Render shadow of only certain objects

[Jengo_1]

Hi, I have a VFX scene that needs some complex shadow matching -
I have a footage with a wall and floor, the wall casts hard shadows on the floor.
I have a scene with a wall, a floor and some mannequins (the mannequins will be composited into the footage). The wall and the mannequins cast shadows on the floor the blends together.

I'd like to render a shadow pass that only has the mannequins shadows on the floor - but here's the trick, I need the wall to still cast the shadows on the floor because the mannequin shadow has to blend in with the wall shadow to look realistic.
If I remove the wall - the shadows the mannequins cast on the floor are too hard.
Here are some screenshots - I want to keep the shadow style they cast on the floor but not keep the whole shadow the wall casts, if i keep it too then it doubles with the shadow already existing in the footage and it becomes too dark.

Here's how the render looks when i remove the wall:

And this is how it looks when I composite the shadow without the wall - it's too hard and doesnt blend realistically with the shadow on the floor:

Here's what happens when I keep both shadows in the render, the floor shadow gets too dark:

I feel like it should be a pretty easy fix, like a shadow mask or something like that, this seems like a pretty common VFX compositing need.

Thanks!

[Jengo_1]

Nothing? I'm sure this is a fairly common VFX question

I think i made it more complex than it is - Here's what I need:

I need to render two overlapping shadows and be able to have a pass render for only one of the shadows (for compositing)

Thanks! (aoktar, beppe, any creative or direct way to do this?)

[bepeg4d]

Hi,
probably the easiest way should be to hide the wall, and add an Object tag to the mannequins inside the shadow of the wall, and disable the Shadow Visibility option
ciao Beppe

[Jengo_1]

Hey Beppe thanks for your reply!
But if I turn off shadow visibility then they won't cast any shadow right?

This is the mannequins cast a reduced, softer shadow when combined with the wall shadow (unlike if the wall is not there, they cast a hard long shadow cuz of the sun).

I ended up rendering the whole scene once as it is with the wall, and used a object color mask to get the mannequins only
Then rendered the whole scene again without the wall, and with a large softbox above the mannequins to fake-soften the shadows. It kinda worked ok for what I needed, but def not a viable solution for more serious projects.

Am I mistaken thinking that this is a very common VFX need? Mixing 3D shadows with existing shadows in the footage?
Or is Octane for C4D not really advisable to use for these VFX work?

[jayroth2020]

Regarding VFX work, there are certain things that are either voided altogether or completely rebuilt digitally, precisely for the reasons you are encountering. Octane is perfectly suited for this, but you will need to modify your approach to the situation (as any VFX team would.)

You might need to break this out in render layers and mess about in the compositing portion of the process. I have scene whole scenes like this rebuilt digitally (projection mapping is a great asset in cases like these).

Have you researched how shots like these are put together?

[Jengo_1]

Jayroth, thanks for the reply my man

I see what you're saying, I'm barely deal with VFX and as a practice have started this series of videos with some people I work with. They had the idea of implementing objects in the environment around their studio in downtown LA and I thought it would be fun.

Besides guiding the person who shoots these vids on how much to move and generally how to approach the shot, we haven't planned any shot really. It's all shot on iPhone too.

It's very lowkey and am just discovering thru this project the depths of VFX, so here's another thing learned! Although it makes a whole lotta sense to have to ability to export mask passes for specific shadows doesnt it?

[jayroth2020]

You could just as easily render just the particular mannequin and a shadow catcher plane, which is actually a better choice, as everything is then under your control. I am not an engineer, so I cannot comment about extracting individual shadows for unique passes, but given the visual complexity of a typical shot, I doubt that would be a practical thing to do, especially when the option I just mentioned is so straightforward. Until AI steps in and makes us all obsolete, I think that would be the best way to get what you want here...

[Jengo_1]

Hah, AI will never make us obsolete my guy! It's just a tool (crossing my toe fingers here)

But yeah, I feel you, only reason rendering the mannequin shadows only was that their shadow becomes way too hard without mixing into the wall shadow. When two shadows are combined they become a different shadow, rather than two individual shadows applied on top of each other.

But yeah, I guess I'll have to get creative with it until they figure out a way to mask out a shadow out of several mixed shadows.

Thanks for the help brother