Any tricks for using G-buffer passes

[Adam H. Stewart]

I know that you cannot save an individual alpha channel for the various G-buffer passes in Vray - so what's the trick for using say a shadow pass which appears white over a black background how do you comp it into your Diffuse pass.
The way I have it set up at the moment in AE is I import the shadow pass turn it off and create a solid. I then create a black solid and set it's alpha channel to the luminosity of the shadow pass.
It works pretty well for shadows, but how would you comp say a GI pass, that has colors - so the solid technique would be of no use.
I know there has to be a better way - any idea's ?

Thanks as always.

[Morbid Angel]

usually for shadow pass its better to use vrayMatte with alpha set to -1, but if you want to use gi pass for example you can add it or multiply it.

[Adam H. Stewart]

Interesting

Interesting I'll give that a shot. I have another question maybe you can help me with. When I render using VFB, selecting the Shadow layer only, Vray also saves out a seperate RGB_Color and alpha pass for me, without me selecting the layer. Is this normal ?

Thanks for the tip on the Vraymatte - I'll give that a shot.

[Morbid Angel]

hmm...I dont know. Vlado has suggested not to use VFB at this point as it is buggy. I never use it, and produce all the pasess manually, this way I have total control over every pass, plus gbuffer does not anti-alias some of the pases like reflection, normals and some other pases.

[Adam H. Stewart]

G-buffer on hold

Maybe I should put it on Ice for now. A shame as I have spent many days getting it to work with Deadline . If only Vray had a similar system to the standard Scanline - it would be next to perfect.

Thanks for your help.

[vlado]

See this page on information on how passes are combined together:

http://www.spot3d.com/vray/help/VRay...buffer.htm#ex2

Basically passes are added together - which is why you don't need any alpha when combining them.

Best regards,
Vlado

[vlado]

On another note, the RGB and alpha channels are always generated - no matter which other channels are selected.

Best regards,
Vlado

[Adam H. Stewart]

Now I'm getting it

Now I'm getting it - I think . So if I wanted to composite the sadows only onto a background plate I would need to have the raw lighting and the shadow pass combined with the Diffuse color inverted ?

- no I'm not getting it I'm still confused.

Lets say if I wanted to remove the shadows from a shot - I would add the raw lighting to the shadows - removing the shadows from the raw lighting - and then multiply this by the diffuse (giving me the Direct Lighting without any shadows) and then add the the Global illunimation (I have no reflections or refractions) which would be the same as the the Real RGB color channel - without the shadow - that would work right ?

Somebody tell me I am not going nuts - this is confusing as all hell.

[vlado]

Re: Now I'm getting it

Lets say if I wanted to remove the shadows from a shot - I would add the raw lighting to the shadows - removing the shadows from the raw lighting - and then multiply this by the diffuse (giving me the Direct Lighting without any shadows) and then add the the Global illunimation (I have no reflections or refractions) which would be the same as the the Real RGB color channel - without the shadow - that would work right ?

Yep, exactly. You can control shadow strength by darkening the Shadows channel before adding it to the Raw light.

Best regards,
Vlado

[vlado]

Re: Now I'm getting it

Now I'm getting it - I think . So if I wanted to composite the sadows only onto a background plate I would need to have the raw lighting and the shadow pass combined with the Diffuse color inverted ?

Take the Shadows pass, invert it, and multiply it by the background.

Best regards,
Vlado

[J_man]

Re: G-buffer on hold

If only Vray had a similar system to the standard Scanline

You could ue scanline to create your shadows and mattes, it will match up perfectly with your VRay rendering.

J¬

[percydaman]

Re: G-buffer on hold

If only Vray had a similar system to the standard Scanline

You could ue scanline to create your shadows and mattes, it will match up perfectly with your VRay rendering.

J¬

keep in mind, that wont really work great if your using vray's 3d motion blur.

[Adam H. Stewart]

Motion Blur

Yeh the motion blur is kililng me on rendertime, but Damn it looks great. I will have to post some images when I am allowed, I have managed replaced a CG book with a live action book and you really can't tell. The Art Director (she is super fussy) just came in and was delighted

My frames are pretty big - oversized for 2D tracking, 1300x731 and my settings are damn high - better to be safe then sorry with the old noise issue in GI. I would hate to have waited 40 hrs to have the bloody thing look all grainy. A more skill Vray artist would tweak the settings down but I prefer to crank it up and be safe.

The solution I have come up with for the shadow pass is pretty simple - invert it and then use the luminance channel for the matt. Seems to be working fine.

Thanks for all your help.