Blending Normal Maps?

What is the best way to blend two normal maps together? Why can’t I just add two normal maps together in Photoshop? I heard that to combine two normals together, you need to add the positive components and subtract the negative components, then renormalize. Looks right to me… Why shouldn’t I be using Overlay (or a series of Photoshop blend modes) to blend normal maps together? I want to add detail to surfaces. How does one combine normal maps in real-time so that the detail normal map follows the topology described by the base normal map?

If this is something you’ve heard before, something you’ve asked yourself, check out this article, written together with Stephen Hill (@self_shadow) on the topic of blending normal maps.

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