Matte setup - exclude environment

[Art48]

Hi,
I got a simple scene set up with a car on a plane and lighting through VRaySun and it all looks perfect.
For post production purposes I render the background seperate from the car.

Now usually this is a fairly simple setup so that I can get the cars effects (shadows, GI caustics, GI) in the car image itself and just drop it over the background in photoshop.

Here comes my problem: How can I make the environment (VRaySky) make all the stuff it does but NOT be visible in the reflections of the ground plane (which is set to matte object, since I only want the car visible).

This stuff needs to render tonight so I am in kind of a hassle here :-/



1: This should go away!
2: Shadow should stay; not neccassarily in alpha but in the RGB channel
3: the little white specs will later be GI caustics, I need them, too
4: I guess GI from the car onto the background should stay, too.

Can you help me out here?
I need the environment do all it usually does BUT not be reflected in the ground plane matte object.

[LudvikKoutny]

Matte objects should never reflect environment. Do you use domelight? There was a bug (now resolved in beta 2 i think) which caused dome light to reflect in matte objects even though it should not.

[Art48]

Thanks for the fast reply, nope, no dome light in there just plain VRaySun and Sky.
BUT I do have VRayAerialPerspective enabled with "affect environment rays" checked. Cant disable it though since it changes the look of reflections of the horizon in the car paint.
I just saw that my alpha channel looks as follows, it kind of looks correct but I dont know if I will get the GI caustics in there, too. But at least the reflected environment seems to be way less or even gone when looking at that alpha itself:



Does this look correct to you? I'd just go with "As Transparency" in Photoshop then?

[LudvikKoutny]

Aside from noise, that does look correct. But chances of compositing anything correctly in Photoshop are very slim, as Photoshop sucks at everything compositing related.

You have to save Alpha separately, using separate render channels output in V-Ray settings. Then, your alpha will be saved with colors. Then, you will invert the alpha and multiply it on background, that will give you shadows of exactly same color as in beauty pass. That will also take care of subtractive part of compositing - shadows. Once you do that, you will have to add your beauty pass rendered against black BG on top of that alpha multiplied on top of that backplate. That will take care of the additive part of the compositing, such as GI bleeding and caustics. Once you do those two things, you will get 1:1 same result as if you rendered everything together right in 3ds Max.

[Art48]

Wow, if that is correct you've been a huge help, Ill test that right away with a smaller resolution image. Not sure if I understand you correctly though: Are two renderings still sufficient? BG and Car as I've shown it above? In the end I would need to seperate it into 3 parts right? BG, Car (without ground effects) and ground effects (without car)?

[Art48]

Sadly I cannot invert anything in PS with 32 bit. And I am correct in assuming it only works in 32bit? Tried it in 8 bit, failed. The car looks same, but effects on background not. Is this even doable in PS? :-/

[LudvikKoutny]

Yep, looks sufficient.

Just grab yourself a free version of Fusion. To save your hair from being pulled out

[Art48]

Got the free version of fusion, just tried it and I still cannot seem to get a pixel perfect match. Basically nearly all works, but shadows. I dont get as deep shadows especially in contact areas. Apart from that comp needs to be done in PS since free version of fusion doesn't support 6k images.

Well, I think actually when I tried in PS to just "over" the two images, background and the car beauty with "as transparency" I don't get the colors in the shadows as you said but in the end it may be less hassle. But some kind of way to do this without magic tricks in photoshop would be really nice -.-