you'd have to be pretty dexterous to be able to use function keys for that. Why not just let users bind the keys however they like ?
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I used to have such a set up before in another indie game where players can rebind keys to whatever they want but due to the game's design and complexity in setting up the instruction screen and tutorials it can get really out of hand at times. So in the end we decided to remove it and ended up with only one control layout but multiple keys assigned to one action. Besides, the decision that seals it was because of the porting to consoles for that game, therefore rebinding keys got abandoned with that move. Hence with that experience and the design decision in this game, I have prepared as many possible control layouts including an Xbox controller (works in any control layout). Well, a lot of indie developers take control input and its integration into game design for granted so I can't blame them since its like an entire topic on its own. I may not want to implement rebind able controls for this game but I will certainly look into making a better setup that integrates with future game.