According to Microsoft, the most reliable way to retrieve the monitor's aspect ratio is to get the desktop resolution and calculate the aspect ratio of that. This, of course, presupposes that the user is using the ideal resolution, which they may well not be. Personally, I would take the *highest* resolution that comes back with EnumDisplaySettings and calculate the aspect ratio of that as I think it's less likely that the monitor's top resolution is a non-native aspect ratio than that the user has set a desktop resolution with a non-native aspect ratio.
I mean I kinda suspect a lot of people with 1280x1024 monitors will have set 1024x768 as their desktop resolution, if only because anything about 1024 is slow in games and thy don't want to have the monitor switching every time they play a game. I could be wrong.
I mean I kinda suspect a lot of people with 1280x1024 monitors will have set 1024x768 as their desktop resolution, if only because anything about 1024 is slow in games and thy don't want to have the monitor switching every time they play a game. I could be wrong.