Front and back material transparency (aka light invisible object)

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Dade
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Front and back material transparency (aka light invisible object)

Post by Dade » Fri Mar 15, 2019 11:12 am

I have ever rejected the idea of having light invisible objects because it wasn't possible to have a consistent and coherent implementation when tracing rays both backward and forward (i.e. BiDir, PhotonGI, etc.).

After having gone trough many different possible solutions, I landed on the idea to add split support for object transparency (aka opacity in BlendLuxCore) for front and back faces. Before it was possible to define only a single type of transparency.

The properties are:

Code: Select all

scene.materials.<material name>.transparency.back =  0 0 0
scene.materials.<material name>.transparency.front = 1 1 1
"scene.materials.<material name>.transparency" is still supported for backward compatibility and will set both to the same value.

How this is related to light invisible objects ? Because you can set an object to have full transparent back faces and full opaque front faces. This is a sphere with a spot light inside:

r1.jpg

And this achieves the result of having "light invisible" object while working in all cases (i.e. Path, BiDir, PhotonGI, etc.).

This new feature can also be used for a lot more and/or in some creative way, for instance, this is a sphere with a checkered texture back face transparency:

r2.jpg
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marcatore
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Re: Front and back material transparency (aka light invisible object)

Post by marcatore » Fri Mar 15, 2019 11:56 am

it seems great...we have to test it :)

Anyway, thank you Dade trying to hear our requests.

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lacilaci
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Re: Front and back material transparency (aka light invisible object)

Post by lacilaci » Fri Mar 15, 2019 12:25 pm

I'm a bit confused. Is this an object or material property? Can this be used to create real thick glass object that let's light through(no shadows or caustics)?

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Re: Front and back material transparency (aka light invisible object)

Post by Sharlybg » Fri Mar 15, 2019 12:35 pm

lacilaci wrote:
Fri Mar 15, 2019 12:25 pm
I'm a bit confused. Is this an object or material property? Can this be used to create real thick glass object that let's light through(no shadows or caustics)?
Me too ( i'm confused) but search for your post to understand as you're used to this kind of thing with corona. But here it seem quite different.
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lacilaci
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Re: Front and back material transparency (aka light invisible object)

Post by lacilaci » Fri Mar 15, 2019 12:49 pm

Sharlybg wrote:
Fri Mar 15, 2019 12:35 pm
lacilaci wrote:
Fri Mar 15, 2019 12:25 pm
I'm a bit confused. Is this an object or material property? Can this be used to create real thick glass object that let's light through(no shadows or caustics)?
Me too ( i'm confused) but search for your post to understand as you're used to this kind of thing with corona. But here it seem quite different.
Yes, well it looks like that but from an artist perspective I'm not sure what this is about.... Can I hide any object from light? If so, can this solution be applied to reflections as well? Can this be expanded to make links between objects and lights(Hide objects only from specific lights)? Or furthermore make object or lights be hidden from reflections?

These are the "archviz" type of features that are very useful, so far I think only corona and vray can do it (maybe some others too, idk.) There was some talk about this on rightclickselect but it seems that recently cycles devs are trying hard to ignore any suggestions on new features no matter how well it was proven to be useful. This seems like a great oportunity for luxcore to "fill in" what cycles can't imho...

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Sharlybg
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Re: Front and back material transparency (aka light invisible object)

Post by Sharlybg » Fri Mar 15, 2019 1:30 pm

i start to understand. It that light can pass through object but only from the inside. Can be use to create efficient light bulb i think :idea:
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Re: Front and back material transparency (aka light invisible object)

Post by lacilaci » Fri Mar 15, 2019 1:36 pm

Sharlybg wrote:
Fri Mar 15, 2019 1:30 pm
i start to understand. It that light can pass through object but only from the inside. Can be use to create efficient light bulb i think :idea:
No, because if it only can go through backface then you still cannot use thick geometry, so you might just aswell use archglass... If it can go through the whole object, then you can make complex looking lightsources (in theory)

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Dade
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Re: Front and back material transparency (aka light invisible object)

Post by Dade » Fri Mar 15, 2019 1:54 pm

lacilaci wrote:
Fri Mar 15, 2019 12:25 pm
I'm a bit confused. Is this an object or material property? Can this be used to create real thick glass object that let's light through(no shadows or caustics)?
It is a material property and it was already available but it wasn't possible to set a different value for the front and back of a face. If you are are looking to the front or back of a face it is defined by the face normal.
lacilaci wrote:
Fri Mar 15, 2019 12:25 pm
Can this be used to create real thick glass object that let's light through(no shadows or caustics)?
No.
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Re: Front and back material transparency (aka light invisible object)

Post by pixie » Fri Mar 15, 2019 9:10 pm

Is it something akin to backface culling?

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Dade
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Re: Front and back material transparency (aka light invisible object)

Post by Dade » Fri Mar 15, 2019 10:01 pm

pixie wrote:
Fri Mar 15, 2019 9:10 pm
Is it something akin to backface culling?
The result is the same (if you place your point of view on the ray traced).
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