Anisotropic Filtering (AF) is a method to improve the image quality of OpenGL textures at angles and eliminates aliasing while reducing blur and enhancing detail. It was believed that the Intel Mesa driver already supported Anisotropic Filtering on Linux, but when Ian Romanick was investigating EWA AF, he discovered it might have not been working at all.
The EWA algorithm for anisotropic filtering is supposed to result in higher image quality and with Ivy Bridge and newer hardware can be implemented in the driver by setting an extra bit, but without this extra bit it turns out AF might not have been working at all. Ian Romanick wrote in a patch earlier this month, "Volume 4, part 1 of the Ivybridge PRM says, 'Generally, the EWA approximation algorithm results in higher image quality than the legacy algorithm.' Using a classic anisotropic filtering 'tunnel' demo, it appears that there is *no* anisotropic filtering on IVB without this bit set."
Kenneth Graunke of Intel also found that AF didn't seem to be working right up until this point. "*No* anisotropic filtering is a surprising result, but I can confirm that enabling the EWA algorithm significantly changes the rendering in one test I've looked at. Without it, you get something like a polar rose, and with it, you get a circle (which is the expected result)."
Before getting too excited over enhanced image quality out of the Intel Linux graphics driver, there is obviously a performance hit with AF being bandwidth intensive. Now that AF is properly working for the Intel driver, Graunke acknowledged, "It also seems to be a 37% performance hit in that test."
Those developer comments in regards to this EWA AF patch can be found on the Mesa-dev mailing list. As of this afternoon, the small patch has been merged to mainline Git for Mesa 10.2-devel that makes for working anisotropic filtering in the open-source Intel Linux driver.