Acceleration Based Player Movement

Question

Ok, so I am making a first person shooter game and I am currently working on movement that looks and feels good. I want to incorporate acceleration based movement for the player so that he has to accelerate to max speed and decelerate to minimum speed. Acceleration will happen when you have the key pressed and deceleration will happen when you let go of that key. The problem is that there are some instances where you switch from moving forward to moving backward where no deceleration is needed because you could potentially be moving at double speed in the reverse if you did. Does anyone have a good implementation of how to accomplish acceleration based movement that works well?

Answer 1

There's something weird about your concept of the issue since you say, "could potentially be moving at double speed in the reverse." This should not be the case with an acceleration-based solution.

You will have a double playerVelocity value in your Player class. To keep it simple for now, imagine just forward/backward velocity. Every frame, you add your acceleration value (PLAYER_ACCELERATION) to the player's velocity until it reaches a maximum - the speed at which you generally want the player to run (PLAYER_MAX_SPEED). This should only take a handful of frames unless you want the controls to feel very "floaty". (You don't for a modern FPS.)

If the player reverses direction, you add negative acceleration. If they are moving forward at full speed and reverse direction, it will take them twice as long to get to "full speed reverse" than it does from standing still.

If the player releases all buttons, you add acceleration towards zero. He will slow down until he stops.

An implementation:

final PLAYER_MAX_SPEED = 1.0f;
final PLAYER_ACCELERATION = 0.2f
double playerVelocity = 0.0f;

public void update() {        // Called every frame, and assuming constant fps

    if (forward_button_down) {

        // The player is holding the "move forward" button
        playerVelocity += PLAYER_ACCELERATION;
        if (playerVelocity > PLAYER_MAX_SPEED) {
            playerVelocity = PLAYER_MAX_SPEED;
        }

    } else if (backward_button_down) {

        // The player is holding the "move backward" button
        playerVelocity -= PLAYER_ACCELERATION;
        if (playerVelocity < -PLAYER_MAX_SPEED) {
            playerVelocity = -PLAYER_MAX_SPEED;
        }

    } else {

        // Neither forward nor backward pressed
        if (playerVelocity < 0) {
            playerVelocity += PLAYER_ACCELERATION;
            if (playerVelocity >= 0) {
                playerVelocity = 0.0f;
            }
        } else {
            playerVelocity -= PLAYER_ACCELERATION;
            if (playerVelocity <= 0) {
                playerVelocity = 0.0f;
        }
    }
}

Another thing you'll probably want to do is make the player accelerate much more quickly toward zero than he accelerates beyond zero, otherwise your character will feel unresponsive.

If you use something like this for forward/backward, and another variable for left/right, you'll have to limit the combined speed (bust out your geometry), otherwise your character will move diagonally-forward faster than he moves only-forward.