How do I make projectiles?

Question

I am totally stumped on this one. I'm using C++ and SFML 1.6 for a game I'm developing, and I have no bloody idea. How do I make projectiles (like bullets)? I just don't understand it. It could be my lack of sleep but I don't know.

So my question is how do I create a Sprite that moves in a definite direction based on where the mouse is? (Think of a top down shooter with mouse aiming)

Answer 1

Easiest solution: If the mouse is at Mx,My and the ship is at Sx,Sy then calculate the direction from the ship to the mouse: Dx=Sx-Mx Dy=Sy-My

Now normalise D (this means scale it so that it's length is one):

DLen=sqrt(Dx*Dx + Dy*Dy)
Dx/=DLen;
Dy/=DLen;

Now Dx is the distance you want to move the bullet on the x axis in order to get bullet speed of 1.

Thus each frame you move the bullet like so (position of bullet: Bx,By Speed of bullet: Bs [in pixels per millisec] Frame time Ft[in millisec])

Bx=Bx+Dx*Bs*Ft
By=By+Dy*Bs*Ft

This give you a bullet that moves towards the mouse position at a speed independent of the direction of the mouse or framerate of the game.

EDIT: As @MSalters says you need to check for the DLen==0 case when the mouse is directly above the ship to avoid division by zero errors on the normalise

Answer 2

One way to do it is to make the bullet face the mouse and then move it across the x and y axis by using trigonometry to find the hypotinuse from the angle. I don't think i explained this very well, so here the code to make a sprite move from its rotation:

void sMove(sf::Sprite& input,float toMove, int rotation){
bool negY = false;
bool negX = false;
int newRot = rotation;
if (rotation <= 90){
    negY = true;
}
else if (rotation <= 180){
    //newRot -= 90;
    negY = true;
}
else if (rotation <= 360){
    newRot -= 270;
    negY = true;
    negX = true;
}
float y = toMove*cos(newRot*PI/180);
float x = toMove*sin(newRot*PI/180);
if (negY){
    y = y*-1;
}
if (negX){
    x = x*-1
}
input.move(x, y);
}