Shadow pass slow, what do tune?

[olika]

Hi again,

just getting used with the 3.3 Vray, so far so impressive
but now one thing i can`t solve:

I´m rendering a pretty simple product shot, just a lamp (looks like a box nothing special) hanging on the wall.
When i try to make a shadow pass (Make Object invisible nothing more fancy)
Vray comes to a grinding halt when it comes to render the shadowed area(where the objects normally sits)

Can someone point me towards a way to go? With the new setup i`m kinda lost between all the settings and tweaks we had to solve it, and how to do it in 3.3.?

the normal render with the Object visible renders totally fast and clean....

[Morbid Angel]

it might be a bug with light cache, if you turn it off (f.e. just bruteforce) does it render fast?

[olika]

thanks dmitry! yes seems related to gi...
2 questions from latest experience with 3.3:

-when you open a old scene and press yes change to new settings, light and mat subdivs are greyed out but show old values (f.e. light with 128 subdivs)
that`s relevant? or should you reset the old subdivs too? to maybe 4-8

- whats the latest easy workflow to extract a proper and fast shadow pass from easy product render (white white white...) (i feel ashamed but kinda stucked right now )

[Morbid Angel]

for your first question, I think it does not matter which values you had in the lights/shaders they are ignored completely.
for your second question, do consider using state sets. Its the most easiest system in max to date which allows you to edit the scene without creating scene copies etc. You can override any value per state set, and create what you need. To produce a shadow, what you need to do (what I usually do) is turn off gi, so direct lights only, set your shadow casting object to be visible to camera off, set your plane (shadow receiving object) to be vray properties matte, alpha -1, shadow, affect alpha on.

[olika]

Thanks again Dmitry,

jep i have kinda like the same workflow for shadows as you described yours.
But still i have some unsolved questions left...
I´m still unsure if the GI does contribute to the shadow? I mean turning it off, different shadow?
And second, when i`m using the described workflow (object not visible to camera, ground as matte,some plates around also not visible and alpha -1,)
it results in a very different sample rate for the ground plane set to be matte.
In the attached imge of the sample rate pass you can see the difference, also slows down of course.
Why is that?

[Morbid Angel]

It depends on how deep you want to go with the comp of this. Technically your shadow is composed of direct and indirect shadow. Direct shadow - from lights. Indirect shadow - from bounce and gi in general. For this to be properly exported and implemented, you need to have regular exr format enabled where both gi and direct shadow can be stored in the alpha. The gi shadow will always be colored, so then you need to run through special expression setup in comp to apply the bounced gi color on to the floor.

This is a bit complicated. What compers usually like to do is control that on their own so what I usually do is provide them a more of a fake and simpler way where you render only direct shadow from your key light source and then contact shadow from dome light or on some case occlusion.