Joe Taylor Joe Taylor - 3 months ago 14
Javascript Question

Smooth character movement in canvas game using keyboard controls

Right, I'm creating a sidescrolling endless space themed game using canvas and javascript. I'm controlling a spaceship just by using the up and down arrows and I want to implement some kind of movement easing so that the ship doesn't just stop dead when I let go of the keys. I've looked around and haven't found anything plus my own attempts just aren't working, this is what I've tried...

Jet.prototype.checkDirection = function () {
if (this.isUpKey) {
this.drawY -= this.speed;
if (this.speed < 5) {
this.speed += 0.1;
}
}
if (this.isDownKey) {
this.drawY += this.speed;
if (this.speed < 5) {
this.speed += 0.1;
}
}
if (!this.isUpKey) {
if (!this.isDownKey) {
if (this.speed >= 0) {
this.drawY -= this.speed;
this.speed -= 1;
}
}
}
if (!this.isDownKey) {
if (!this.isUpKey) {
if (this.speed >= 0) {
this.drawY += this.speed;
this.speed -= 1;
}
}
}

Answer

You just want to apply some friction. Its pretty easy. You can do something like the following.

this.speed*=0.98;

The lower the value (0.8, 0.5, etc) the faster you will slow down.

I provided a demo where you can move around and will gradually slow down. Go ahead and play with the value and see how it affects it.

Live Demo

var canvas = document.getElementById("canvas"),
    ctx = canvas.getContext("2d");

canvas.width = canvas.height = 300;

var x = 150,  //initial x
    y = 150,  // initial y
    velY = 0,
    velX = 0,
    speed = 2, // max speed
    friction = 0.98, // friction
    keys = [];

function update() {
    requestAnimationFrame(update);

    // check the keys and do the movement.
    if (keys[38]) {
        if (velY > -speed) {
            velY--;
        }
    }

    if (keys[40]) {
        if (velY < speed) {
            velY++;
        }
    }
    if (keys[39]) {
        if (velX < speed) {
            velX++;
        }
    }
    if (keys[37]) {
        if (velX > -speed) {
            velX--;
        }
    }

    // apply some friction to y velocity.
    velY *= friction;
    y += velY;

    // apply some friction to x velocity.
    velX *= friction;
    x += velX;

    // bounds checking
    if (x >= 295) {
        x = 295;
    } else if (x <= 5) {
        x = 5;
    }

    if (y > 295) {
        y = 295;
    } else if (y <= 5) {
        y = 5;
    }

    // do the drawing
    ctx.clearRect(0, 0, 300, 300);
    ctx.beginPath();
    ctx.arc(x, y, 5, 0, Math.PI * 2);
    ctx.fill();
}

update();

// key events
document.body.addEventListener("keydown", function (e) {
    keys[e.keyCode] = true;
});
document.body.addEventListener("keyup", function (e) {
    keys[e.keyCode] = false;
});
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