My Linux system has a graphics card that identified itself as "[AMD/ATI] Kaveri [Radeon R4 Graphics]".
If I run it with the default/generic driver (blacklist radeon and fglrx modules), fonts on X look great - letters are solid black or solid white and easily readable.
If I use either the open-source "radeon" or the proprietary "fglrx" driver, text becomes ugly - for example, the top bar of the letter "T" will be made up of some black pixels and some gray pixels. This doesn't seem to be specific to any one desktop environment or program, the text gets ugly anywhere. There may be subtle differences in the way images are displaying too, but it's hard to tell for sure.
I've confirmed that screen resolutions and DPI are the same in all cases, and looking at output from fc-match it doesn't look like there's any differences with hinting/anti-aliasing/etc. being turned on/off.
The ugly text is exactly the same (down to the same pattern of the grey dots in the letter "T") whether I use "radeon" or "fglrx", which leads me to believe it's not actually an issue with the driver, but with something X is doing. Eg. X is saying "I have detected that there is a driver capable of using $feature, I will now turn $feature on." and whatever that feature is has the side-effect of making text look ugly.
I know there are a bunch of guides out there for how to make Linux fonts look nicer (not having much luck with those so far), but what I would really like to know is what X is doing differently when I have the radeon or fglrx module loaded -- if I could just get it to stop doing whatever-that-is and display my text the same way it does when treating my card as a generic card, the text would look great.
Any ideas what X might be doing differently between these scenarios and how to get it to stop?