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Comment by esperent | Hacker News Reader

> Game developers are in love with screenspace ambient occlusion (SSAO).

I work in 3d and I think I can say with confidence that developers are not in love with SSAO. It's just that real ambient occlusion for moving models is both really hard to set up and would totally blow the performance budget for most games, while in most 3d engines you can add SSAO in couple of minutes and it costs ~1ms per frame.

That will hopefully change as graphics cards that support raytracing become more common. But in the meantime, we know SSAO is a hack. We know it doesn't look physically correct, we only use it because (if done well) it looks better than the alternative, which is no ambient occlusion, or ambient occlusion only on static models.

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The article may have been about SSAO, but it applies equally well to "real ambient occlusion", because raytracing voxels assume that corners "absorb" light like a black hole.

The photographs prove however, that corners in the real world do NOT absorb light, meaning that even "voxel ambient occlusion" and other "realer-models" are fake.

What people generally mean by Ray traced AO is just an approximation for path traced global illumination. It doesn't just work on the basis of corner=dark, and can be set up to be realistic.

It doesn't really have much to do with voxels, either. RTAO operates directly on the scene geometry (through an acceleration structure), or an approximation of it.

The true holy grail we are approaching and what game developers always wanted is ray traced global illumination, which is basically the perfect solution.

https://i.natgeofe.com/n/874df281-d3e0-489a-98c0-6b840023b82...

Here is a photograph of New York City.

Here is a CGI of a bunch of boxes with "Ambient Occlusion": https://upload.wikimedia.org/wikipedia/commons/9/91/AmbientO...

You can see that "Ambient Occlusion" ultimately comes down to "corners are darker" heuristic. It doesn't matter if you're doing "Ray Tracing based Ambient Occlusion" (see https://docs.unity3d.com/Packages/com.unity.render-pipelines...), or "Screenspace", or whatever. The heuristic is: "corners probably have less light", and the design of the algorithm works backwards from that.

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Now look at New York city again: a collection of boxes in the realistic sun. Carefully look at the corners.

Its actually kind of rare for corners to be darker in the real world. Every shadow must be calculated. Some corners are darker, while others are touched directly by the sun and have no darkening effect at all.

IMO, Ambient Occlusion "looks good" for the same reason that cartoons / anime "looks good", it instantly provides contrast in a slightly unrealistic fashion, which is more important for high-speed video games. The human eye can see the contrasts and better understand geometry with the "fake shadows" helping out.

The idea is, if the light is indirect or ambient, ambient occlusion will reduce indirect light. If the light is direct, then ambient occlusion will have no effect.

Which is exactly what you see in your picture of New York. If the corner is in direct light, then there is no difference from any other surface - or it is even more illuminated from radiance. If the corner is in indirect light, then yes, corners are less luminous.

It's not rare at all for corners to be darker in the real world. Compare a wide open, but shaded street, to a narrow alley. The alley will be much darker.

This is exactly the idea of modern AO with ray tracing. By raytracing AO, you can actually see how "open" each pixel is to the indirect light source. If the pixel is under direct light from some source, do not affect this component of illumination. If it is under indirect light from another source, be it the sky, or the walls, or some very large area light, reduce this indirect illumination.

So if the corner is directly lit by the sun, there is no darkening at all using RTAO and a PBR renderer.

It is indeed in the name. Occlude the ambient light component.

It's not a questions of "every shadow must be calculated". It's a question of approximating global illumination, or radiosity.

I can confirm that when the article was written, bad SSAO was everywhere :-)

In games where I've worked as a graphics programmer we have mainly used SSAO solutions such as HBAO and GTAO.

I wasn't really a fan of AO during the original craze but I really like these modern screen space versions.

For anyone else who was not born with the ability to instantly know what an acronym stands for, it appears that HBAO is horizon-based ambient occlusion while GTAO is ground-truth ambient occlusion.

Yes, sorry, that is correct. I guess we all get blind to the jargon we use in our daily lives.

For anyone interested the original GTAO article[0] is very well written and gives a really good insight into how these techniques work

[0]: http://iryoku.com/downloads/Practical-Realtime-Strategies-fo...

Is this the Quantum Foam wallpaper you were talking about awhile ago?

https://512pixels.net/downloads/os9-5k/Quantum-Foam.jpg

Hm, yes, about two years ago I believe. May I ask how you ended up following up on this now?

I googled something and found that old thread, but there was no other way to message you (that I know of).

Ok cool! Thanks for the tip.

AO is only half of what's needed, you need full GI for realistic corners. One phenomenon that the article didn't point out, but you can simulate trivially easy in Blender, is that corners that are perfectly lit are actually lighter, due to more reflected light reaching them. To observe, just set two grey planes at 90 degrees to each other and add a point light at medium distance. Render with Cycles. Pay attention to corners in well lit places with plain smooth diffuse walls like conference rooms and you will be able to spot it in real life too!

You don't really need full GI. RTAO in Unity for example does take into account direct light and treats it differently to get the described effects, as well as some degree of indirect lighting so some corners do look comparatively brighter in some places.

At the time that the article was written, SSAO was usually way overdone in games. It's an effect used because it's cheap, okay, but then it should be toned down, not turned up to 11 and drawing attention. I do remember the ridiculous fuzzy black bars in corners.

Same as it ever was. Remember when lens flares were The Cool New Thing and ridiculously overblown in every game that came out? Or when it was reflections, or shadows, or bloom, or...

Lens flares in games were always stupid, because they imply that you're looking at the scene through a camera, which breaks the immersion. I mean, they might as well also have a boom mike occasionally pop in from the top of the screen while they're at it.

I absolutely agree with you—but I think the same about lens flares in cinema, and given how entrenched they are in that realm, I'd say the rest of the world disagrees.

Now, yes, unlike video games, movies are in fact (usually) filmed with cameras. However, lens flares are still an artifact—they don't exist in human vision, and we know how to get rid of them. We just choose not to.

Don't give them any ideas! Actually I can see that being funny as an easter egg in CS or something.

Also, speaking as someone who wears glasses and has a bit of astigmatism, flares/haloes around lights aren't necessarily stupid or unrealistic (although the classic TV camera lens flare, admittedly not so defensible).

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