OpenGL/DirectX: How does Mipmapping improve performance?

Mipmaps are useful at least for two reasons:

• visual quality - scenes looks much better in the distance, there is more blur (which is usually better looking than flickering pixels). Additionally Anisotropic filtering can be used that improves visual quality a lot.
• performance: since for distant objects we can use smaller texture the whole operation should be faster: sometimes the whole mip can be placed in the texture cache. It is called cache coherency.
  • great discussion from arcsynthesis about performance
• in general mipmaps need only 33% more memory so it is quite low cost for having better quality and a potential performance gain. Note that real performance improvement should be measured for particular scene structure.

see info here: http://www.tomshardware.com/reviews/ati,819-2.html

You are no doubt aware that each texel in the lower LODs of the mip-chain covers a higher percentage of the total texture image area, correct?

When you sample a texture at a distant location the hardware will use a lower LOD. When this happens, the sample neighborhood necessary to resolve minification becomes smaller, so fewer (uncached) fetches are necessary. It is all about the amount of memory that actually has to be fetched during texture sampling, and not the amount of memory occupied (assuming you are not running into texture thrashing).

I think this probably deserves a visual representation, so I will borrow the following diagram from the excellent series of tutorials at arcsynthesis.org.

     

On the left, you see what happens when you naïvely sample at a single LOD all of the time (this diagram is showing linear minification filtering, by the way) and on the right you see what happens with mipmapping. Not only does it improve image quality by more closely matching the fragment's effective size, but because the number of texels in lower mipmap LODs are fewer it can be cached much more efficiently.