AO node in Cycles - Color Output

The problem with your proposal is that what you want to do is non-linearly mapping the result of an AO evaluation, e.g. with a Color Ramp. This just doesn't work without taking a good amount of AO samples before feeding the ramp, every time the shader is evaluated . The shader would become extremely expensive, that's why you really want to pre-compute the occlusion on a texture or a vertex map.

Lukas says what you want is not possible, but I'd rather say it is impractical. Having said that, having just a single AO sample per shader evaluation (maybe with distance and falloff) that averages out over multiple passes may still be useful - this is what I believe Octane does. You just can't use this with a Color Ramp, you will get exactly the problem that your image shows: wiki.blender.org/index.php/File:Osl_ao_issue.png

Probably they use the same trick as the OSL AO shaders floating around on the web, returning either 0 or 1 depending on a random ray - for mixing shaders, that works, but not for more advanced setups. Alternatively, you can of course always bake AO and use it as a texture.

Yeah I usually bake AO to use it like this. It also makes great dirtmap masks and so on.

let's say it's impossible for a color node ... the shader one still can have a distance parameter, to use it in indor scene for exemple

@lukasstockner97 I understand that 'Real AO' color input cannot work in path tracer but for procedural texturing purposes trick similar to this OSL script will be just fine. This feature even don't need do take account of other objects in scene to reduce computation cost (but option for that can be nice). In most cases baked textures for single asset contain only "self AO" and thats enough for creating procedural masks.

Not taking into account the other objects in the scene would likely not reduce computational cost significantly - raytracing a BVH is logarithmic in complexity, after all.

You say tricks "similar to the OSL script" would be fine, but then you will still face this issue: wiki.blender.org/index.php/File:Osl_ao_issue.png

You won't be getting a "sharp" dividing line, which would be a requirement for even a basic wear-and-tear shader.