Graphics in Video Game: Ambient Occlusion

Ambient occlusion is the computer graphics effect of approximating how light radiates in real life, especially off what is considered non-reflective surfaces. Ambient occlusion, as the name suggests, is an ambient effect and globally illuminates. The soft appearance created by ambient occlusion is similar to an overcast day.

To calculate ambient occlusion, rays are calculated in every direction from the surface. If a ray collides with another surface, like a wall or another object in the scene, there is no significant change. However if that ray reaches the background, sky, or something else that was set by the programmer, the brightness of that particular surface will increase. As a result, points surrounded by a large amount of objects will appear dark, and objects in the open will be more white. As a final result, the ambient occlusion is then combined with the final textures of the scene to create the effect. The example below illustrates ambient occlusion that was done in Maya and implemented in an OpenGL program.

In this image, ambient occlusion is calculated and stored to a texture. This texture is then combined with the original texture in Photoshop and then added back to the original object. This way it provides realistic lighting to the object in question.

Screen space ambient occlusion

In video games, ambient occlusion, also known as screen space ambient occlusion (SSAO), is a method of calculating ambient occlusion in real-time! The first use of SSAO was for the game Crysis in 2007. Click here for a demonstration of SSAO in Crysis.

SSAO is calculated differently then most ambient occlusion methods simply because of how much potential calculation exists. With SSAO, the depth and surface normal are rendered to a texture in the first pass. In the second pass, a screen-size quad is rendered. In the pixel shader, samples are taken from the neighboring points in the scene. These points are then projected back to screen space to sample the depth by accessing to the texture in first pass. By doing this, we check if the depth sampled at the point is closer or further away than the depth of the sample point itself. The closer the depth sample, the darker the surface as something is covering it.

SSAO in Starcraft 2

Using this method of calculating SSAO could potentially be hundreds of calculations per pixel. With the more calculations done, the better quality the scene will look. To save on computation, some static objects can have a "baked" ambient occlusion texture while others can be constantly updated.

While SSAO is excellent for providing better quality, it has several flaws. One problem with SSAO is that it tends to include artifacts. For example, objects that are outside the screen do not contribute to the occlusion and the amount of occlusion depends on the camera position and viewing angle. Also, the higher the resolution, the more calculations that need to be done. Even small changes in resolution can have big problems.

To reduce artifacts, it is good to blur slightly as it will eliminate any noise left by the SSAO. A Gaussian blur will work for this. Then, upon adding lighting such as Phong, it will create a very nice looking object.

Conclusion

SSAO is an excellent method of calculating realistic lighting in a scene. Since first used in Crysis, it has seen use in many different games. In the future, perhaps we will use better looking and more intense uses of SSAO.