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I am trying to move an AI spaceship side to side. I would imagine using sin would solve this but I am struggling. So far this is what I have. This just moves the spaceship right forever as it is called in an update function which is called every frame. I just want the spaceship to move right a certain amount then do the same to the left, and repeat forever. Thanks

m_velocity = Vector2D(200,0);
Vector2D friction = FRICTION * m_velocity;
m_velocity = m_velocity + friction * frametime;
m_position = m_position + m_velocity * frametime;
marktilbrook
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  • Do you want to model an [harmonic oscillator](https://en.wikipedia.org/wiki/Harmonic_oscillator) or just having the spaceship bouncing between the boundaries? – Bob__ May 06 '21 at 09:34
  • Just want it to move back and forth after a few seconds. – marktilbrook May 06 '21 at 09:58
  • Then an oscillator may be the easiest solution: https://godbolt.org/z/rhqabd4j3 – Bob__ May 06 '21 at 13:57

2 Answers2

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As you said, you can do this using sine.

The values of sine go from -1 to 1.

So if you want to move your spaceship between position x₁ and x₂, you have to scale and translate it:

x = (x₁+x₂)/2 + (x₂-x₁)/2 *sin(ωt)

Where the first term moves the middle of the sine to the middlepoint between the two values.

The second term scales the amplitude of the sine, so it exactly reaches x₁ and x₂.

ω is the angular frequency which is a measurement of how fast/slow the thing moves.

t is the current time(accumulated since the start).

QuantumDeveloper
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if your m_velocity 2D vector is the direction your object is moving (turned) and you want side steps then you can simply turn the vector by 90 degrees. In 2D its very simple:

side1.x=+m_velocity.y;
side1.y=-m_velocity.x;
side2.x=-m_velocity.y;
side2.y=+m_velocity.x;

now you got 2 vectors pointing to the left and right (which depends on your coordinate system) so just scale it to match your side speed.

Another option is to get the angle from your speed and use sin,cos:

ang = atan2(m_velocity.y,m_velocity.x) +/- (M_PI*0.5);
side.x=side_speed*cos(ang);
side.y=side_speed*sin(ang);

However this is less accurate and much slower. Which side it is depends on the +/- Also I assumed your angles are in radians. If in degrees instead change (M_PI*0.5) to 90.0

However you are using Newton d'Alembert integration without acceleration. Its more realistic to apply the the control stuff (thrusters) to Acceleration instead of speed ... and have the turn angle ang already stored or in form of transform matrix or basis vectors for rendering anyway.

For more info see:

[Edit1] so you want zig zag like movement without forward motion

try this:

m_velocity = Vector2D(200*sin(2.0*M_PI*f*t),0);

where f is frequency of movement in [Hz] and t is elapsed time in [sec]

To make it more prone to rounding errors use

m_velocity = Vector2D(200*sin(fmod(2.0*M_PI*f*t,2.0*M_PI)),0);

However even this will not be OK after t saturates to the point it can not be incremented ...

When put together with time:

static float ang=0.0;
m_velocity = Vector2D(200*sin(ang),0);
ang=fmod(ang+(2.0*M_PI*f*dt),2.0*M_PI);

where dt is time interval in [sec] of your timer or whatever event you use to invoke this code

Spektre
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  • This is not what I am looking for. Thanks – marktilbrook May 07 '21 at 07:33
  • No, I literally just want a space ship to move right for a few seconds and then left for a few seconds and repeat. Literally that's it. – marktilbrook May 07 '21 at 07:46
  • @marktilbrook what you describe is applying force impulse ... that is the acceleration stuff I was writing about ... in the linked QA you can see it in code ... so you just set the acceleration for few frames then stop it ... If you want zig zag then you need to control zigzag ... maybe you shoud draw an image describing what you want – Spektre May 07 '21 at 07:47
  • There is an spaceship in the middle of the screen, how do I make it move left, and then move right, then left, then right, then left, then right, then left, then right, then left, then right, then left, then right, then left, then right, then left, then right, then left, then right, then left, then right, etc.. – marktilbrook May 07 '21 at 08:00