Light Path Expressions

[vfxronin]

Aloud I'm happy with the Current AOV set that comes with V-ray i really wish that we could have LPE in V-ray, it would really help so we don't have to modify the shaders or hack them plugins build a custom AOV that uses the LPE. Sometimes i find my self really wanting to tweak some Aov or i have to make couple of Random AOV to make a particular AOVs! i know its memory intnsive but Ram is so cheap this day that it not really a issue.


as to any speed advantage and gain! I'm not sure.

any one ells has any thought on this?

[yolov]

What would that do, can you elaborate? Isn't that the same set of render elements that are already available? To my understanding, which is easy to be wrong it would be having another render element and defining what channel it would represent. Example: Add a custom channel and tell it to be the specular contribution of lights to the scene? I read some more on the topic and there's some more advanced stuff with this, I'll put it up for discussion.

[vlado]

i know its memory intnsive but Ram is so cheap this day that it not really a issue.

I don't know why you think it's memory intensive (it's not). Memory is the least of my concerns; but render times might be a problem (i.e. would you accept 2 or 3 times slower renders?)

Best regards,
Vlado

[vfxronin]

That interesting! form the very little experience iv had with it and Feed back what i heard fro Juggle Book in Prman 20/21 was that
they where taging the sample and when render was finished would do the math and split it in to AOV channels which was the issue with the memory concomption , ill need to reconfirm this From friend how worked on the show! so the Slower render times in news to me!

unless they are approaching it differently!

[jasonhuang1115]

Is this LPE thing tied to separating GI contribution on per light(s) basis? If yes, I wonder if it has to do with what GI method users are using. IIRC, someone commented that this would be doable but maybe implemented for GPU RT first because its GI is BF+BF. Would love to be corrected.

[joconnell]

LPE was brought in to Iray first I think? It means you can be really specific about render elements - for example you'd be able to get a pass of only the second reflection or gi bounce in your scene as it's own pass, or only the reflections of a specific object in another specific object as it's own pass. The possibilities are kind of bananas, I'd say heavy photo retouchers would love it but I'd imagine it'd be hellish to keep track of all the bits needed to complete a rebuild if you started using them.

https://www.youtube.com/watch?v=2M-nDVUDD1c

[jasonhuang1115]

I think it would be good to have this as an option with users knowing the render time hit. So when it's needed, users turn it on (or set it up), take the render time hit, and gain the control he wants in post.

[vfxronin]

That interesting! form the very little experience iv had with it and Feed back what i heard fro Juggle Book in Prman 20/21 was that
they where taging the sample and when render was finished would do the math and split it in to AOV channels which was the issue with the memory concomption , ill need to reconfirm this From friend how worked on the show! so the Slower render times in news to me!

unless they are approaching it differently!

yep! just to confirm this was basically correct. I spoke to the guy how written The LPE for Jungle book they where using it to split light ID aka "light select" it didn't increases the render time to that extend but was Memory was a issue. on some show they had render Tranmistion aov but there was no need for this movie.

[vlado]

But you already have the light selects, even without LPEs?

Best regards,
Vlado

[vfxronin]

i think the favouritism with LPE to generate Light Selects may be because it docent add to the render time, light select will add to the addition render time docent it? pleas correct me if I'm wrong with this.

also the freedom to write new aov passes with out digit in to the shaders.


thanks .

[vlado]

i think the favouritism with LPE to generate Light Selects may be because it docent add to the render time, light select will add to the addition render time docent it? pleas correct me if I'm wrong with this.

No, the light select does not add to the render time; it simply spits out information that the renderer is already computing anyways.

Best regards,
Vlado

[vfxronin]

So i would like to make a few corrections to my original request. As benign in soho I'm luck to be a Drink away from al the right pepole and in my case one floor.
So after getting a 30 min lectures in LPE! by the Man how implemnted. i can now understand that in the Case of PRMan its completely written around it so this is how there able not to get any speed hits in there renders. So I'm wondering if Vlado is open to Alternatives or V-ray own version of this that will work well in V-ray. i guess the challenge would be to eliminate speed hit from AOV passes and free dom to creat customs aovs!

thanks you.

[joconnell]

AOV's in vray don't add any speed hit as they're part of what vray will calculate to make the beauty pass anyway. Extratex passes where there's additional sampling being done on top of the beauty will have a speed hit (so for example a utility ambient occlusion pass which is sampled separately from the beauty) but stuff like light select,lighting, gi, spec, reflection, refraction and so on is pretty much free.

LPE stuff (those random specific transmission / object specific element passes you mention) on the other hand isn't in vray at the minute so I'm not sure what overhead that has.

[vfxronin]

AOV's in vray don't add any speed hit as they're part of what vray will calculate to make the beauty pass anyway. Extratex passes where there's additional sampling being done on top of the beauty will have a speed hit (so for example a utility ambient occlusion pass which is sampled separately from the beauty) but stuff like light select,lighting, gi, spec, reflection, refraction and so on is pretty much free.

LPE stuff (those random specific transmission / object specific element passes you mention) on the other hand isn't in vray at the minute so I'm not sure what overhead that has.

hey. aaaaa AOV do have a speed hit! there not free!( okey nothing crazy like 2x but still a speed hit)

And as to to light select is dosent support refraction! is there a way to get that added?

[t.petrov]

LPE will have an effect on the rendering performance for sure, because we'll have to evaluate if a path matches given LPE.
The more expressions you have and the more complex they are the slower it will be.

The real question is which practical problems you can solve with LPE and you cannot solve with the current hardcoded AOVs.
If you answer this question, then we can think about some solution or even consider implementing LPE.
Until then we have no big incentive to invest time and effort in implementing LPEs.