Problem with Matte Objects and GI / Lightcache

[olika]

Seems that something changed in the matte object behavior, can`t remember having this issues in the past.

It`s a pretty simple setup, Objects on a groundplane, lit by a domelight, problem occurs with and without a hdri in the domelight.

Everything is fine, Shadow details fine as you can see,
but as soon as i change the groundplane to a matte objects (which never changed light calculation in the past i think),
the light solution gets screwed with a lot of unclean calculations, leaks, splothces and stuff (pictures to the right side)

Tested it with two different scenes now, not sure where to optimize settings,
maybe someone can point me to the right direction.
Is it GI, is it matte settings?

[olika]

Update:
Seems the behavior is connected to the retrace option in the lightcache settings.
When unticked everything is fine, but why?

[svetlozar.draganov]

It looks like a bug - is there a chance to attach one of those scenes for investigation since I wasn’t able to reproduce the issue?
Let us also know the exact V-Ray/3dsMax/OS versions.

[olika]

For sure, you can replicate the problem very easily (just used a teapot in that example).
Also i used the new universal settings (hopefully correct ;-)) but i think it`s the same with the optimized approach (props akin!)

http://we.tl/IahqMrev24

[svetlozar.draganov]

It seems there is something to do with the new Leak Prevention feature, if you turn it off by set Leak Prevention to 0 the artifacts disappear.
The case has been forwarded to our Developers Team for further investigation.

[olika]

Hi Svetlozar,

hmmmm so time has passed, now i run into the same problem again ;-(
But now the leak prevention zero trick doesn`t seem work anymore.

Again it`s the render against pure white problem.
Object on ground plane lit by hdri super simple.
When i use the groundplane as solid everything is fine with the go and stuff,
but a soon i change the groundplane to be a matte (äh my way of achieving render against pure white…maybe that is wrong from beginning, but hey would love to have a animation rendered out in just one pass without shadows extra pass and so)
the GI turns crazy where the object sits on the ground, so it`s always flicker on those fine details areas.
Any tips on that? sorry for resurrecting this old thread/problem but still having issues with it.
It´s vray 3.3 in use and tried different light cache settings but with no good results so far…
Thanks in advance, if there is interested i can upload the scene to investigate

[svetlozar.draganov]

We well need a scene for this since, would it be possible to upload one?

[kagemaru]

Have a look at the vray object parameters “No GI on other matte”, it reminds me that situation.
By default is off, it means that gi calculations won’t occur between matte objects so if you have say an element that greatly contributes with its gi to the global lighting, you will see a big difference in the final output.

[olika]

Sure, i`m glad you`ll have a look onto the scene.
There`s two thing going on, first not sure whats up with the lightcache, watch the little contact shadow from the cardboard to the groundplane, kind of fuzzy,
but it only is that way when you turn on matte on the vray groundplane.
Matte turned off and domelight on its fast and clean shadow.
Matte turned on and domelight on its slow and poor gi
Matte turned on and domelight off its fast but also poor gi.

https://we.tl/YodSdtuUA9

[tashko.zashev]

Thanks for sharing your scene file.
Would it be possible to update your V-Ray version to: 3.40.02 ?
I just completed a few render tests (frames 0-15 / V-Ray 3.4) and found that this LC setup works best for this project >>

Furthermore it might be a good idea to switch to BF/BF. I don’t see any big difference in the render time for the first 15 frames.

[olika]

Hi Tashko,
thanks for investigating with me on that stuff.
Unfortunately i`m not able to update right now because of some running projects.
But i was fiddling with your suggested LC settings, what do you think is the problemmaker? leak prevention?
But hey i am still struggling with what i`m afraid of is a really beginner thing to do, but i`m not able to solve it completely.
I`m hoping i can catch this this time with you guys!
Attached are 3 renders of the scene i was sending you too.

First one is rendered with groundplane set to matte and the background/env is standard BLACK.
- Render is then saved, open in Photoshop, mask with it`s alpha and put a white solid behind it (correct workflow?)

Second one is just rendered with the groundplane as standard solid white plane
- Render is what you see, openend in PS no alpha here to mask

Third one is rendered with groundplane set to matte and the background/env is set to WHITE.
- Render is then saved, open in Photoshop, no need to mask, area around objects is already pure white.

As you can see, all three renders give different results to me in Photoshop

First one, with what i think is the way you get advised to all the time, render on black BG, is the one i unlike the most because of it`s dirty looking dark shadow areas. Here is where become unsure, thats the way you should do it but looks ugly when i try it…hmmm? what i`m not getting?

Second one is way more brighter in shadow areas, seems to me that more GI is considered or kept, not so dirty.
But cause of no alpha of shadow you need to light the groundplane as good as you can to come close to pure white without overexposing.

Third one looks fine cause all area around is pure white and thats what i want or try to achieve
But here comes the trouble again, see posts above about LC makes problems with flickering in detailed, contact shadow areas, when groundplane is set to matte.

I`m ashamed i`m still bothering you with this problem but i feel so stupid or noobish on that one…

[olika]

Addition:
What i do right now to get a satisfied result is to render one time with GI on, everything is visible, ground plane is solid but with alpha -1.
Thats what i call the “beauty pass”.
Second, i render the same but this time with GI off, object is invisible for camera, ground plane is matte.-> “shadow pass”
Then when it comes to compose it, i duplicate the beauty pass so the upper layer is masked with the alpha of the object only. So object can be graded individually.
The underlaying layer is then masked with the alpha of the “shadow pass”.
So i`m not taking the shadow pass result as the shadow layer but instead take the beauty pass and mask it with the alpha of the shadow layer.
Why doing that?, for me it seems to be the best solution so far to keep all the bright GI shadow results with color bleeding etc. in the comp but also be able to have a transparent shadow layer underneath and shadows are kind of “bordered” with the mask and not running endlessly.
(Resulting rendered objects in that case should be placed on white background with shadows but close to the object so you don`t have intersecting them with the canvas so you don`t have a wide canvas and be able to layout them pretty close together. No picture borders wanted because of long shadows)
Can you follow? Hard for me to write that down in simple words, but what i`m asking here (cause i`m a one-man show, no desk neighbor to ask ;-))
Does my workflow make sense at all or do i just don`t see the mistake i make.
Not sure if its more related to GI Stuff (LC discussion above) or a question to background color or its more wrong compositing what i`m doing not right.
For me (and the pitty is i have to create stuff on white background all the time aaaah feel so stupid need a clean solution)
right now it all seems way to complicated how i approach it, what about the rest out here?
Also to mention, most of those renders are not artwork it`s just poor paid simple product shots but many, so a easy solution with as few as possible render passes would be the goal.