Aside from the fact that you can control the bump mapping per shader with a bump node, are there any differences between these materials?
As far as I can tell, they are the same. Is the displacement socket just a convenience thing? Is there a performance advantage?
Theoretically they are meant for similar purposes and functionally there is overlap when used in simple cases.
However, when used for extreme bump mapping, the bump map node produces much better results:
Left: Texture plugged into displacement output Right: Bump map node plugged into shaders
The other difference is of course that the displacement output can be used for actual geometry displacement (when when using the experimental feature set):
Left: Bump displacement Right: True displacement
Both spheres use the same material with the texture plugged into the displacement output. The sphere on the left uses the default Bump displacement method, while the sphere on the right uses the True method:
Note that the sphere on the right has a Subsurf modifier on level 4 to give it enough geometry to displace. There is a Use Subdivision option in the displacement settings, but I found this causes some wireframe artefacts.
Finally, using the Bump node allows you to choose not to have a bump map for certain shaders. For example you may want the gloss on the material to appear like a glossy coat or varnish, while still having a bump map for the underlying diffuse shader:
Left: Same displacement for all shaders Right: Different displacement for each shader
For reference, here is the sphere with no bump or displacement
In theory they should be different.
I am not sure of the naming convention in blender now as it changes quite a bit. However here is my rendering of the definitions:
Plugging the image directly into the displacement socket treats it as a displacement map (much heavier, moving actual geometry about).
Using a bump, gives you a bump map.
As you can see the output of the bump node is a normal. You plug this normal into material and it will change the normals of the surface according to the image. Giving it the feeling of being engraved when the actual surface stays flat. (more efficient than displacement).
Having said that keep in mind that if you go close to the surface you will notice that it is flat and not "bumpy".
Usually one would use a combination of both... displacement, for the bigger details, and normal/bump mapping for the finer detail.
The best test is to try and increase the strength of both, go down close to the surface and see what happens.
The point is so that, if you want to, you can use a bumpmap in one shader and another one entirely in another shader. If you plug it in the output, it will be used for the whole material.
