Setting a correct angular velocity to a canvas object - html

Question

I am building a space shooter game and would like the ship to fire rockets at the direction of the cursor. Therefore, I grab the radian value of the angle it should fire at, multiply it by the ship's speed and set it's x and y velocities respectively.
I have this as a Bullet class:
function Bullet(x, y) {
this.x = x;
this.y = y;
this.rotation = 0;
this.width = 6;
this.height = 3;
this.color = utils.getRandomColor();
this.speed = 80;
}
And here is the function which updates the movement of all instances of the bullet class:
function drawBullet(bullet) {
var dx = mouse.x - bullet.x,
dy = mouse.y - bullet.y,
angle = Math.atan2(dy, dx);
bullet.vx = Math.cos(angle) * bullet.speed;
bullet.vy = Math.sin(angle) * bullet.speed;
bullet.x += bullet.vx;
bullet.y += bullet.vy;
bullet.draw(ctx);
}
It starts okay, going in the right direction and velocity and stuff. But as soon as it reaches the mouse, it stops dead there and starts flickering. NOW, I realise that this is because of the way I am getting the angle, using the mouse position as a value - the problem is that I can't figure out a way to use just the angle for the velocity, not the distance to the mouse position. So it doesn't slow down.
All suggestions are welcome, thanks in advance!

Answer 1

If you don't need homing missile type behavior just pass the mouse coordinates when you create the bullet.
Example:
new Bullet(shooterX, shooterY, mouseX, mouseY)
I included an over engineered stack snippet but the relevant part is below.
var Bullet = function(x,y,tx,ty){
this.speed = 15;
this.x = x;
this.y = y;
var radians = Math.atan2(ty-y, tx-x);
// we now have our velX and velY we can just refer to
this.velX = Math.cos(radians) * this.speed;
this.velY = Math.sin(radians) * this.speed;
}
Bullet.prototype.update = function(){
// just update by our previous calculated velX and velY.
this.x += this.velX;
this.y += this.velY;
};
var canvas = document.getElementById("canvas"),
ctx = canvas.getContext("2d"),
width = 250,
height = 250,
output = document.getElementById("radians"),
output2 = document.getElementById("degrees"),
cX = 0,
cY = 0,
mX = 0,
mY = 0,
bullets = [];
canvas.width = width;
canvas.height = height;
canvas.addEventListener("mousemove", function (e) {
mX = e.pageX;
mY = e.pageY;
});
var Ball = function (x, y, radius, color) {
this.x = x || 0;
this.y = y || 0;
this.radius = radius || 10;
// makes our x and y the center of the circle.
this.x = (this.x-this.radius/2);
this.y = (this.y-this.radius/2);
// how far out do we want the point
this.pointLength = 50;
this.px = 0;
this.py = 0;
this.color = color || "rgb(255,0,0)";
}
Ball.prototype.shoot = function(tx, ty){
bullets.push(new Bullet(this.x, this.y, tx, ty));
}
Ball.prototype.update = function (x, y) {
// get the target x and y
this.targetX = x;
this.targetY = y;
var x = this.x - this.targetX,
y = this.y - this.targetY,
radians = Math.atan2(y,x);
this.px = this.x - this.pointLength * Math.cos(radians);
this.py = this.y - this.pointLength * Math.sin(radians);
// -y will make 0 the top, y will 0 us at the bottom.
output.textContent = radians;
output2.textContent = radians/Math.PI * 180
};
Ball.prototype.render = function () {
ctx.fillStyle = this.color;
ctx.beginPath();
ctx.arc(this.x, this.y, this.radius, 0, Math.PI * 2);
ctx.closePath();
ctx.fill();
ctx.strokeStyle = "rgb(0,0,255)";
ctx.beginPath();
ctx.moveTo(this.x, this.y);
ctx.lineTo(this.px, this.py);
ctx.closePath();
ctx.stroke();
};
var Bullet = function(x,y,tx,ty){
this.speed = 15;
this.x = x;
this.y = y;
var radians = Math.atan2(ty-y, tx-x);
this.velX = Math.cos(radians) * this.speed;
this.velY = Math.sin(radians) * this.speed;
}
Bullet.prototype.update = function(){
this.x += this.velX;
this.y += this.velY;
};
Bullet.prototype.render = function(){
ctx.fillStyle = '#000';
ctx.beginPath();
ctx.arc(this.x, this.y, 2, 0, Math.PI * 2);
ctx.closePath();
ctx.fill();
};
var ball1 = new Ball(width/2, height/2, 10);
canvas.addEventListener("click", function (e) {
ball1.shoot(e.pageX, e.pageY);
});
function render() {
ctx.clearRect(0, 0, width, height);
ball1.update(mX, mY);
ball1.render();
bullets.forEach(function(b){
b.update();
b.render();
});
requestAnimationFrame(render);
}
render();
ol{list-style:none;}
<canvas id="canvas"></canvas>
<div>
<ol>
<li>
<span>Radians : </span><span id="radians"></span>
</li>
<li>
<span>Degrees : </span><span id="degrees"></span>
</li>
</ol>
</div>

Answer 2

Add a new property on bullet that stores the angle of motion, initialize it to -1. Then, on the very first drawBullet call, check if it has been initialized first. If not, set the angle...
function Bullet(x, y) {
this.x = x;
this.y = y;
this.rotation = 0;
this.width = 6;
this.height = 3;
this.color = utils.getRandomColor();
this.speed = 80;
this.angle = -1; // New, angle property initialized to -1
}
function drawBullet(bullet) {
if (bullet.angle === -1) { // Only pull the mouse cursor and get an angle
var dx = mouse.x - bullet.x, // If it hasn't already done so.
dy = mouse.y - bullet.y,
angle = Math.atan2(dy, dx);
bullet.angle = angle;
}
bullet.vx = Math.cos(bullet.angle) * bullet.speed; // Re-use the angle value.
bullet.vy = Math.sin(bullet.angle) * bullet.speed;
bullet.x += bullet.vx;
bullet.y += bullet.vy;
bullet.draw(ctx);
}