Make a bullet move towards the mouse position

Question

Before you give me a link to another post look first, just in case.

So I'm making a top down game where the person can shoot around, I want the bullets to move towards the mouse, I have tried this:

angle = (float) Math.atan2(pmx - x, pmy - y) * 180 / Math.PI;
x += this.getVelX() + (Math.cos(angle * Math.PI / 180) * 15);
y += this.getVelY() + (Math.sin(angle * Math.PI / 180) * 15);

The pmx means previous mouse position where I clicked, and the same for pmy. But this doesn't work properly look at this screenshot, the gun is pointed towards the mouse (the screenshot doesn't show the mouse) and the yellow squares are the bullets.

How it moves the bullets is the goes towards the mouse about 50 pixels then curls down to the left and inverted for the other side.

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After looking at Theraot's post Section B:

int speed = 15;
int velocityX = pmx - x;
int velocityY = pmy - y;
length = Math.sqrt(velocityX * velocityX + velocityY * velocityY);
velocityX *= speed / length;
velocityY *= speed / length;

x += velocityX;
y += velocityY;

This has worked to move it the direction of the mouse, but a new issue has occurred whether this is my issue or not I don't know, as I'm not that good at maths, what it does is moves in the correct direction and when it gets to the mouses old position where it was clicked it stop moving:

If there is a way to stop this from that method can someone tell me if not which Section should I use instead?

Answer 1

First: you have your arguments to atan2 backwards. If you check the docs, you'll see it takes y first, then x

But why go to all the work to take the angle, convert it from radians to degrees, convert it back to degrees, then convert it back to a vector? This is a pile of extra trig you don't need.

instead, when you spawn the bullet, calculate its velocity like so:

dx = mouseX - x;
dy = mouseY - y;

length = Math.sqrt(dx*dx + dy*dy);

dx /= length;
dy /= length;

this.setVelX(dx * speed);
this.setVelY(dy * speed);

Do not repeat the above on every tick unless you want a homing bullet that can change direction in flight.

Instead, each frame, all you need to do is continue moving the bullet along the straight line of its velocity, using the same stored velocity rather than re-calculating it:

x += this.getVelX() * deltaTime;
y += this.getVelY() * deltaTime;

I've changed your velocity integration here, assuming you want to move straight at the mouse at some fixed speed, and adding a deltaTime adjustment so your speed stays consistent regardless of framerate.

Answer 2

Do you want bullets to be shot in the direction of the mouse?

When the player clicks (Version A)

1. Compute the angle from the avatar position

   angle = Math.atan2(mouseY - avatarY, mouseX - avatarX);
   

2. Compute the velocity vector

   speed = 15;
   velocityX = speed * cos(angle);
   velocityY = speed * sin(angle);
   

3. Create the bullet with that velocity

When the player clicks (Version B)

DMGregory is right, we do not need trigonometry.

1. Compute the velocity vector

   speed = 15;
   velocityX = mouseX - avatarX;
   velocityY = mouseY - avatarY;
   length = Math.sqrt(velocityX * velocityX + velocityY * velocityY);
   velocityX *= speed / length;
   velocityY *= speed / length;
   

2. Create the bullet with that velocity

Addendum:

The idea is to compute the velocity vector (velocityX and velocityY) once before you create the bullet (thet is, on click), then create the bullet, and store the velocity in the bullet.

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On update

For each bullet, move it according to velocity:

x += this.getVelX() * delta;
y += this.getVelY() * delta;

Where delta is the time from the last frame.

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Do you want bullets to be chase the mouse?

On update, do it all

angle = Math.atan2(mouseY - y, mouseX - x); // x and y of the bullet
speed = 15;
velocityX = speed * cos(angle);
velocityY = speed * sin(angle);
x += velocityX * delta;
y += velocityY * delta;

speed = 15;
velocityX = mouseX - x; // x is of the bullet
velocityY = mouseY - y; // y is of the bullet
length = Math.sqrt(velocityX * velocityX + velocityY * velocityY);
velocityX *= speed / length;
velocityY *= speed / length;
x += velocityX * delta;
y += velocityY * delta;

Note: know that by changing velocity, technically the bullet has an acceleration. The code above ignores that fact.

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Do you want to rotate the weapon towards the mouse?

On update, set the orientation to this angle

angle = Math.atan2(mouseY - avatarY, mouseX - avatarX);

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Placing the bullets at the end of the gun

You might want to create the bullets at (avatarX, avatarY), however, it could be better to create at (avatarX + cos(angle) * r + offX, avatarY + sin(angle) * r + offY) and adjust r, offX and offY to make the bullet come out of the point of the gun.