Editing S3 Texture Compression (section)

== Codecs ==
There are five variations of the S3TC algorithm (named '''DXT1''' through '''DXT5''', referring to the [[FourCC]] code assigned by Microsoft to each format), each designed for specific types of image data. All convert a 4×4 block of [[Pixel|pixels]] to a 64-[[bit]] or 128-bit quantity, resulting in compression ratios of 6:1 with 24-bit [[RGB color space|RGB]] input data or 4:1 with 32-bit [[RGBA color space|RGBA]] input data. S3TC is a [[lossy data compression|lossy]] compression algorithm, resulting in image quality degradation, an effect which is minimized by the ability to increase texture resolutions while maintaining the same memory requirements. Hand-drawn cartoon-like images do not compress well, nor do [[normal mapping|normal map]] data, both of which usually generate [[Compression artifacts|artifacts]]. [[ATI Technologies|ATI]]'s [[3Dc]] compression algorithm is a modification of DXT5 designed to overcome S3TC's shortcomings with regard to normal maps.  [[id Software]] worked around the normalmap compression issues in ''[[Doom 3]]'' by moving the red component into the alpha channel before compression and moving it back during rendering in the [[pixel shader]].<ref>{{cite web|url=http://www.gamershell.com/news/14362.html|title=DOOM 3 Video Requirements|last=Duffy|first=Robert|date=July 27, 2004|website=|publisher=Gamershell.com|url-status=dead|archive-url=https://web.archive.org/web/20080103155922/http://www.gamershell.com/news/14362.html|archive-date=January 3, 2008|access-date=2012-01-25}}</ref>

Like many modern image compression algorithms, S3TC only specifies the method used to decompress images, allowing implementers to design the compression algorithm to suit their specific needs, although the patent still covers compression algorithms. The [[Nvidia|nVidia]] GeForce 256 through to GeForce 4 cards also used 16-bit interpolation to render DXT1 textures, which resulted in banding when unpacking textures with color gradients. Again, this created an unfavorable impression of [[texture compression]], not related to the fundamentals of the codec itself.