GI vs. Ambient Occlusion

Started by N-drju, May 19, 2014, 02:05:28 pm

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N-drju

Please explain me what is the difference between global illumination and ambient occlusion. I can't seem to figure it out.

My simplistic view is that GI is ultimately a good thing, but many early morning and late dusk scenes turns out horribly dark in many shadowed spots with GI applied. :P

Ambient occlusion on the other hand makes day brighter (so to say :D) but is not fully as effective and real in terms of shadows.

What's the key to understand all this?
"This year - a factory of semiconductors. Next year - a factory of whole conductors!"

archonforest

May 19, 2014, 03:32:55 pm #1 Last Edit: May 19, 2014, 03:37:28 pm by archonforest
See this on GI:
http://en.wikipedia.org/wiki/Global_illumination#List_of_methods

And this on Ambient:
http://en.wikipedia.org/wiki/Ambient_occlusion

Hope it helps :D

Best to render the same scene with the two method and then switch in between and see what happens with the shadows..etc
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N-drju

Yes, that's what I did. Right now I'm working on a, let's say, promo image for my... book. :D

I placed human model in a shadowy area between the trees. Ambient occlusion turned out bright, not to say optimistic. The problem of course was that shadows were rather poorly rendered. However, global illumination encompassed my human model in almost complete, gloomy darkness with a setting of RD=3 and SQ=4! Trees don't cast that sort of shadow... Come ON!

The differences between the images are rather minor.
"This year - a factory of semiconductors. Next year - a factory of whole conductors!"

archonforest

In this case post your file so people can take a look of it.  ;)
Dell T5500 with Dual Hexa Xeon CPU 3Ghz, 32Gb ram, GTX 1080
Amiga 1200 8Mb ram, 8Gb ssd

Hannes

When you're using GI you can increase the strength on surfaces of the environment light to make the shadowy areas brighter. Try small steps.
GI is the more natural way. AO can be enough in some cases, but I'd always stick to GI especially with the GISD feature.

dandelO

May 20, 2014, 06:52:12 am #5 Last Edit: May 20, 2014, 06:55:40 am by dandelO
I like to use a GI light for surfaces and, if it's not needed I just drop its Strength in Atmosphere down to '0'. That speeds up renders by a huge amount, it's  good for bright daylight scenes where atmosphere shadows aren't so important.

Sometimes, though, you won't want to do this. Disabling GI in atmo is really, really fast but maybe you need some atmosphere Envirolighting, you could try with any fill-lighting to brighten the atmo, another tip that some people liked when I posted it before was: Use two Envirolights. Disable Atmo Effect on one(GI), disable Surface Effect on the other(AO). That will give you envirolight for both areas, at a much cheaper cost. AO comes out very strong in atmo and surfaces, try setting the respective Envirolight(AO) strength to about '0.1 to 0.25.'

While I'm posting lighting tips, not strictly GI related but. Soft shadows can take a long time to render, especially in heavily atmospheric renders. Chances are, you won't see the benefits of high SS samples on your surfaces because you'll hit 'Stop' on the renderer because it's taking too long, the atmosphere is likely making use of your Soft Shadow samples, that you're unlikely to ever see shadows from.
Simply duplicate(ctrl/cmd+D) your sunlight. On the original sun, uncheck Lights Atmo.
On the new one uncheck Lights Surfaces and Soft Shadows.
That should speed up some rendering, a lot!

N-drju

Okay, so I made a final render with GI settings of 3 and 8 and supersample prepass checked. The image turned out nicely and not too dark. I did increase sun power though to 3.1 (because originally opted for something like 2.5) and the enviro light power on surfaces to 1.1. I think that will do. Tree shadows look credible so, no harm no foul.
"This year - a factory of semiconductors. Next year - a factory of whole conductors!"