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Screen Space Ambient Occlusion or SSAO is a method to compute an approximated AO pass in real-time!

This is done by using the depth information of a 3D object or scene that has been stored from the Z-(depth) buffer of the renderer.

Go ahead and try the interactive demo or see a video example!

The technique was introduced 2007 at Siggraph by Crytek Studios and is becoming an often used feature within next generation live-3d engines or cg/video post applications.

After playing with the ray traced inside-out AO and having to realize that it´s not quite doable with Pixel Blender, I turned to SSAO and quickly got some nice results.

In short it´s all about importance sampling.

SmartSSAO looks up the depth information of 12 surrounding samples and checks if they get occluded to average the AO value.

The disadvantages lie in the nature of SSAO being “screen spaced” which means that it can´t take objects from the off into account, and the result has to be blurred to reduce the inevitable noise.
Also there is no ray bouncing, so classic AO is visually superior – but hey… who can argue with “real-time”?

View another video of the buffer and the solo SSAO pass without/with blurring.
Please note that those strange looking anomalies on the buddahs arms originate from the badly ranged zmap I produced in a hurry.


To sum it all up:

SSAO makes it possible to have AO in places where it normally isn´t possible – e.g. real-time engines (yes – even for Flash/Papervision), or within a video post process.

All that’s needed is the Zbuffer of a 3D rendering.

You can download the Shader here or get it packed with an AFX example.


Hint: As said before, the SSAO pass needs to be blurred in order to clear out the emerging noise – but don´t overdo it, as the blurring will “bleed” the result over to unwanted areas.
This could be targeted by a special SSAO blur filter that would again have to take the depth map into account.


Click here to see this screenshot as video

Update:

I feel like I should adress the X,Y-offset params of the filter which can be used to tweak the “shadow direction”.
“Shadow direction” in quotes as (SS)AO is all about indirect lightning so this is a real “tweak” or an unrealistic param – which I thought might come in handy in some cases.

I was asked if the shader was performant enaugh for flash – and even if it looks a bit ugly codewise as I had to get rid of a loop – it surely is.

So I updated the package and made this small interactive flex demo :

Be sure to try unchecking “color SSAO?” and “show SSAO?” to get an impression.

You can grab the source here.

Update 2:

The filter now features an additional sampling radius parameter which controls the dispersion and allows to adjust it to the desired level of detail – this should also make it more useable for papervision.

I hope this little experiment will be of use to somebody out there and would love to hear any suggestions.