How do AOVs work?

[yusrig]

Hello. I am trying to get Custom AOV outputs to work. My understanding so far is as follows:

When I create a custom output in my Mainpass Output node I can pass in any AOV type I want from the mesh nodes. But then I need to pass this into the Render Layer as a new output so that it can be accessed in the Render nodetree and then in turn be passed into the Screenpass nodetree. I think I have wired this up correctly (correct me if I am wrong) and should be getting custom AOVs in my render. But I am not. I attached screenshots of what I did below. Can someone help me figure out why this isn't working as I expect it to?

P.S. I am not an advanced user and some of the detailed concepts are a little confusing for me. If you could add context would be great!

1. First I need to create a custom output in my mesh graph:
   
2. Then I need to make sure in my render layer I connect that to render layer output so I can then access it in the render graph
   
3. In the render graph I pass it into the screen pass which I have added custom input and output for this custom AOV.
   
4. Here is what my screen pass looks like:
   
5. But when I render i only get a black image with no data under the custom output
   

[Warrenayiss]

I'm not skilled in this, in fact, I used your explanation to reproduce the bloom effect shared in #347 so I will tell you where what I did differ from what you explained.
In step 3, I didn't change the default ScreenPass I created a different ScreenPass node that I put between it and SuperSamplingAA

Then made a completely new Screen node tree

Hope this help you at least a little bit
And thank you teaching me about AOVs

[yusrig]

Oh man, am I dumb! Yes, your solution helped, creating a new Screen pass node tree was required to get the AOV to work, instead of using the same Screenpass. But more over I made a critical mistake in my Mainpass output. I used a Vec3 instead of Vec4 for the custom AOV output and thats why it wasn't rendering anything (I think...).

Anyway using a separate screenpass helped. So I guess I could make multiple Screen pass materials to be different AOVs and get different passes for compositing later.

[pragma37]

You should be able to connect RenderLayers.shadow directly to RenderOutput.shadow.
I think the only reason your initial setup didn't work was because of the vec3 output (If each layer output has no transparency then they can't be composed together, so you only see the result of the last layer, which is probably empty).