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Eleonore Leger avatar image
Eleonore Leger asked ·

Best way to output transmission and specular aovs for multiple overlapping transmission objects ?

Hi everyone,


On the project we are currently working on (a tv series), we need to render some shots containing several asset with transmissive parts (dragonflies' wings, glass parts...) that tend to often overlap each other.

Here is the thing :

For the comp, we need to have full control over transmission (that means having transmission only, without seeing objects that are behind or inside transmissive parts), but also specular reflections on these parts, to stay coherent with the environment and see close objects reflected in it.

So, for now we tried different approaches :

- we tried to use LPE, as in theory we should be able to output only transmission rays... but... I didn't manage to get it work, as I'm really new to LPEs and finding deep examples about it around the net is quite difficult... any advice about this approach will be usefull !

- we tried using trace sets, but it's not really solving the problem on itself, as we still need to "see" solid objects between 2 transmissive object as matte, else the objects in the back won't be masked at all... so we needed to combine it with ray switch to get what we need

- using ray switch seems to be an interesting solution, but it implies that we add it in every shading network on all assets... so for now we tend to keep that as a last resort

- finally the solution we have now is... to render those 2 AOVs in 2 separate render layers, in order to have one renderlayer that overrides every objects but glass with an aiMatte shader, to render a clean transmission pass, and another renderlayer to output only the specular of the transmissive objects.


The former option is the one we stick with for now, but it's not really optimized, as it doubles our render time, especially considering theses passes are rendered with DOF and motion blur.

And also, this solution feels quite... inelegant.

So, any suggestion on the way to proceed when dealing with this kind of case would be great !


Thanks in advance :)

mtoaaovaovstransmissionrefraction
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Lee Griggs avatar image
Lee Griggs answered ·

Have you tried transmit_aovs?

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Eleonore Leger avatar image
Eleonore Leger answered ·

Hi Lee

Thanks for the super rapid answer =)

In fact when I was trying to understand LPEs I already tried this one (as I kind of tried every suggested LPE that seemed to match my case haha...) but it's not exactly what I need.

Maybe my question wasn't really clear, let me explain myself with an quick (and ugly) example :


What i'd like to have (ideally) is an aov containing transmission (but not the beauty of objects inside/behind), but also reflection of the environment over the glass (here, the green ground).

Something like this :

I did this using some ray switchs and an aiMatte :

but I was wondering if there would be another way (ideally some magic LPE), as this raySwitch option implies for us that we rework our entire workflow...

Unless it's possible to apply a global raySwitch to desired objects ? (but I've never heard of it)

Thanks for your time btw =)


transmission-01.jpg (90.4 KiB)
transmission-02.jpg (52.1 KiB)
1601304700542.png (118.2 KiB)
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Eleonore Leger avatar image
Eleonore Leger answered ·

Hi Lee

Thanks for the answer =)

In fact when I was trying to understand LPEs I already tried the one you're talking about (I kind of tried every suggested LPE in the docs that seemed to match my case haha...) but it's not exactly what I need...

To clarify my question, let me explain myself with an quick (and ugly) example :

What i would like to have (ideally) is an aov containing transmission (but not the beauty of objects inside/behind), but also reflection of the environment over the glass (here, the green ground).

Something like this :

I did this using some ray switchs and an aiMatte, but I was wondering if there would be another way (ideally some magic LPE), as this raySwitch option implies for us that we rework our entire workflow (we need to add it to every shading network)...

Unless it's possible to apply a global raySwitch to desired objects ? but I've never heard of it

Maybe there isn't such a solution, but I was just curious.


Thanks for your time


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Lee Griggs avatar image
Lee Griggs answered ·

Hi Eleonore, there is a specular/transmission LPE example here:

https://docs.arnoldrenderer.com/display/A5AFmUG/Introduction+to+Light+Path+Expressions

More information about LPE's can be found here (with examples):

https://docs.arnoldrenderer.com/display/A5ARP/Light+Path+Expression+AOVs

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