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I'm making a top-down space shooter where the player controls a ship and can aim and shoot using the mouse cursor. How can I fire bullets from the ship at the angle from the ship to the mouse cursor?

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marked as duplicate by Josh Petrie Aug 10 at 4:33

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Is it that you need help calculating the angle based on the mouse cursor? –  Nate Jun 7 '11 at 20:41
    
Yeah, sorry if my question is somewhat vague in that regard. And also how to move the projectile based on the angle, that too I need. –  Tristan Dube Jun 7 '11 at 21:42
    
Wait, you're asking us about basic trigonometry, or the means to implement it? –  jco Jan 21 '12 at 20:45

2 Answers 2

up vote 17 down vote accepted

If I understood your problem properly, you just want to shoot a bullet towards a mouse position. Here is how I would do:

First of all, you must find the movement required for the bullet to get to the mouse, like so:

Vector2 movement = mousePosition - bulletStartPosition;

Then, you should normalize it to have a vector with a length of 1 so that you can hold a vector which tells you in which direction to go, like so:

movement.Normalize();

But here you have a little problem, if the direction is equal to (0, 0) (meaning that the mouse is on the bullet start position), then you'll divide by zero, so make sure you check for that with the last piece of code:

if (movement != Vector2.Zero)
    movement.Normalize();

So, you've got the movement required to move towards the mouse. You have to keep a Vector2 within your bullet class which holds the Direction of the bullet.

What's next? You have to actually move the bullet!

In your bullet update code, do the following:

bullet.Position += bullet.Direction * bullet.Speed * gameTime.ElapsedGameTime.TotalSeconds; // multiply by delta seconds to keep a consistent speed on all computers.

Where bullet.Speed is a float representing the bullet's speed in units per second.

Basically, here are the things to change:

Inside you bullet class, add a float Speed and a Vector2 Direction.

When shooting, set your bullet.Direction to mousePosition - bullet.Position and safely normalize it (by checking for equality with Vector2.Zero first).

When updating your bullet, do the following: bullet.Position += bullet.Direction * bullet.Speed * gameTime.ElapsedGameTime.TotalSeconds;.

It should work.

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2  
+1 Vectors are a wonderful thing. Learn them! –  doppelgreener Jun 7 '11 at 22:43
    
Thanks, that did the trick ! Awesome link on the vectors too, definitely something I'll read soon. –  Tristan Dube Jun 8 '11 at 1:27
    
what is movement used for after you normalize it? I dont see it being used. –  Raptrex Aug 23 '11 at 18:40
    
@Raptrex: Actually movement is what I called the Direction later on. I just called it movement in the code to first show what it represents. It is basically just the Direction vector. –  Jesse Emond Aug 28 '11 at 2:13
1  
@JesseEmond Welcome to the vote to close privilege, use it wisely. –  Byte56 May 17 '13 at 19:44

If you're having trouble calculating the angle, you can use this:

Vector2 target = mousePos - startPos;
float angle = Math.Atan2( target.Y, target.X );
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+1 Keepin' it simple. –  Nick Wiggill Jan 21 '12 at 16:40

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