Mip mapping
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Mip Map
What is a Mip Map?
Mip maps help reduce aliasing in-game when a textured surface is rendered.
The texture is resized multiple times to make "mips," smaller versions of the texture. These smaller versions are swapped or blended with the original texture in order to keep as close a ratio as possible between screen pixels and texture pixels. When unfiltered texture pixels get smaller than screen pixels, the pixels shimmer like crazy as they change from color to color. Mipping helps solve this problem.
Most game engines will create the mips automatically, if mips don't already exist in the texture file when it's loaded. If you create the mips yourself, you can do some interesting texture tricks, by taking advantage of the fact that the textures are swapped by distance... fogging, sharpening, fading out, etc. If you're using the DDS format the NVIDIA DDS plugin for Photoshop is an easy way to edit mips.
NVIDIA says: "Whether you're using mipmapping or not, you should never use "nearest" filtering, which causes texture swimming artifacts when textures are minified. Using mipmaps allows you to create accurately minified versions of your textures, so that they look as good as possible in the distance. Mipmapped textures only require 33% more memory than their non-mipmapped counterparts, so it is wise to trade off the small amount of extra memory for the vastly improved image quality. In addition, today's GPUs are optimized to handle mipmaps very efficiently, so the performance impact of using mipmaps is minimal."
NVIDIA whitepaper [Get Small: Understanding MIP Mapping] by Kevin Bjorke (4MB PDF).
NVIDIA DDS plugin for Photoshop
The NVIDIA DDS plugin for Photoshop has a great interface for testing MIP settings.
Sharpening Settings window
Each of the numbered boxes at the bottom corresponds to one of the mip levels... "Image" is the top mip level, and they descend from there.
The number of boxes you need to adjust depends on how large your original image is. For example a 256x256 map only has nine mip levels: 256,128,64,32,16,8,4,2, and 1 (1x1). So you would only have to adjust the boxes from Image to 8.
Takes a lot of testing to see what works best. Something that helps is the 3D Preview button, just make sure to adjust the Preview Settings first so it doesn't have to make a ton of different compressions each time you change a setting.
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