Using ambient occlusion to fake GI - help

[Gary Mc Ginty]

We would like to composite an animation of a operating Theatre with moving people. At the moment its flickering even on high settings.

 

We got advice to use ambient occlusion to fake GI.

 

We know how to set up the ambients occulsion which we tried and it is working well.

 

what we dont know is what to use for the other passes?

we have tried to just turn off the GI but the results are not great. The rendered scenes were very dark.

 

Any idea of how to setup the other passes and what would be the best settings for the compositing?

 

 

 

thanks

[Nic H]

Your ambient occulusion pass is wrong. How are you doing it? It looks like a environment override something?

 

You need to use Render Elements Vray_ExtraTex with vray dirt as the map, adjusting the dirt parameters to suit your scene.

 

Then use multiply in post and adjust opacity to suit.

 

_ _ _

 

Also, you shouldnt be getting flickering in that scene. Seeing as though you are compositing anyway why dont you do the people seperately using vray matte object?

Edited July 30, 2008 by nicnic

[STRAT]

yes, wrong AO pass method. in Cinema you give the complete scene a luminous white material and run AO over that.

 

also, your initial pass should be a standard beauty pass. i dont know what you've done there. AO is just an added overlay thats lightly comped over in post.

[Gary Mc Ginty]

thank you for the fast reply.

 

We went through some AO tutorials saying we should use an override material with a Vraylight material and a VrayDirt inside. this is what we have done for the AO.

we can not find the render element Vray_ExtraTex, is this in Vray 1.50?

 

 

and could you please explain what to do for the first pass? we should not use the GI so we turned that off but then we have not got enough light in the scene.

 

and for using Vray matte shadow, the moving people sometimes get flickers, that is why we wanted to try it with AO.

 

 

 

thanks

[Fran]

Hi Gary,

 

Add a few omni lights around the scene that are set to ambient only. Then render your direct light/color pass so that it will look like what you have only it will relieve the dark areas. Then comp that with the A/O pass you've already done. In Photoshop, adjust the opacity and brightness of your AO layer until you like the results.

[Crazy Homeless Guy]

As Strat said, your base pass should be a standard GI beauty pass. AO is designed to enhance the GI pass, not replace it.

 

Using AO you should be able to use lower settings on your GI pass, therefore speeding it up, but still bring out some of the detail once you apply the AO pass.

 

I don't think your AO pass is as bad as what is being implied. You should still be able to get something decent out of it if you do a level adjustment on it in Photoshop.

 

Also, try applying the pass using Multiply instead of Overlay.

 

I attached the corrected AO pass in my attachment. Basically, you want the layer transfer mode set to multiply. Then, the areas that are white, or near white in the AO pass will not effect the image, but the areas that are dark will add detail to the image at the contact points, corners, or other places where there is darkness.

[Nic H]

thats not an AO pass either.

 

it should basically be even black and white, with black on surfaces where they are closest to other surfaces, spreading to white on 'open' areas.

[RyanSpaulding]

You cant fake GI by adding an AO pass to a non-GI render. To fake GI, you need to add lights on the bottom of the model...maybe a few and set them below the floor....then exclude the floor from casting shadows. That will get you closer to your desired result.

[STRAT]

You cant fake GI by adding an AO pass to a non-GI render.

 

yeah you can

[Mr-JosE]

Its quite simple really. Make a 100% illuminated standard material, add a v-ray dirt map to the diffuse slot, and adjust the settings of the v-ray dirt map to suit. The occluded colour should be black (or any colour you want shadows) and the unoccluded colour should be white. Adjust the radius to control the spread of the AO pass. Nice and easy.

 

For your render settings; on the global switches rollout turn off everything in the lighting group. Use the new AO material in the override material slot.

And make sure GI is turned OFF.

 

Hit render and you will have your AO pass.

 

For you to use it as a replacement for GI you need to make a diffuse and a direct light pass using the render elements controls. Setup your scene as you would normally, without GI, and activate the vray_diffusefilter and vray_lighting filters.

 

Once rendered load them into PS. Make two groups, one for AO, and other for Lighting. In the AO group you should have your diffuse pass with the AO pass on top of it set to multiply blend mode. Set the groups blend mode to linear dodge (add). In the direct lighting group you have the diffuse pass with the direct lighting pass on top of it, with the blend mode set to multiply too. Place the AO group above the direct lighing group.

 

With this setup you can control how much 'GI' you have, tint the GI, tint the direct light etc, etc.

 

Hope this is clear.

[Gary Mc Ginty]

Hi everyone,

 

Thank you again for the new comments and the help.

 

I have searched the internet to get the answer but could not find anything really detailed.

 

Mr-JosE, thank you very much, your explaination seems very clear and you are exactly answering my question.

I will try this and let you know how I am getting on but I am sure it will be fine.

[RyanSpaulding]

yeah you can

 

Hmn...explain the concept to me then...because the way I see it, you'll have a non GI render which will be dark in non-lit areas...and the AO pass tends to darken areas not lit by light. Would you invert the AO pass and use that to add brightness the the dark areas?

 

Yo estupido.

[Dave Buchhofer]

Hmn...explain the concept to me then...because the way I see it, you'll have a non GI render which will be dark in non-lit areas...and the AO pass tends to darken areas not lit by light. Would you invert the AO pass and use that to add brightness the the dark areas?

 

Yo estupido.

 

because you also need the 'DIFFUSE filter' pass to use as the base. diffuse filter is just the color with no lighting applied. you'll then want to add in your direct lighting, and multiply your AO pass on top

[RyanSpaulding]

Ah ok...I was thinking of it differently...I was specifically talking about his lit, non GI image like his 1st one...and then adding AO to it. I guess I missed the part about it being a straight diffuse render 1st.

[STRAT]

yeah, you cant just switch off gi, hit render, then add an ao pass. as you say, you'd just make the darker areas darker. you'd be lacking your gi environmental lighting.

 

you need to either run a gi pass using zero samples so you get the lighting levels/atmos correct, or switch off gi completely and put in lighting to compensate. then run the ao pass and comp it. Vray's gi is generally very fast anyway, especially if it's pre-cached at 50% original size. probably faster than rendering a separate ao pass.

 

here's a quick rough and ready test i did to illustrate -

 

 

01) beauty pass - no gi or AO at all. just flat scanline style diffuse and lighting

 

 

 

 

02) AO pass - pure AO, rendered in scanline mode

 

 

 

 

3) beauty & AO comp - the AO pass is layered over the diffuse pass in photoshop. The layer mode is turned to MULTIPLY.

 

 

 

 

4) full gi render - the original was rendered in full gi mode too.

 

[zipzip]

Hi SRRAT, How do you get a GI pass with 0 sample , can you explain it a little more, i am trying to do what you said ,but i am not sure if i am doing it right. Thanks.

[STRAT]

you're asking a question of software and techniques which are nearly 3 years old. times and methods are rapidly changing. if i were to reply to this thread now i'd highly recommend you use full-on correct gi methods. AO as a primary gi method isn't needed or recommended these days. gi is, in most cases, a lot faster than ao is anyway. AO should only really be used as a subtle enhancement to your lighting, if you even need it at all.

[Crazy Homeless Guy]

AO should only really be used as a subtle enhancement to your lighting, if you even need it at all.

 

+1

[zipzip]

thanks.

[sdds]

I dont no if this is a bit late for you but if you are using the newer version of vray, sp5 there is an ambient only light that can be used to fake gi for parts of your aniamtion. We use this all the time for moveing objects. As for faking gi overall it can be a very tricky process and not easy to do in vray. you need many lights to fake colour bleed and bounce with ambient only lights, then use ao for soft shadows, this method needs good post work, colour correction to get it to work though.

[zipzip]

Thanks sdds, I run a quick test with vray ambient light and put a vray dirt map in light map slot, it seems it is working ,but i haven't test it for complex scenes.

 

thanks again for the tip.