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There are 2 ways to add detail to a 3D model. One is to physically change the shape of the model by adding more vertices. The other is to fake the illusion of extra vertices with normal maps.

Normal maps are often used because they are more efficient and don't add more vertices to the mesh. They can also produce quite realistic results.

If this is the case, then how should a 3D modeler decide which method to use to add detail to a mesh and when? Why even bother modifying the mesh to add high-level details if normal maps can create the illusion of depth just as well?

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    $\begingroup$ Hello :). Normal maps work great for adding shading details, but the silhouette of the mesh stays flat. This may look unrealistic in closeups, when using rimlight or when looking at shadows. $\endgroup$ – Jachym Michal Oct 6 '20 at 5:47
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    $\begingroup$ "Just"? How do you create those normal maps? $\endgroup$ – user253751 Oct 6 '20 at 16:53
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It depends.

When it is not going to view in detail and the geometry of vertices isn't important.

As you said, there is no extra vertices created, which mean, if there isn't a geometry, rendering engine can't render on that pixel. Commonly seen in early game when we can't afford high vertices computing.

enter image description here

As a same result, AO, shadow, light bounces cannot be accurately calculate due to the rasterized geometry (Although there is some trick that can compute them in additional process).

So how do we decide which one to use:

  • Is our scene need to be realistic as possible as we can?
  • Can the rendering engine/hardware afford such high vertices? Or high resolution normal map?
  • Can we obtain the normal map?
  • Do we have time to bake normal map? (especially you don't want to waste time on tweaking your map and vertices. eg. making animation)
  • Is the rendering time important? (Realtime Game, VR, AR, news animation)

There is much more thing that we should consider about, including what pipeline are the artist using, which might be the most important factor in current decision. And I don't think an short Q&A post can cover all the issue in this problem.

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For example, you cant 3D print a normal map as well as you cant set some physical actions to a normal map. In short. If the surface is just for looking and for not so important parts or if there is just no noticeable difference, use normal map. It can shows some geometry or shadows and light bounces not so realistically.
If you expect to use the surface for using it like a real world object, then may be there can be some reason to actually model the geometry that you can need in animation, simulations or printing. The same goes for displacement maps i would say.

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