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angle_difference

NOTE: The GameMaker:Studio function of the same name produces the same results and obsoletes this script.

This function determines the smallest relative angle between two directions, a value between -180 and 180 degress. This information has many uses but it is very often used for decision making and AI.

For example, by measuring the angle between the direction an instance is facing and the direction to a target, one could determine if the target is within the "cone of vision" of the instance.

// Determine if a target is within a 90 degree cone of vision.
directionToTarget = point_direction(x, y, target.x, target.y);
angleToTarget = angle_difference(directionToTarget, direction);
if (abs(angleToTarget) < 45) {
    // The target can be seen!
    fire_missile_at(target);
}

Let's say the attacking instance has fired a heat-seeking missile at the target. The difference in angle between the direction the missile is heading and the direction to the target can determine how the missile needs to turn to reach the target. Each step the missile can be made to gradually turn towards the target by simply adding a fraction of the measured angle to the direction the missile is heading.

// Turn towards the target by two degrees (or less if necessary).
directionToTarget = point_direction(x, y, target.x, target.y);
angleToTarget = angle_difference(directionToTarget, direction);
direction += sign(angleToTarget) * min(2, abs(angleToTarget));
angle_difference(angle1, angle2)
Returns the relative angle [-180..180] between the given angles.
COPY/// angle_difference(angle1, angle2)
//
//  Returns the relative angle [-180..180] between the given angles.
//
//      angle1      1st direction in degrees
//      angle2      2nd direction in degress
//
/// GMLscripts.com/license
{
    return ((((argument0 - argument1) mod 360) + 540) mod 360) - 180;
}

Contributors: xot, EyeGuy

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