Handling input with GLFW

Question

Here is the definition of my keyboard callback method I'm using to check for user input for a 2D basic game. However I'm having a problem handling certain simultaneous key presses.

For example, If I hold the Right and Up arrows keys, the player moves 45 degrees toward the top right of the screen as it should. Then, while still holding the up and right keys, if I press Space (which fires a projectile), that works as well.

However, if I hold the Left and Down arrow keys, the player moves as it should, but when I press Space, I get no input response, so I can't fire a projectile when moving down and left. All other movement + fire projectile combinations work, just the down and left doesn't... I can't figure out why. Any ideas?

if (key == GLFW_KEY_LEFT)
{
    GameController::getInstance()->getPlayer()->changeKeyPress(GLFW_KEY_LEFT, action);
}
else if (key == GLFW_KEY_RIGHT)
{
    GameController::getInstance()->getPlayer()->changeKeyPress(GLFW_KEY_RIGHT, action);
}
else if (key == GLFW_KEY_UP)
{
    GameController::getInstance()->getPlayer()->changeKeyPress(GLFW_KEY_UP, action);
}
else if (key == GLFW_KEY_DOWN)
{
    GameController::getInstance()->getPlayer()->changeKeyPress(GLFW_KEY_DOWN, action);
}
else if (key == GLFW_KEY_SPACE)
{
    GameController::getInstance()->getPlayer()->changeKeyPress(GLFW_KEY_SPACE, action);
}
else { }

Answer 1

Rollover is the property that allows a keyboard to properly register many key presses at once. Keyboards are wired in rows and columns. Even if the keyboard is not square, the individual keys are in a roughly square matrix of wires connecting their switches. The controller connects one row and then tests to see which of the columns are hit. Some key combinations "shadow" others. The controller can tell when the input is ambiguous and send no keys.

Better keyboard use diodes arranged to avoid the ambiguity and thus support "full rollover", although in practice USB limits you to all the modifiers plus 6 distinct keycodes at once.