How can I render out a pass that contains my 3D model with an alpha channel?

[vanja_kapetanovic]

Is there a way that VRay can render out a pass that contains my 3D models separated. Like if I have an airplane and than some smoke animation and stuff...Can I export my airplane on one pass so I can composite that airplane with different color and stuff later. Like to have it on its own pass with everything else in the scene transperent (alpha channel), is that possible?

[fixeighted]

I don't quite understand. If you know what an alpha channel is then why not simply use that?
Unless I am misunderstanding and you want to do something more complex
If you don't quite understand alpha channels then it really is a simple thing.
Save your image/image sequence with a transparency, i.e. a png or other format that supports it and any geometry or volumetrics will be white/greyscale, while the empty backround e.g. transparency will be....transparent

[vanja_kapetanovic]

Yes but question was more I think in the realm of Object ID thing. I would like to be able to separate my models in post. So if you have 2 cgi characters fighting and you want to be able to see only one of them at the time in post, there should be an option where program is able to know "ok that's a separate geometry it's 2 things not one" and assign them a number or something. Idk I took a break off of 3D and vfx for a few years and it's possible I am overestimating where we are with technology. I just thought it would be possible as it's all inside the computer so who cares right. It's like "green screen" effect for 3D models. It would be neat if possible. There is a workaround with masking and than replacing the masks and what not so it can be done that way I guess. It's not even neccessary it's this NUKE generation with node compositing and stuff that kind of started doing this. Most movies are made without any passes back in the day. Like the best ones are just done right so they didn't need 300 folders with .exr's to get it right. so I am sure I dont need this at all I was more wondering whats the limits today with exporting and thats why I experiment wiith this.

[fixeighted]

Ok, so in that situation you can use cryptomattes and have them work on materials/layers etc. With the added benefit of working with motion blur etc. (though not transparency).
You can use multimattes if it's less complex.

[vanja_kapetanovic]

fixeighted Thank you for your solution. So I have a scene of a burning plane crashing from the sky. It's a model of a plane (contains multiple parts) and there is a PhoenixFD simulation going on. I was gonna use passes such as Diffuse, Lighting, GI,Slef Illumination+Atmosphere (for phoenix fx), refraction (for lights inside of the plane cabin and window lights, reflection and maybe specular. So essentially I was wondering how would I go about exporting just an airplane ONLY on one layer or pass or whatever it is called and than if I had like a city under that would be another layer I would want or If I had a ufo model chasing my plane that would be a third layer and so on. It's like when you HIDE everything BUT A specific model. That's all I wanted. To be able to not mess up everything once I start color correcting my airplane. It's so hard to separate elements, they have 5 methods and none worked so far (Material, Object, Render...). How do you use CRYPTOMATTES ? Thank you

[fixeighted]

This isn't really too tough. The passes that you render are entirely up to you, depending on what you want to do with the image itself in post - just read up on the passes in the docs, as things have likely changed since you last used them.

For the compositing tasks, with cryptomatte, just add cryptomatte as an element and it will automagically create perfect masks for each unique object in the scene. Just search online and there are many videos showing how it works for whatever software you use.

Best idea is to get a frame that you can test it all out on, then post again if there are problems.

[vanja_kapetanovic]

@fixeightd I don't think what I ask can be done. So far no one had a solution for this and also I know it's my lack of knowledge but that function of VRay pass where VRay just renders out a selected object can definitely not be done without masking and different kind of manipuation in post. Cryptomatte gave me similar result like all the other render object ID matte. It's several colors each assigned to an object rather than an object itself. I get it now. It can not be done the way I thought..So I will have to learn whatever way there is to isolate elements of the render. I don't really know how to use colors to isolate objects so that's why I said it's tough. If you look at my simple test rener I wanted to have 4 passes . PASS 1: SPHERE....PASS 2 FLOOR, PASS 3 CUBE, PASS 4 TUBE. Instead I get colored image of elements. I really don't know what am I suppose to do with this so maybe I should research how do use color in post to separate elements as that's obviously how it's done. VRay can not export elements ITSELF. I think at this point that's a fact as I actually probably used every single render pass offered by VRay and of cours none worked for that given task.

[vanja_kapetanovic]

This is what I was hoping to get. Each on separate layer including it's real color, material, light received and such. I can do it by hand but I wiill have to render my whole video from the beginning every time I select a new object and it took me 2 days to finish my actual render (not this one the one I am trying to render out actually) so if I have 4 objects it would take me roughly 8 days to get these passes. I see now why everyone is saying make sure you really need those maybe you can get away with less passes hahahaah. OOf coure, you guys are right I definitelly don't have 8 days for 300 frame render so, I will just separate maybe one element but bbasically yeah, this would be a great new feature for VRAy 6 or 7 to add.

[fixeighted]

You are correct; there is no magic solution that gives you what you want. It all takes an understanding of the shot and what specifically you want to have control of later.
This takes some organisational work and a knowledge of how to composite elements.
It honestly is not that hard to do anything these days and some quick searches will lead you to videos or other resources to guide you through the basic procedures.

If you want more specific help then if you post concisely, using an actual real-world example along with the problem you are having, then there are many here who can/will offer solutions
The above example is simple. You can separate each object by making the other objects matte objects, or by using a material wrapper. This way you don't need to mask anything.

[zoranm]

For these sort of shots I find very useful Render mask options. You select what you need to render by layer or include list and vray will automatically set up the rest of the scene to matte.
Z.

[squintnic]

This is not a specific vray feature, its a 3dsmax and pipeline feature you are after.
I had to do this recently and provide unoccluded and stacked objects with alphas to make a massive PSD where the layers could be moved around to suit the final client use.

check out this - it can do what you want.
https://renderstacks.com/

i think you are not listening to people and over complicating what is really a simple comp solution. if you want everything on seperate layers with shadows and reflections and everything you will be rendering in layers and rendering a LOT of frames. hope you have deadline ready for this task!

learn basic vray and basic alpha based compostiting first, we havent even seen what you are trying to actually do lol