[Radiance-general] trans material - tspec comparison

[Andrew McNeil]

Ah, good point with the roughness. The only way I know how to get a good
value for the roughness parameter is trial and error it until it looks
about right in a rendering - which is probably why I avoid using it. If
anyone has an good method for setting roughness please share!

Andy


On Wed, Aug 13, 2014 at 4:38 AM, Jan Wienold <jan.wienold at epfl.ch> wrote:

>  Hi Shri and Andy,
>
> I think you can "spread" the "direct" component when applying a roughness
> to the trans material. Then, the scattered light is not lambertian any
> more. You can control the scattering "width" by the roughness.
> You can see the difference then if you look directly onto the material
> (smaller or larger spot towards sun direction).  Getting different patches
> on the wall etc. is quite hard I guess. The reason for this is , that as
> soon as the "direct part" is scattered (when applying roughness), it is
> handled by the ambient calculation and it is not part of the direct
> calculation any more.
>
> Using a BSDF with a fine resolution is in fact the best way to handle this.
>
> good luck,
>
> Jan
>
>
> On 08/13/2014 01:26 AM, Andrew McNeil wrote:
>
> Shri,
>
>  In the images you sent I suspect that the sunpatch is over exposed, so
> it appears white in all three renderings. If you create a falsecolor
> rendering you'll likely see that the sunpatch has different luminance
> values (make sure to set the scale sufficently high).
>
>  One of the drawbacks of using Radiance's trans material to model
> translucent fenestration materials is that the transmission is divided into
> strictly specular and diffuse. The specular component is not scattered at
> all, and the diffuse component is completely lambertian. You might have
> expected the sunpatch to soften and broaden as the diffusion increases, but
> with the trans material the shape of the sunpatch doesn't change as
> diffusion increases, it just reduces in intensity.
>
>  To study diffusion like this, you'll really want to use a BSDF. Probably
> also a tensor tree BSDF so you get sufficient accuracy in the sun patch.
> And unfortunately LBNL hasn't measured BSDFs for diffusing glazing, so I
> can't offer an immediate solution.
>
>  Best,
> Andy
>
>
>
>
> On Tue, Aug 12, 2014 at 3:51 PM, Shrikar Bhave <shrikarbhave at gmail.com>
> wrote:
>
>>   Hello All,
>>
>>  For tspec comparison,  I rendered the same scene for four different
>> tspec values of 10%, 50%, 90% and 100% (ideal diffuser).
>>
>>  I was expecting varying amount of diffusion, but I am getting puzzling
>> results. only 100% (ideal diffuse) value returns no direct sun-patch. All
>> the other values show a clear sun-patch in the space.
>>
>>  Any clue why this is happening? Would you expect such results?
>> Everything else is constant in the model. (images are attached)
>>
>> Thanks,
>>
>> Shri
>>
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