existing 'shadow area'? The light, of course.
> Otherwise, there would be an illogical 'void' in that area, filled with...
> what??  ;-)
> 
> Duh. It took me 'only' ten days to realize this... :-[
> 

LOL
I am still pondering the issue; higher dimension bypasses. :-)

-- 
Thomas

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On 23/11/2017 07:53, Thomas de Groot wrote:
> I am still pondering the issue; higher dimension bypasses. :-)

That's the way it is looking to me. ;-)

-- 

Regards
     Stephen

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Thomas de Groot <tho### [at] degrootorg> wrote:


> I am still pondering the issue; higher dimension bypasses. :-)

Well of course.
It's a bypass!  Gotta build bypasses...

https://www.youtube.com/watch?v=HNmIQX_ImgM

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