Blender indeed can’t bake displacement from selected to active. It’s a well known limitation, which a lot of people are waiting to see fixed. Blender’s bake system is one of the older and more limited parts of the software.
It still can be used, but you have to know this issue in advance and plan the workflow for it. If you want a displacement map, you will need to convert your object into a multires sculpt.
If your high poly sculpt was created from a clean subdivision and not a bunch of random triangles (dyntopo), it will be easy to do by rebuilding a new low poly mesh from the high detail sculpt.
Take your high poly sculpt as a starting point.
Put a multires modifier on it.
Use the multires modifier’s “unsubdivide” button to reconstruct the lower levels.
If you haven’t made UVs for the mesh before, you can now. Multires unsubdivide actually changes the base mesh even though the modifier is still on it, so you can go in edit mode and UV unwrap the lowest level without having to apply the modifier.
You can now bake from multires.
When you are done baking, you can set the multires to its lowest level, apply the modifier and you now have a low poly mesh with all the maps baked.
If you are dealing with unmatching or messy topology that can’t be unsubdivided, then you would have to use a shrinkwrap modifier to project the sculpted detail on a clean low poly mesh so you can then have it as a multires sculpt.
But if (and only if) both meshes use the same UV space projection, it’s possible to bake the position from the HighRes and then subtract the Position from the LowRes surface and the result is a nice Displacement Map.
It’s a great tutorial and the first to show up on Google, but in my case depth info had to come from another mesh and not a texture. @etn249
Thank you, but in my case I had to work with two very different distinct models, so a multires modifier wasn’t an option. @kkar
Yeah, I’m not entirely sure why they decided to remove it at all. @robin.hohni
This is probably as good as it gets. Thank you very much. I also really like how they’re saving the depth info to an attribute so it works independently of UVs. @Piotr_Adamowicz
I wouldn’t say baking is 100% horrible, but I’m definitely looking forward to any improvements the devs have in store in that area. Hopefully we’ll see a re-designed baking system within the next five years. @Secrop
That was my initial plan, but it turned out the high res mesh didn’t have any UVs at all.