books on comping with vray

[flipside]

Hi,

I'm looking for books/tutorials on comping with Vray. By comping I mean splitting the rendering in various layers like diffuse, reflection etc... How to use it, when to use it, when to use which layers etc...

But also comping as in seperating the BG from the foreground objects, rendering them in several passes so you can replace the background in post, or edit it in post. Usually when I try things like that, my shadows with alpha channel contain parts of the background/floor so I can't replace the background with something else. Or GI shadows don't get included, or become a lot darker etc...

So i'm a noob in comping!

I'm asking here in the hope some people have gone through the same process, and can point me to good books so I can skip all the bad ones

Thanks for any info.

wouter

[joconnell]

In terms of compositing the bets book for technique is the art and science of digital compositing by ron brinkmann of shake fame - it's not a software tutorial book, more a theory one. The chapter on integration techniques is by far the best few pages you can read on compositing. That aside what program are you planning to comp in? If it's after effects or photoshop then after effects studio techniques is great, otherwise it's gnomon and fx phd for nuke, fusion and shake. There's no vray stuff as such aside from chris nichols dvd but it does render elements comping rather than breaking up a render into elements or layers - Vlado did something about that though with combustion or fusion but all other compositors can do the same - http://www.spot3d.com/vray/help/150S...rials_anim.htm

[Morbid Angel]

there was a tutorial by chis nichols about comping, very well explained, but since the new forum its gone somewhere...

[RErender]

I've been looking around too, haven't found anything that I would call comprehensive. I'd love to find an example of how to comp area shadows together from two separate layers to avoid any halos..

[joconnell]

Yeah they're a bit of a pain in the arse alright - you've gotta cut out the shadow areas of 3d objects with masks if they overlay on top of real shadows and so on...

[dlparisi]

there was a tutorial by chis nichols about comping, very well explained, but since the new forum its gone somewhere...

He means this one...
http://www.chaosgroup.com/forums/vbu...ad.php?t=32485

[Morbid Angel]

yes...that one

[joconnell]

shimazake's photoshop importer has the guts of what you need by the way - http://www.chaosgroup.com/forums/vbu...3&postcount=15 - it lists the stacking order and blending modes you need to use and if anything photoshop is a good start since it'll apply to any other comper.

[sv]

I know this is slightly off-topic now, but i might as well ask this here seeing as how you guys know a lot about comping and i can't get to the bottom of it...
Also is Vray now missing the coverage channel because i'm running into problems when recompiling the elements and thought i might need it / it's been suggested to me. I know Vlado has said before that the only relyable method of comping is ADD, and avoid multing stuff, but people do this day in day out so can anyone point out what's wrong here:

Vlado says that you can't combine any elements with an operation other than 'add' reliably. I have found that the edges appear at the very first stages in this diagram, when multiplying RawGI and Diffuse - how do you get around this?

render:


comp:


difference:


diagram:


By the way - this is done with a trial version of nuke, which has a 30 day lock. I really want to get into this so am trialing the software, but can't afford it if i can't get this to work!

Many thanks in advance if you find the time to reply to this.

[Seraph135]

I know Vlado has said before that the only relyable method of comping is ADD, and avoid multing stuff, but people do this day in day out so can anyone point out what's wrong here

Vlado is right. You cannot get correct results when multiplying these passes. The problem is extra noticeable if you have reflective objects, but will still happen even if you don't. The anti-aliasing is the main problem. Say you have a 100% reflective object. In the diffuse pass it shows up black, and in the RawGI pass it also shows up black. Multiplying them together makes all the shared anti-aliased edges get darker. Even if your scene has no reflective objects you will still end up with different values along anti-aliased edges in your scene. So it will likely never be 100% exactly the same as if the whole scene was rendered together. You might be able to get it close enough in some cases though. If you don't use raw passes and just use passes you can plus then you can get the same exact output as Max.

The information on this page is a little mis-leading.

http://www.spot3d.com/vray/help/150S...erelements.htm

It says that multiplying RawGI and Diffuse together will give the same result as the GI pass. For the most part that is true, but the difference is that when your compositing passes your dealing with multiple frames that are all anti-aliased...Inside Vray (Please correct me if I'm wrong Vlado) all the math is done and then the image is anti-aliased.

So if you render your passes higher res without any AA, then resize them back down at the end of your comp you could use raw passes.

Tim J

[sv]

Thanks Tim.
So it might be better to render the passes Self-illumination + Direct lighting + Global illumination + Reflection + Refraction (all added); as the only operation is add rather than complicating it with elements that require multing. hmmm ok.

With the above difference result, i suppose you could try to use this luminance and invert it, and try to use this to darken the poorly Anti-ailised edges a little to try and blend them a little better....bit messy, - what about a sub-pixel element and a coverage element as Thorsten once suggested to me? You could use these in the same way presumably to mask or maybe even just darken these troublesome edges?

Would this be possible? Excuse my ignorance if not - still trying to learn / teach myself like a lot of others.

[Seraph135]

With the above difference result, i suppose you could try to use this luminance and invert it, and try to use this to darken the poorly Anti-ailised edges a little to try and blend them a little better....bit messy,

Well...Your going to have only limited success with that. You can only get that to work if the result of the difference is added back...If it needs to be multiplied then your still going to have problems. For example the method below will make the (Diffuse * RawGI) and GI match exactly.

(Diffuse * RawGI) + ((Diffuse * RawGI) Difference (GI))

what about a sub-pixel element and a coverage element as Thorsten once suggested to me? You could use these in the same way presumably to mask or maybe even just darken these troublesome edges?

I wasn't able to get this to work. But maybe I wasn't doing it right. If Thorsten is around perhaps he can explain that one.

Tim J

[sv]

I wasn't able to get this to work. But maybe I wasn't doing it right. If Thorsten is around perhaps he can explain that one.

Tim J

umm - I might be confusing things here - lack of knowledge i'm afraid....I'm not really sure that a subpixel or coverage element could do this - in either case they aren't included in these current builds so can't test it.

Thanks for the pointers Tim - but in the event of these edges, what is best to do? Would you just have to avoid re-compositing these elements causing the problems? Perhaps just use mattes and Mtl_Selects to isolate parts of the image? Seems like a shame to me....any way around it?

[Seraph135]

any way around it?

Actually there is. Don't render the raw elements...create them in the comp from the diffuse, GI and Lighting passes. If you do the math its actually quite simple. I Don't know why this didn't occur to me sooner. I just used this same method (but on different passes) on a project at work recently.

If (Diffuse * RawGI = GI) then (GI / Diffuse = RawGI). You can also divide the Lighting by the Diffuse and create the Raw lighting pass.

Then you can work with those passes and put them back together again to create the correct final output. If you do this you will likely need to post multiply the alpha again since dividing the RGB channels will (sorta not exactly) un-premultiply the image. But you would need to un-premultiply the image anyways if you were working with RAW passes to avoid halos around your matte.

This method does not work for the Raw reflections or refractions though.

Tim J

[sv]

perfect - thanks so much.
Will look forward to using / trying this.