Rotate sprite along the angle between two objects

Question

Im trying to make flames follow my object, I set the direction for my moving object using the points between it and another object (using static destination for testing).

No matter what I do I cannot use the radians (which I use to set the direction), translate them to degrees, then rotate my flames so it looks like they are following it (like a comet).

What I try:

sprite.setPosition(pos.x, pos.y);
radians = Math.atan2(0 - pos.y, 540 - pos.x); //set radians
degrees = radians * 180 / MathUtils.PI; //calculate degrees
fireSprite.setPosition(sprite.getX(), sprite.getY());
fireSprite.setRotation((float)degrees); //rotate according to degrees

I move the object like this:

pos.x += 200 * Math.cos(radians) * delta;
pos.y += 200 * Math.sin(radians) * delta;

Am I missing something in the math? I feel like it should work.

An example of result from calculations:

radians: -1.8745297149164084, degrees: -107.4026382478209
radians: -1.1776894171295331, degrees: -67.47663130103584
radians: -2.1756410640936705, degrees: -124.65504723907603
radians: -1.2244578042111383, degrees: -70.15626242087843

The flames face the wrong direction, and not in a any sort of pattern I can use to debug my problem.

Answer 1

You can try taking the vector from the ship (or whatever you have) to the target position, then get the angle of that. libGDX's Vector2 class has most of the methods you need.

Something like this might work for you;

// Get these from where ever you have them
Vector2 shipPosition = new Vector2(100, 100);
Vector2 targetPosition = new Vector2(0, 540);

// Calculate the vector from the ship to the target
Vector2 fromShipToTarget = new Vector2(targetPosition).sub(shipPosition);

// Get the angle of the ship-to-target
float angle = fromShipToTarget.angle();

// Set the origin so that the sprite is rotated around its center, not its corner
fireSprite.setOrigin(fireSprite.getWidth() / 2.0f, fireSprite.getHeight() / 2.0f);
fireSprite.setPosition(shipPosition.x, shipPosition.y);
fireSprite.setRotation(angle);

The angle method on Vector2 will give you the angle of the vector towards the positive y-axis so if you have two points (0, 0) and (1, 1) the above example will yield a vector of 45 degrees.

Answer 2

Here is the code I use to get the angle of a 2d vector, hopefully it is what your are looking for. (using our own library functions, most of which should be obvious)

float zVec2f::getAngle () const
{
    //straight up, 0.0f degrees
    zVec2f up(0.0f, -1.0f);

    //angle between up vector and our vector
    zVec2f normal = *this;
    if ( normal.normaliseSafe() ) // returns true if it was possible to normalize vector
    {
        float angle = acosf(zDot(normal,up));

        //compensate for the fact that acos only
        //gives us 0->Pi (mirror along 180 degree line)
        if ( x < 0.0f)
        {
            angle += (2.0f * (zPIf - angle));
        }
        zASSERT(angle>=0.0f);
        zASSERT(angle<=zPI2f);

        return angle;
    }
    return 0.0f;
}