>
> This comment was made in the other thread (from 2002) as well I think, but I
> don't quite understand it. If you look at this example from the POVRAY
> documentation: http://www.povray.org/documentation/view/3.6.2/99/, there is this
> tornado effect which is visible in front of a white background. Could this
> tornado not have been made red? If so, that seems similar to what I want to do.

Okay, 'smoke' not tornado. Maybe this is fundamentally different, but is what
gave me the idea that absorptive media could be used effectively in front of a
white background.

Post a reply to this message

Brad <bra### [at] gmailcom> wrote:
> Okay, 'smoke' not tornado. Maybe this is fundamentally different, but is what
> gave me the idea that absorptive media could be used effectively in front of a
> white background.

Note that absorbing media does not take light into accoung in any way.
(The media will cast shadows, but the media itself will not be affected
by light.)

OTOH, if all you want is a cone of visible media, you can do that with
absorbing media.

-- 
                                                          - Warp

Post a reply to this message

On 10/03/2013 5:47 PM, Le_Forgeron wrote:
> If you have an empty bag and put in it 2 g of grass... you have a bag
> with 2 g of grass.

So all you need now is some cigarette papers and some roach material.

Brad, if your background is white and you put a white object (or a white 
spotlight) in front of it. You will not see a difference in colour. 
White + white = white as (unless you are using an output that accepts 
values greater than 1) the output will be clipped to <1,1,1>.

Try using a background that is a very light grey.

-- 
Regards
     Stephen

Post a reply to this message

Am 10.03.2013 19:07, schrieb Brad:
>
>>
>> This comment was made in the other thread (from 2002) as well I think, but I
>> don't quite understand it. If you look at this example from the POVRAY
>> documentation: http://www.povray.org/documentation/view/3.6.2/99/, there is this
>> tornado effect which is visible in front of a white background. Could this
>> tornado not have been made red? If so, that seems similar to what I want to do.
>
> Okay, 'smoke' not tornado. Maybe this is fundamentally different, but is what
> gave me the idea that absorptive media could be used effectively in front of a
> white background.

Absorbing media - yes. Scattering media - no.

Post a reply to this message

>
> Absorbing media - yes. Scattering media - no.

Thanks for the help everyone. I appreciate all the comments.

Post a reply to this message

clipka wrote:

> Absorbing media - yes. Scattering media - no.

What about the extinction part of scattering media? I would
have expected this to cause some absorption if the background is
white, but the media itself receives less light to scatter.

Post a reply to this message

Am 11.03.2013 02:25, schrieb Christian Froeschlin:
> clipka wrote:
>
>> Absorbing media - yes. Scattering media - no.
>
> What about the extinction part of scattering media? I would
> have expected this to cause some absorption if the background is
> white, but the media itself receives less light to scatter.

That would do the job, too - but why bother with scattering media when 
you can have the same extinction effect with absorbing media?

Post a reply to this message

> Hello,
>
> I'm trying to create an image with scattering media, on a white background. This
> topic seems to be similar to that posted by someone in 2002
>
(http://news.povray.org/povray.general/thread/%3CXns92C3BC15D86CCseed7%40povray.org%3E/),
> but I did  not see a solution in that thread.
>
> I can create the image I want with no problem with a BLACK background (see this
> image: http://tinypic.com/r/1sh8gw/6). While my input *pov file is rather large,
> below are the (I think) essential elements of creating that image:
> ---------------------------------------
> #declare color_light = rgb <2.00, 2.00, 2.00>;
> #declare color_background = rgb <0.00, 0.00, 0.00>;
>
> light_source { light_location color color_light media_interaction off shadowless
>   scale light_scale }background { color color_background }
>
> light_source { <80, 15.516/2, 65.259> color color_light spotlight point_at <5*43
> ..628/6., 15.516/2, 65.259/2> radius 10 falloff 5 tightness 20 media_attenuation
> on}
>
> box {
>     <0, 0, 0>, <80, 15.51625, 80>
>     texture {pigment { color rgbt <1,1,1,1>}} hollow interior { media {scattering
>   {1, 0.07 extinction 0.01} intervals 5 samples 300,300}}}
>
> ---------------------------------------------
>
> I wanted to make a white background, so I made color_background rgb < 1,1,1> and
> then tried to modify the spotlight light_source by setting the spotlight color
> to something like red. However, this is what the outputted image looks like in
> this case: http://tinypic.com/r/10prshz/6.
>
> You can see in that image that there is some-sort of red spotlight action going
> on, in that the atomic positions in the spotlight have some mixture of red color
> on them, but definitely the image does not have the scattering effects present
> with the white background.
>
> What am I missing? Thanks for any pointers you can give.
>
> Best,
> Brad
>
>

Your media seems to comform to the obsolete sampling method 1. The 
default is method 3. With that method, the followings apply:

Your media will render faster if you omit intervals 5. It default to 
intervals 1 and should be left alone. For example, intervals 1 samples 
1000 renders faster than intervals 10 samples 20 for 5 times as many 
samples.
Also, the second samples parameter is NOT used.

Against a white background, the colouring can easily get drowned out, 
especialy when using a very smal extinction value. Remove extinction 
0.01 and use the default of 1. This will darken the background.

For all light_source, "media_atenuation" defaults to ON, no need to set it.

As other pointed out, you can use absorbing media.
You can also use a background that is not totaly white.

Scaling a light is the same as multiplying it's location by the scale value.


Alain

Post a reply to this message

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

>
> For all light_source, "media_atenuation" defaults to ON, no need to set it.

Really? If so, I didn't know that. But the docs for "Atmospheric Attenuation"
say this:
"Normally light coming from light sources is not influenced by fog or
atmospheric media. This can be changed by turning the media_attenuation on for a
given light source on."

Or did you mean media *interaction*? That DOES default to ON.

Post a reply to this message

> Alain <kua### [at] videotronca> wrote:
>
>>
>> For all light_source, "media_atenuation" defaults to ON, no need to set it.
>
> Really? If so, I didn't know that. But the docs for "Atmospheric Attenuation"
> say this:
> "Normally light coming from light sources is not influenced by fog or
> atmospheric media. This can be changed by turning the media_attenuation on for a
> given light source on."
>
> Or did you mean media *interaction*? That DOES default to ON.
>
>
>

fog YES. fog never cause light atenuation. Regular absorbing and 
scattering media, contained or not, always intract with your lights 
unless you explicitely set media_attenuation off.

It may be another leftover from some early versions, like no later than 
version 3.0, maybe version 2...
My first brush with scattering media date back to version 3.1, and I got 
shadows from media as well as media self-shadowing. I've NEVER used 
"media_atenuation on" and all my scattering medias cast shadows unless I 
set extinction to zero.

In the same optic, ALL light_source, except shadowless ones, have this 
default photons block:
photons{refraction on difraction on area_light off}

The documentations erroneously tell it's all OFF.


Alain

Post a reply to this message