Multi pass rendering question

[alex321]

HI
I m pretty sure this question has been asked and i apologize for making you the experts repeat. But I noticed in the Vray Options the VFB Channels with layers such as diffuse channel, background, alpha, shadow, lighting etc...
Is it possible to do multi pass rendering then? How can I make it work such as if I render a scene and then go on in Photoshop and open my diffuse file, light , shadow files etc...
how can I change the diffuse color in Photoshop with all the layers put together?

Thanks a lot and regards

Alexandre Galin
www.alexandregalin.com

[dalomar_cg]

Re: Multi pass rendering question

I am by no means an expert here, but I'll take a stab at what your asking at anyway...

What you're looking for is compositing, which is basically taking all of the components of a given rendering (diffuse, lighting, shadow, reflection, etc) and assembling them together via certain operations (addition, subtraction, multiplication) to create the final result. This is something that is really beyond the capabilities of photoshop (it can do some of it, but not all of it AFAIK), and is more about programs like After Effects or Combustion as these programs allow for much more control of how these different layers are assembled.

Here's the "problem"...this whole system only really works if you can have all of this assembled together in the right manner...before that point doing things like changing diffuse colors may not really be possible. Enter the other issue...we don't have all of the channels that are needed. We're actually missing certain channels such as reflection and refraction, which I believe is an issue with there being a gap in the V-Ray SDK. You can get all of those channels, but you will have to render to VRimage and then export all of the channels from them.

All of this being said, it would be possible to take certain channels from the VFB and use them, selectively, in photoshop. Personally, what I've done on a number of occasions is to use a shadow pass within photoshop to get a matte/shadow affect for a groundplane... you could also use a GI pass to add a bit more contrast so some areas as well...

The long and short of it (at this point...and from my view) is that there are some capabilities that can be gained from doing multipass stuff with VfR. But for the most part, full blown compositing is going to be something that is probably going to require a drastic change in your V-Ray workflow and possibly some software that you don't have. If you have some specific questions about what you're trying to accomplish that might be more helpful to see if there's a solution for it within the current capabilites of things...Hope this helps.

[Micha_cg]

Re: Multi pass rendering question

A friend tell me, that Vray 1.6 for max will support a full psd channel output. Damien, do you know something about it?

[dalomar_cg]

Re: Multi pass rendering question

With vfMax 1.5 you can have VRimage file write directly to an EXR file and that will have all of the channels with it, so you won't have to convert the VRImage file...is that what you're talking about? I doubt you would be able to write all of the channels directly to a psd for the simple reason that psds can't handle hdr data, which is very much needed good compositing.

[alex321]

Re: Multi pass rendering question

Thanks Damien for your comments
It does give me an idea of what can be done.
Multi pass is something I would like to do more often now . I don t have much experience but it seems a great way to make changes in a very short time.
Thanks again

Alex

[Micha_cg]

Re: Multi pass rendering question

Thanks Damien, yes, without HDR data it is not so useful. Only the extended version of Photoshop allow 32bit layers. But 16bit psd could work quite good too. I'm curious, an all-layer-output-image could be a great helper.

[dalomar_cg]

Re: Multi pass rendering question

Yes, it would...you could also setup a batch file that would automatically extract the layers from the vrimg file and into an exr through photoshop. That would probably require a little trickery (a bit more than I can do), but I'm sure its feasible.