Shaking objects near correct position (with video)

Question

All right, so I'm testing two box objects. One is standing fixed in a position and another at the beginning of the program goes to the first one. And at the end the latter should stand completely over the first (so the center of both is equal).

Here's my method in the Object class:

Old code

void Object::moveTo(Point _p) { // _p is always the center of the other object we want to go to

    // adjust speed, don't worry it's not important here, I guess
    if (current_speed == 0)
        current_speed = speed/16;
    else if (current_speed >= speed)
        current_speed = speed;
    else
        current_speed *= 2;

    // we want to calculate the new center of the
    // our object (the one that run this method)
    Point center;
    // distance(x) calculate the distance between the center point of the object and
    // the x point passed
    if (distance(_p) <= current_speed) {
        // we don't want shaking things!
        center = Point(_p.x, _p.y);
    } else {
        // move diagonally
        center.x = getCenter().x + current_speed*std::cos(angle(_p)); // angle calculates the angle between the center of the object and the _p point
        center.y = getCenter().y + current_speed*std::sin(angle(_p));
    }
    // since we draw sprites by the top left corner we need to convert
    // the center point to the correct top left point
    center = toPosition(center);
    sprite.setPosition(center);
}

New code

Notice that the origin of the sprite is set to be in the center so setPosition will correctly set the center and that getCenter() returns sprite.getPosition() which is the center point.

void Object::moveTo(Point _p) {
    Point center;
    //std::cout << distance(_p) << std::endl;
    if (distance(_p) <= speed) {
        center = Point(_p.x, _p.y);
    } else {
        center.x = getCenter().x + speed*std::cos(angle(_p));
        center.y = getCenter().y + speed*std::sin(angle(_p));
    }
    sprite.setPosition(center);
}

This is what happens: http://youtu.be/Ngra3FXFe0A.

The distance keeps going from 15.xxx to 16.xxx and I don't know why.

Testing surprise

• Doing distance(_p) <= 10 instead doesn't make it shake. With 9, 8... or minor it shakes.
• Defining a minor speed the object gets nearer the correct position before start shaking.
• Setting the white cube to follow the player (yellow sprite) it moves like a jagger: http://youtu.be/9FjRfHHprEQ
• Here's a recent video: the 3 little squares points (its the specific top left corner) to: blue = box center, red = player center, green = top left corner of the box. As you can see they are correctly calculated (this test was made with speed of 1, while the others were made with a greater speed): http://youtu.be/iFAWGssk7NI

Distance function:

float Object::distance(Point _p) {
    return sqrt((pow((getCenter().x - _p.x), 2) + pow((getCenter().y - _p.y), 2)));
}

Angle function

float Object::angle(Point _p) {
    //  (180 / PI = 57.3065)
    return 57.3065f * atan2(getCenter().y - _p.y, getCenter().x - _p.x);
}

Why is that?

Answer 1

It's clear that the simple 'new' code posted doesn't have a significant problem - the angle calculation looks correct, and as soon as the object would get within range, the position will be set to _p.x, _p.y, which is exactly where it needs to be. The only reason this would ever jitter is one of the following:

1. current_speed hits zero when distance(_p) is non-zero (perhaps due to floating point error), meaning the sprite moves a full amount again - this seems the most likely problem in the 'old' code, but you don't show how you calculate current_speed, and it's not even in the 'new' code;
2. distance(_p) changes even when _p and the sprite's position are constant;
3. _p is not constant, and something is periodically changing it, even when the object appears to be still;
4. the following sprite's position is not constant, despite being set to equal _p - perhaps something else moves the sprite;

One way you can reduce the jitter is to remove the "move or don't move" test and instead scale the movement amount proportional to the distance to the target. You don't need an angle at all - just calculate the movement vector (your current argument to atan2() in angle()) and if the length of the vector is more than your speed, scale it down to that size. Otherwise, leave it as it is. Then add that vector to your sprite's position to apply the movement. It's impossible to overshoot that way because the length of the movement is never bigger than the distance to the target.

But don't do this until you find the problem in your code that is causing the issue, otherwise you'll just be hiding a bug rather than fixing it!

Answer 2

In this part:

    center.x = getCenter().x + speed*std::cos(angle(_p)); // angle calculates the angle between the center of the object and the _p point
    center.y = getCenter().y + speed*std::sin(angle(_p));

I think you want current_speed instead of speed.

Answer 3

Do you know guys what the problem was? This: 57.3065f. This makes the angle() function returns degrees. And this was bad because std::sin and std::cos expects radians to be passed.