How to composite ambient occlusion pass?

[magneto]

Hi,

I am trying to render out an object in passes and then composite them in the COP context but I am not sure how to comp the ambient occlusion.

I watched Peter Quint's latest Passes in H12 tutorial but in his example the AO was grayscale. Mine has some color tint. Not sure if this is improper?

Also my main diffuse render looks darker, while AO looks much cleaner and well-lit (skylight). I rendered the AO pass using only the skylight (sole light).

SO the skylight is only rendered for AO. When I render the main passes (direct_diffuse, direct_reflect) with the skylight, then color correcting this works just like I want. But I don't know how to do the same thing using a separate AO pass?

Peter Quint also does the same, and combines the AO afterwards. But when I do this, like I said the final image appears very dark in certain areas. Should I be using more lights for this purpose?

Thanks

EDIT: Can I also have objects that are not visible but contribute to the lighting? I have a floor object, when it's visible actually makes the main object much brighter which is what I want, but I don't want this floor object visible in the main pass. I added it as Forced Phantom, but then the result is the same as if the object is invisible, so it doesn't seem to contribute to the reflections the same way when it was actually visible.

Edited May 21, 2013 by magneto

[bloomendale]

imo:

multiply your ao only with ambient (indirect) light pass and add the rest (it's why we have A in AO).

if you multiply it with the whole light set you'll get more "dirt" in some areas cause direct passes already has their occlusions (O from AO) - shadows.

[magneto]

Thanks, so I shouldn't render out direct lighting? I thought AO was indirect light pass? I probably need to research more about these, but I don't have much comp experience.

[anim]

if you are talking about AO pass created by Envlight or VEX GI Light shader then it simulates direct lighting of evenly lit sky dome

AO mode ignores sampling of color from environment map and therefore is a little faster, but less accurate and cannot be used to light the scene from HDRI because of mentioned smplification

but apart from that Direct Lighting mode and AO mode of env light will give you proper light pass and therefore they are composited by simple addition to other diffuse light passes (you can dim or brighten each pass before adding them together)

and of course you should use one or another, Direct Lighting is preferred since it's more accurate

if you have pure white AO pass (created by Surface Shader), you need to multiply it first with Color Pass (or Paint Pass or Albedo pass, everybody calls it differently, but it's pure color pass without lighting) to get colored AO pass which is scene lit just by uniform sky dome (same as Env Light AO gives you) then composite it as diffuse light pass (using add)

many people use AO pass as "artistic" dirt pass by various multiplying on top alchemy, this usage is up to you, but that is only if you are going for such feel, since that usage is non-physical

[magneto]

Thanks anim. In Peter's example he was using direct lighting for the main pass and AO for the AO pass. If you have a color skylight set to direct lighting or AO for example, is it ok to use this in comp or should you try to be neutral for skylight/envlight and try to get the tint from sunlight, night light by using separate lights and then comp them in the end?

[anim]

I tend to setup my render as closely to final output as possible so if I want colored environment or HDRI map I will use already colored version for my environment light since it contributes everyvhere not just direct diffuse (but specular refraction indirect diffuse and volume bounces as well) in comp to get full control over tint of the light you need to export per light specular refraction, indirect etc, but too many passes may be nightmare for the comp and usually not very necessary

it's as well easier for me to tweak color of the lights and other properties directly in live render view than trying to tweak that in comp where there may be too late to do some major tweaks if necesarry (like moving lights, changing attenuation etc)

it as well depends on your pipeline, you can outsource a lot to comp if you want

I don't render AO nowadays, it was good technique when direct environment light was expensive to compute

so if you already have direct environment light then AO is just a bonus if you really like that pass and have some creative use for it in your scene

[magneto]

Thanks anim, that makes a lot of sense. I will have to do more rendering and comp to get a better understanding but it's clearer now