Hello everyone,

I noticed recently that the mtlx ambient occlusion node was no longer accessible in Houdini 21 (https://www.sidefx.com/docs/houdini/nodes/vop/mtlxambientocclusion.html). Is there any kind of replacement for it? Having pixel AO on the fly in materials is incredibly valuable to make quick improvements, are we just meant to either bake to attributes (which is limited by the geometry) or bake actual maps in cops (which is much longer, requires UV, has a limited resolution...)?

I use AO in materials so much in my day to day work, I'm a bit confused... Thanks !

The ambient occlusion node was removed from MaterialX, which is why it doesn't show up in Houdini 21. Karma has the ability to generate an AO AOV without doing anything in the material. There is a checkbox in the Karma Render Settings LOP to enable this AOV.

(I'm parroting something I heard yesterday so I may be wrong about this - please post again if I'm inadvertently lying)

While I'm not particularly sad to hear that the materialx AO is gone (it was too barebones to be really useful in shader networks) it also means we now have no way of doing it aside from switching back to VEX based shaders. That, of course, only works on cpu and isn't really an option in my opinion. An AO AOV can't replace that either, that is for an entirely different purpose. We definitely need a different solution in Karma that is more useful and works on both CPU and GPU renderers.

For those who still need this node. In my case, I used project initially created in H20.5 (and later edited in H21) with AO node in one of shaders. So I created Recipe just with this single node and it works!

added HIP

What is the use case for this with modern renderers? I was surprised to see it at all after not having messed with 3D for years, since I remembered it only as a sort of ugly hack to get around the inability of old "kinda sorta pathtraced" renderers to converge in a reasonable amount of time and generate something that resembled shadowing with any level of fine detail at the same time, although I stopped using ambient lights for anything pretty early on (but I admittedly was just doing things for fun and not on deadlines where simplifying the lighting may have been crucial to being able to get effects for a movie rendered in the next decade)

As soon as dome lights and HDRI maps showed up I thought all of that had died since those (or just generated gradient maps doing the same thing like sky lighting) are the only way to generate light coming from all directions that isn't broken in a physical context unless your entire scene takes place in a sphere with walls made from lights... and shadows enabled on those doesn't seem to affect speed much. It did when they first showed up with CPU rendering and a 2 processor system. TBH I didn't realize it was ever used anymore until I saw that node.

I'm almost certainly missing something so if anyone is bored enough to explain what the modern use is (games and exporting shader networks to them maybe?) I'd appreciate it. My last big jump was from the 3ds max scanline render or mental ray to maxwell render and the results were so much better I was willing to accept the 24 hours renders took to converge to a reasonable level... but again, I was doing stills. Some more recent games allow pathtracing / raytracing to be used instead of AO for shadowing from the sky dome even if they don't do anything with raytracing otherwise since it's gotten cheap enough to do that with on most cards, so even that use seems like it's on the way down and only kept going by the ridiculous overpricing of GPUs that the combination of NVidia's near-monopoly and the boom in computationally expensive spambots brought on.

AO texture nodes are not used for lighting but for shading. You can make pretty quick and easy procedural dirt with it for example.