Can't get bullets to shoot at correct angle?

Question

I've been having some trouble with trying to get a bullet to fire at angle that I put in. I am using eclipse with java.

My code:

x += (int) (spd * Math.cos(dir));
y -= (int) (spd * Math.sin(dir));`

The feel like the reason it's not working is because it is casted to an int which possibly makes it so inaccurate. But in order for it to draw the rectangle it needs ints.

When inputting dir as 0 it's fine and shoots to the right. The problem is when I put in 90, instead of shooting striaght up it shoots off a little to the left.

Any idea on how I can fix this? Thanks!

Answer 1

No, you're making a classic mistake: Java trig functions need radians, not degrees. It's not 90 that you should pass; it's π/2.0.

So be sure to convert angles in degrees to radians by multiplying by π/180.0.

This is true for C, C++, Java, JavaScript, C#, and every other language I know. I cannot name a single language that uses degrees for angles.

double radians = dir*Math.PI/180.0;
x += (int)(spd*Math.cos(dir));
y -= (int) (spd * Math.sin(dir));`  // I don't know why you do this.  Funny left-handed coordinate system.

Speed is the magnitude of the velocity vector. The equations, as written, only express velocity as (vx, vy) components.

If you want displacements, you'll have to multiply by a time step:

vx = speed*Math.cos(angle);
vy = speed*Math.sin(angle);
dx = vx*dt;
dy = vy*dt;
x += dx;  // movement in x-direction after dt time w/ constant velocity
y += dy;  // movement in y-direction after dt time w/ constant velocity

If there's acceleration involved (e.g. gravity), you should calculate the change in velocity over time the same way.

Answer 2

Keep coordinates as floats, cast them only in the call to your drawing method (or cast them into another variable). You need to keep the precise values for the next iteration.

Answer 3

Yes, you need to keep the angle as floats.

But more likely, you said you're using 0 and 90. Math.cos() and Math.sin() take radian values. If you're passing in degrees, you'll definitely have things headed in the wrong direction. Convert from degrees to radians, and your directions will likely be much better.

Answer 4

One problem is that round off error propogates when you increment x and y for each iteration. One posssible solution is to compute the x and y coordinates based on a counter instead:

x = (int) (time * spd * Math.cos(dir));
y = (int) (time * spd * Math.sin(dir));

Here I use the variable name time for the counter to indicate that it is some measure of the amount of time that has passed since the bullet was fired.