: It looks weird. And why is it so dark!?
First of all, do you have at least "thousands of colors" selected in the graphics prefs? Better still is "millions of colors", but you cannot use these textures with only 256 colors.
Secondly, have you tried adjusting the lightness in your graphics prefs? You can also change brightness in game, but I forget which function keys do this in AO.
Third, do you have any 3D models in your AO folder? This once threw me off when I downloaded a beta of 3D fighter models for Infinity. They do look cool, but they aren't compatible with the high-res textures and they make everything look very, very dark if you combine the two.
Fourthly, you might want to try adjusting your OpenGL settings. The only things you need checked are Do Map in OpenGL (you won't have map mode if you don't), Do Flashes/Tinting in OpenGL and Pipe 2D Graphics through OpenGL (again, no map mode without this). You can also check Make Static Effects Look Flat if the graphics card in your old iMac is just too limited to handle things. As a last resort, you can use the checkbox at the top to replace your landscape textures with a solid color, but I'd rather do without high-res textures than do that.
Hope this helps.
