Asteroids game c++ inertia - c++

Question

I'm trying to make my asteroid keep moving once I press a button.
void Ship::applyThrust()
{
_v.setX(_v.getX() - sin(2 * M_PI * (_angle / 360)) * 2.5);
_v.setY(_v.getY() + cos(2 * M_PI * (_angle / 360)) * 2.5);
}
This is the formula I have to help my ship move.
The _v.setX and _vsetY updates the X and Y position
The M_PI is just 3.14.
The _angle is how much of a rotation I set with my left and right arrow keys and
The 2.5 is how many frames per I want it to move
My ship is moving just fine, the problem is that I'm trying to simulate space inertia and have my ship continue moving.
Any ideas on how would be the logic for this?
Thanks,

Answer 1

Inside of your game loop, you'll need a function that updates the position of the ship based on its x and y velocity. You're close in getting the new x and y coordinates of your ship, but you don't account for the velocity of the ship. When you apply thrust, get the x and y components of your velocity, not your new position. You'll have an additional function to update position which should be called within the game loop at a timed interval - e.g. every time you update the frame. So your applyThrust function should actually update your ship's velocity. That means you'll need to add variables to your Ship class for your ship's position and your ship's velocity if you don't already have them. I'm breaking out the components of position and velocity for simplicity, but you'll probably want to store them in a vector for clarity:
class Ship
{
private float xpos, ypos; // current position of your ship
private float velocityX, velocityY; // current velocity of your ship
}
Then, when you apply thrust, you change the velocity, and remember that applyThrust is only called when the thrust button is pushed, not every frame as the position update is:
void Ship::applyThrust()
{
/*
Assume 1 total unit of thrust is applied each time you hit the thrust button
so you break down the velocity into x and y components depending on the direction
your ship is facing.
*/
// thrustUnit is how much thrust each thrust button press gets you, it's arbitrary
// and can be the 2.5 multiplier you're using in your question
float thrustUnit = 1.0;
// now get the x and y thrust components of velocity based on your ship's direction
// assuming 0 degrees is pointing to the right
float newVX, newVY = 0.0;
newVX = thrustUnit * cos(_angle);
newVY = thrustUnit * sin(_angle); // radian conversion left out to save space
// Update your ship's velocity
updateVelocity(newVX, newVY);
}
updateVelocity will look something like: (note that the velocity is additive so it continues to drift unless a thrust is applied in the opposite direction - the same as if it were in a frictionless medium such as space)
void Ship::updateVelocity(newVX, newVY)
{
// update the x component
velocityX += newVX;
velocityY += newVY;
}
So now you'll also need an updatePosition function that takes into account your ship's velocity. This gets called with each frame update:
void Ship::updatePosition()
{
// add the x component of velocity to the ship's x position
// assuming xpos and ypos are variables in the Ship class for tracking position
xpos += velocityX;
ypos += velocityY;
}
Now the position of the ship changes incrementally according to each velocity component at each frame update. You can also make thrustUnit a member variable to allow for power-ups that can either boost or decrease your thrust component for your ship's speed and being able to control it better with a smaller thrustUnit or giving it hyperspeed with a large thrustUnit.
Good luck with your game!