Screen Space Ambient Occlusion – Application in games

I’ve been working on a screen space ambient occlusion code for a few weeks and I’ve managed to get a working project going, so I feel I’m qualified to talk about it. So let us begin. In computer graphics, ambient occlusion attempts to approximate the way light radiates in real life, especially off what are normally considered non-reflective surfaces. Like in my example here:


Unlike conventional methods such as Phong shading, ambient occlusion is a global method, meaning the illumination at each point is a function of other geometry in the scene. However, it is a very crude approximation to full global illumination. The soft appearance achieved by ambient occlusion alone is similar to the way an object appears on an overcast day.

The first major game title with Ambient Occlusion support was released in 2007, yes, we’re talking about Crytek’s Crysis 1. Then we have it’s sequel Crysis 2 as shown here:


It’s a really cheap approximation to real ambient occlusion, it’s used in many games. It is cheap, although it does draw out a significant overhead compared to running without SSAO on. Mafia II churns out 30-40 fps when I turn on AO on max settings without PhysX, but when I turn AO off it’s silky-smooth at 50-75.

Without ambient occlusion:


Screen space ambient occlusion is improved in Crysis 2 with higher quality levels based on higher resolution and two passes while the game’s water effects – impressive on all platforms – are upgraded to full vertex displacement on PC (so the waves react more realistically to objects passing into the water). Motion blur and depth of field also get higher-precision upgrades on PC too.

With ambient occlusion, note the presence of darker areas. 


 While there may only be three different visual settings, it’s actually surprising just how low the graphics card requirement is to get a decent experience in Crysis 2. In part, PC owners have the console focus to thank here – Crytek’s requirement of supporting the game on systems with limited GPUs and relatively tiny amounts of RAM required an optimisation effort that can only benefit the computer version.

According to the developer, the base hardware is a 512MB 8800GT: an old, classic card that’s only really started to show its age in the last year or so. On the lowest setting (which is still a visual treat), this is still good for around 30-40FPS at lower resolutions such as 720p and 1280×1024. If you’re looking for decent performance at 1080p, a GTX260 or 8800GTX is recommended while 1080p60 at the Extreme level really requires a Radeon HD 6970 or GTX580.

In terms of the CPU you’ll need, things have definitely moved on from the days of the original Crysis, where the engine code was optimised for dual-core processors. Crysis 2 is quad-core aware, and while it runs perfectly well with just the two cores, ideally you should be targeting something along the lines of a Q6600 or better.

Nowadays we have a bunch of DX10 & DX11 games that make use of Ambient Occlusion natively ( via their own engine ), Crysis 2, Aliens vs Predator 3, BattleField 3, Gears of War 2, etc.

Still, there are several new titles without Ambient Occlusion support, and also older games that would benefit from it. nVIDIA came to the rescue a couple of years ago with an option to force Ambient Occlusion on various games by enabling the corresponding option in their drivers Control Panel.

Initially you could only choose from “Off” and “On”, but from the 25x.xx drivers and on you have three options to choose from, “Off”, “Performance” which balances the effect’s application to enhance your image while keeping the performance numbers relatively close to what you had without A.O. enabled, and “Quality”, this option sacrifices performance, often at a massive rate, but gives you the best image quality you can achieve with A.O.

As a result of extensive testing of all 3 settings of Ambient Occlusion in a couple of games, this is our result:

Ambient occlusion is related to accessibility shading, which determines appearance based on how easy it is for a surface to be touched by various elements (e.g., dirt, light, etc.). It has been popularized in production animation due to its relative simplicity and efficiency. In the industry, ambient occlusion is often referred to as “sky light”.[citation needed]

The Terrain Ambient Occlusion system controls the amount of ambient light in a scene. For example, a dense forest has less ambient light near the ground because most of the light is stopped by the trees. In the current implementation, occlusion information is stored in textures and the effect is applied to the scene in a deferred way.

The images below show the difference (in a scene) when Terrain Ambient Occlusion is enabled and when it is not.


The ambient occlusion shading model has the nice property of offering better perception of the 3d shape of the displayed objects. This was shown in a paper where the authors report the results of perceptual experiments showing that depth discrimination under diffuse uniform sky lighting is superior to that predicted by a direct lighting model.

High 32 is highest smooth frame rate (steady 60fps) that I’ve found while testing with a MSI GTX 680 Lightning.


What are we looking at here? Alright, the best example is the vase held up to the candle. Notice the blurry yet real time shadow effect which is cast behind the vase? There you go, that’s SSAO in Amnesia. The thing is SSAO is entirely dependent on angle: if you were to stand behind the vase the shadow wouldn’t show up yet the vase would have a black glow to it. The bookcase, the fireplace and the dresser show other examples of SSAO effects you’ll see throughout the game. SSAO maxed out on High/128 killed my framerate down to 14 FPS and there was little difference compared to the much more playable setting at Medium/64. Overall, the game looks better with some form of SSAO enabled. In some areas it made things look engulfed in some ugly blurry black aura. Still, a good feature to have in a game that uses shadows for its overall atmosphere and I appreciate the effect SSAO tries to achieve yet I think it does a pretty sloppy job at emulating ‘darkness’. Setting this setting maxed out isn’t a good idea either, you’ll actually get less “black glowing” if it’s set to around 16/32. Now if only they had some form of Anti-Aliasing to play with.

The occlusion at a point on a surface with normal can be computed by integrating the visibility function over the hemisphere with respect to projected solid angle as shown below:


where  is the visibility function at , defined to be zero if  is occluded in the direction  and one otherwise, and  is the infinitesimal solid angle step of the integration variable . A variety of techniques are used to approximate this integral in practice: perhaps the most straightforward way is to use the Monte Carlo method by casting rays from the point  and testing for intersection with other scene geometry (i.e., ray casting). Another approach (more suited to hardware acceleration) is to render the view from  by rasterizing black geometry against a white background and taking the (cosine-weighted) average of rasterized fragments. This approach is an example of a “gathering” or “inside-out” approach, whereas other algorithms (such as depth-map ambient occlusion) employ “scattering” or “outside-in” techniques.

Along with the ambient occlusion value, a bent normal vector is often generated, which points in the average direction of samples that aren’t occluded. The bent normal can be used to look up incident radiance from an environment map to approximate image-based lighting. However, there are some situations in which the direction of the bent normal doesn’t represent the dominant direction of illumination.


So that’s SSAO in a nutshell, it was quite difficult to understand let alone implement due to the amount of calculations, as well as the fact that is puts heavy burdens on your processor. But like since the beginning, games sought to be as realistic as possible in order to uphold its role as a medium that immerses players into experience. Screen Space Ambient Occlusion is another means of doing to simulate the nature of lights and shadows. 


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