- When linear filtering normal maps the filter can cause normals to become less than normalized. If this happens then we will have to renormalize the normals in a fragment program each time a texel if fetched.
- Artifacts will arise when trying to use normal maps and a DXT compression format, especially with specular highlights.
- Generating the Z component can increase the rendering quality.
- The 3Dc compression format is better for normal maps than the DXT compression formats.
- DXT1 gives us the highest compression ratio.
- There is a performance gain when working with compressed textures because there is not as much data that needs to be sent down the pipeline.
Source: Nvidia Developer Forum