When you press a key on your keyboard, the operating system (or windowing environment) typically passes three pieces of information to the current application:
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The scan code for the key that you pressed, supplied by your keyboard.
These correspond to the physical location of the key you pressed, and are mostly the same for every keyboard layout.
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Any modifier keys that have been held (eg: shift, AltGr).
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The key code (or keysym) for the key that you pressed, supplied by your operating system.
Your operating system builds this from the configured keyboard layout, the scan code and modifier keys.
This corresponds to the letter printed on the key that you pressed.
Note: An input method editor (IME) is more complex than this -- it will typically intercept the keystrokes before it reaches the application, and then send an IME event containing composed characters.
Note: Some keyboards support "hardware layout switching", which moves keyboard layout selection from the OS into hardware. This typically results in the OS always beliving it has a US-QWERTY keyboard attached.
A game developer can choose how to interpret a key press when assigning it to an action (like move forward):
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The developer can assign an action to a key code (letter).
This will assign the action to a specific letter (eg: W).
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The developer can assign an action to a scan code.
This will assign the action to a specific key position (eg: 25).
If they use scan codes, they can:
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Look up the operating system's key map, and display the key in the UI correctly, or,
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Use a hard-coded key map -- normally whatever the developer uses (typically US-QWERTY).
Using key codes (letters) to assign actions makes the games largely unplayable if you don't have the same keyboard layout as the developer.
The fact that the options menu of a game shows letters, or has QWERTY bindings by default, doesn't indicate whether they use scan codes or key codes. They could also use scan codes without correctly consulting the operating system's keymap.
A developer can make this worse by not allowing you to modify keybindings on first start-up, or inconsistently use scan codes and key codes (eg: the options menu uses key codes, but the game uses scan codes).
This library rewrites incoming input events before it reaches the game, so that it is like the system keymap was set to QWERTY.
There are a few different versions of the library (SDL 1.2, SDL 2.0 and Xlib) which use slightly different approaches to solving the issue.
In effect, any game which assigns actions to key code will now have them assigned to scan codes with a hard-coded keymap.
This does mean the UI will still show the wrong key, but the position of the keys won't be jumbled anymore.
Not noticeably.
This may make a couple of extra library calls, but only on the input handler code. It stays out of the way as much as possible.
The correct way to handle keyboard input is to assign all actions to scan codes, and then reference the operating system's keymap when displaying a key label in the UI.
This way, when you start playing a game for the first time, it doesn't matter what your keyboard layout is, all the tutorials and menus will display it correctly. Most of the time, the key positions will be what the developer intended (eg: WASD-style movement keys will be in an equivalent inverse-T orientation on the 2nd and 3rd row).
All games should support changing the key bindings, even before entering a tutorial. Some players prefer different orientations of movement keys, either because they use a mouse with their left hand, or may otherwise find it difficult to use a default key configuration.
Some developers use scan codes but don't reference the operating system's keymap (or do so incorrectly). The game is mostly playable, but it does mean that keyboard shortcuts in tutorials displayed are incorrectly. This library does not attempt to resolve that issue.
This is a hack around the problem, and doesn't address the issue correctly.
Additionally, it's not possible for one to anticipate all of the layouts that might be used with a game.
You could manually change the keymap before starting each game.
This is tedious, and this library aims to be a "set and forget" solution.