For a game like Solar Striker(idk if anyone knows that game) here is the basic idea:
You can move up and down the screen and the enemies come off the top and come down at your. All the bullets go straight up the screen. No angles or anything. So all you need to do is take away from the y position and then check for collision.
but take a game like Asteroids for example. in that game enemies come from all around and you rotate. well to find out how much x and y to add you would need to calculate the movement based on the angle of rotation. like this:
moveX = sin(angle)*speed
moveY = (cos(angle)*-1)*speed
I did cos(angle) times -1 because the y axis is opposite(+ is down and - is up) then you add that to your x and y values and you get a nice smooth movement at that specific angle.
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