Forums » Help » Need help with Trigonometry

I hope someone could help me solve this problem.

Referring to this thread started 6 months back
http://impactjs.com/forums/help/position-bullets-with-rotating-spaceship/page/1

The thread tells us how to position, the firing position at the center and forward.

But if my muzzle is fire from the side instead. What would be the correct mathematical formula?

Base on the quote from Dom below. I'd like to edit the formula to allow firing from the side forward instead. How do I change the mathematical formula Dom provided?

Quote from dominic
Cool looking game!

You need a bit of math to spawn the bullet at a rotated (to the ship's angle) and translated (forward to the ship's nose) point.

// Find the center of Player and move the bullet foward by 20px,
var forward = 20;
var sx = (this.pos.x + this.size.x/2) + (Math.cos(this.angle) * forward);
var sy = (this.pos.y + this.size.y/2) + (Math.sin(this.angle) * forward);

ig.game.spawnEntity(EntityBullet, sx, sy, {angle:this.angle});

This assumes your player's .angle already is in radians, otherwise just use .toRad() to convert (it's a shorthand for the *Math.PI/180):

var sx = (this.pos.x + this.size.x/2) + (Math.cos(this.angle.toRad()) * forward);
var sy = (this.pos.y + this.size.y/2) + (Math.sin(this.angle.toRad()) * forward);

Also note that I'm passing the player's angle to the Bullet entity in the settings parameter ({angle:this.angle}). This will directly set a .angle property on the Bullet entity in the this.parent(); init.

init: function( x, y, settings ) {
	this.parent( x, y, settings ); // This will write the .angle property to 'this'

	this.vel.x = Math.cos(this.angle.toRad() * this.maxVel.x;
	this.vel.y = Math.sin(this.angle.toRad() * this.maxVel.y;
	
	this.addAnim( 'idle', 0.2, [0] );
	//rotate the angle of the animation sprite
	this.currentAnim.angle = this.angle.toRad();	
},

Again, mind the .toRad() if you need it. Also, you shouldn't have to set .accel when you set .vel, and I'm not sure what the bulletOffSet was for.

Quote from wands
my muzzle is fire from the side instead

I'm not sure I understand your question. What's wrong with the formulas provided? Positioning from the center-out WILL fire from the side.

Hi.

What I meant was to fire on the left or right

The code finds the center of the entity and forward it. It's essentially still firing in the center of the entity. But say if I want to fire on the left or right instead?

I think this is what you mean...

/* How to spawn a bullet to the side */

var distanceFromCenter = 10;

var x = this.pos.x + this.size.x/2; // center of entity x
var y = this.pos.y + this.size.y/2; // center of entity y

// Pick a side.
x = centerX + (this.isFacingLeft ? -1 : 1) * distanceFromCenter; 

// Spawn bullet.
ig.game.spawnEntity(EntityBullet, x, y);

Thanks for the reply.

If this is rotated, will I get a different result?
My player rotates and moves.

Riffing off of Joncom's code:

/* How to spawn a bullet to the side */

var distanceFromCenter = 10;

var x = this.pos.x + this.size.x/2; // center of entity x
var y = this.pos.y + this.size.y/2; // center of entity y

// rotate!
x += Math.cos(this.angle) * distanceFromCenter * (this.flip.x ? -1 : 1);
y += Math.sin(this.angle) * distanceFromCenter * (this.flip.y ? -1 : 1);

// Spawn bullet.
ig.game.spawnEntity(EntityBullet, x, y);

Will test this out. Thank you