Arnold Shadows for Compositing

#1

Hi,
I am using Arnold render for Maya and need some tips for rendering and compositing Shadows. I intended to render my scene with my objects and shadows in separate files. I created a set with a primary visibility override, to hide my objects. The plane uses a shadow catcher shader.
My problem now is that i use 3 directional lights, with shadows turned on and additionally a skydome light. When i render my scene with an aistandard shader on the plane, the shadows looks fine with a sronger shadow on the left and two other much softer and more transparent on the right. However with the shadow catcher shader the 3 shadows looks all on the same density level, even if there should be a difference.
How do you deal in such cases and could someone suggest an other way of rendering shadows?
Maybe also with AOV. I couldn’t find nothing good on the internet.

Thanks in advance!

#2

shadows are allways complicated… cause “shadows = less light”
in feature film shadows are rendered with the environment…
two renderlayer… one environment + shadows and a seperate one with the charakter…
would that work in your case…?

#3

Thanks for the reply.
No it doesn’t work so well to me in such cases, but it could be helpful to understand how this arnold shadow catcher works, cause in other render like vray it works perfektly fine.
When I render it with a normal material, like ai standard shader, everything is looking good, cause the right shadow that is caused bei the left light, becomes transparent and soft, because an other light coming form the right bright the shadow up.
In the shadow catcher is this not the case, cause every shadow has the same density and I don’t understand why.

Thanks!

#4

It sounds like the shadow-catch shader is capturing the shadow buffer, not the lighting. If your shadows are opaque / not being cast by transparent objects, then the buffer is opaque too. The shadows only look different intensities, because there’s overlapping light hitting them. Shadow data is rendered separate from and light data (then composited in the renderer before you see the beauty)

In features we’d probably render each shadow separately and adjust in comp.

Also what I’ve done to collect gi shadows and color at the same time - is to use an all-white, regular shader for the ground / environment in a separate pass, which you multiply in comp. But that gets a little tricky when you get values above 1.0, and non-shadowed areas need to render bright enough, so it does take some effort.

Shadow passes are usually either multiplied over the background beauty render, or used as a matte for color correction to the beauty - is that what you were doing?

#5

Yes I did some tests, and saw that assigning a white material and hiding the gometry, keeping just the floor, worked fine when multiplying it to the BG.
Yes in fact it seems that the Shadow Catcher catches the shadow buffer. How can I use this information in post comp or is there a way to catch the light instead of the shadow buffer?

#6

The only way to use that shadow buffer in comp, since you have overlap, would be to catch the shadows from each light separately ( unless Arnold has something special for that - I’m no Arnold guru, but I don’t think they do, I’ve never seen anything like it ). Vray has a feature that can save individual light contributions in a separate layer, if arnold has that, it should do the trick. Or, if you’re not trying to catch GI, you can make light mattes by changing the light (or shadow) colors to pure red, green and blue.

Using the white floor will probably get you the most accurate shadow levels, with less work than splitting out all the lights. Or maybe someone with more Arnold knowledge can chime in.