Only 2 released games have claimed AO to date (that I know): Crysis and FarCry2. They don't even implement AO which is very costly to performance, but
SSAO. I really don't know how AO or SSAO can be done on drivers. I think it's something that has to be implemented in the game engine. Otherwise the only thing they might be doing is replace every lighting DirectX call with a function (bunch of shader operations and its related DX calls) that calculates AO. And as it is said in the wikipedia depth buffer must be calculated several times. Engines that already have AO probably store depth data when the lighting is being calculated, but most probably others don't, so when that simple lighting call is replaced with the AO one, most probably depth data has to be calculated again along with many other things, reducing performance greatly, ie:
AO supporting engine:
1- Vertex data is calculated.
2- Depth data is calculated and stored in no volatile memory.
3- AO is calculated.
4- The scene is lit using AO data.
5- Color and texure data is calculated with MSAA (which requires depth data).
6- Everything else that needs depth data is calculated.
7- The rendering continues with everything else.
Take into account this just pretends to represent what might be happening and NOT how an actual engine works.
No AO supporting engine:
1- Vertex data is calculated.
2- The scene is lit.
3- Color and MSAA is calculated, requiring the calculation of depth data which is stored in vRAM.
4- Everything else that needs depth data is calculated.
5- The rendering continues with everything else.
Mutated no AO supporting engine:
1- Vertex data is calculated.
2- AO is calculated, requiring depth data to be calculated and stored in Vram.
3- The scene is lit using AO data.
4- Color and texure data is calculated with MSAA, requiring a second depth data pass.
5- Everything else that needs depth data is calculated.
6- The rendering continues with everything else.
Things can get far worse if all the lighting is not done at the same time, which is true for some games where objects and player/npc models are rendered after the environment has been rendered.
To finish I have to make some comments about the screenshots that have been shown in the thread:
- First of all, AO or SSAO are best seen in well lit indoors and preferably with a single light source. The reason is that AO simulates the darker areas created by occluding objects. You'll find very few occluding objects outside, specially in the Fallout3 screens being shown here.
- IMHO the first Crysis screenshot in the OP has already SSAO enabled in-game, with r_ssao=1, as the darker area in the ground, in the bushes and the rocks, suggests. Yeah in other screenshots it's noticeably darker, but if SSAO was completely dissabled there wouldn’t be any. See it on these screens (I found the place
):
r_ssao 1
r_ssao 0
IMHO without SSAO looks like garbage in comparison.
- Finally I would want to ask someone to make a proper screenshot comparison, that is, well lit indoors, with a single light source if possible.
as expected, totally useless feature. corridor sim. will get a bonus , but all the rest stay unchanged.
ps: many recent game use AO , but it's quite subtle .
As you can see above is far from being useless. Of course if you compare a ss with AO to a ss WITH AO enabled too, the difference is very small.
And none in an outdoor or very dark scenes like the Fallout3 ones.
Also I'm very interested to know which games/engines have (SS)AO, I have only heard Cryengine 2 and Dunia (Farcry 2). Oh and Offset, but when will that release?
) and because I have XP most probably AO option won't appear for me anyway. 
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