You'll need to definitely support OpenGL because Mac and Linux don't have DirectX - but you could give Window's users the DirectX option.
You will of course have to use BlitzMax as Nomen said.
It doesn't matter what format you use for your graphics but PNG is good especially if you like to preserve the full quality of your graphics.
You can't use any dll's whatsoever unless it's your Windows version. You can look at using the ? compiler directive to make pieces of your code compile specifically for a given platform. Note that Mac has both X86 and PPC.
You need a machine with the o/s on it for each o/s that you want to make an executable for. Depends what you go with but for example on a Mac you can run OSX + Linux + Windows and make binaries for all three, whereas on most PC's you could do Windows + Linux. You just have to have the o/s installed and be IN that o/s to compile. Also if you want to compile for PPC Mac at the moment it seems you need a PPC Mac, you can't do it from an Intel Mac.
I don't think there's a lot more that you have to do specifically to make things work on all three platforms unless you start to do things with storing files to a user's folder or something like that which is o/s specific. Also if I recall there may be some case-sensitivity in filenames and certain characters which are not allowed, that type of thing. But in terms of behavior `within` the BlitzMax environment everything should work as soon as you compile it.
BlitzMax is actually pretty good at being compatible cross-platform without much modification. I was able to get a smallish application running right away just be compiling it on Windows with no changes to the code at all.
You will of course have to use BlitzMax as Nomen said.
It doesn't matter what format you use for your graphics but PNG is good especially if you like to preserve the full quality of your graphics.
You can't use any dll's whatsoever unless it's your Windows version. You can look at using the ? compiler directive to make pieces of your code compile specifically for a given platform. Note that Mac has both X86 and PPC.
You need a machine with the o/s on it for each o/s that you want to make an executable for. Depends what you go with but for example on a Mac you can run OSX + Linux + Windows and make binaries for all three, whereas on most PC's you could do Windows + Linux. You just have to have the o/s installed and be IN that o/s to compile. Also if you want to compile for PPC Mac at the moment it seems you need a PPC Mac, you can't do it from an Intel Mac.
I don't think there's a lot more that you have to do specifically to make things work on all three platforms unless you start to do things with storing files to a user's folder or something like that which is o/s specific. Also if I recall there may be some case-sensitivity in filenames and certain characters which are not allowed, that type of thing. But in terms of behavior `within` the BlitzMax environment everything should work as soon as you compile it.
BlitzMax is actually pretty good at being compatible cross-platform without much modification. I was able to get a smallish application running right away just be compiling it on Windows with no changes to the code at all.