Game Maker - How do I create homing effect for a projectile?

Question

I'm trying to make a homing projectile for my bullet hell game and I'd need to be able to calculate the angle between the target and projectile relatively to the projectile's angle (0 degrees would be the direction the projectile is pointing). Right now the angle calculation is absolute done using point_direction, but the problem is when the target is at the 4th sector the projectile starts steering the wrong way. Another issue is that if the projectile does a 180 degree turn while chasing the target (or moves down if fired by enemy) the steering direction will get inverted. I have also tried mp_potential_ functions but their pathfinding is too "agressive".

This is what my current code looks like:

if(instance_exists(obj_fighter1)) {
    var target;
    target = instance_nearest(x, y, obj_fighter1);

    if(target != noone) {
        var angle_to_target;
        angle_to_target = point_direction(x,y,target.x,target.y);

        if(angle_to_target < direction) {
            direction -= 2;
        }

        if(angle_to_target > direction) {
            direction += 2;
        }
    }
}

Hopefully this information is enough and is understandable.

score 1 · Answer 1 · answered Aug 21 '15 at 10:08

Okay, a common Game Maker question. The routine I use is below. Looking at it, it could do with a bit of refactoring, but it does work.

var wantDir;
var currDir;
var directiondiff;
var maxTurn;

// want - this is your target direction \\
wantDir = argument0;

// max turn - this is the max number of degrees to turn \\
maxTurn = argument1;

// current - this is your current direction \\
currDir = direction;

if (wantDir >= (currDir + 180))
{
    currDir += 360;
}
else
{
    if (wantDir < (currDir - 180))
    {
        wantDir += 360;
    }
}

directiondiff = wantDir - currDir;

if (directiondiff < -maxTurn)
{
    directiondiff = -maxTurn
}

if (directiondiff > maxTurn)
{
    directiondiff = maxTurn
}

return directiondiff

So you'd call this, and it'll return you a value that you can add to your missile's angle. So if you call it scr_get_angle, your code might then look like this:

if(instance_exists(obj_fighter1)) {
    var target;
    target = instance_nearest(x, y, obj_fighter1);

    if(target != noone) {
        var angle_to_target;
        angle_to_target = point_direction(x,y,target.x,target.y);

        direction += scr_get_angle(angle_to_target, 2);
    }
}

Game Maker - How do I create homing effect for a projectile?

1 Answers1