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# Maintaining speed with touch path

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5 replies to this topic

### #1Richard Goodrum  Members

Posted 13 March 2012 - 11:09 AM

I'm currently making a game similar to the airport controller games already available but I'm having a bit of difficulty keeping the same speed with a path. When the plane is initialised, it is given a velX and velY value that changes the position of the plane, but when the user touches the plane, a vector of points is created that set the position of the plane. My question is, how can I keep the plane moving at the same speed as it was set, instead of just moving from point to point at whatever speed the path is created?

### #2Richard Goodrum  Members

Posted 13 March 2012 - 11:10 AM

Forgot to mention, if the plane is moved after being held down, points are collected and added to this vector.

### #3frob  Moderators

Posted 13 March 2012 - 05:03 PM

Whatever you do will end up being very specific to your game logic.

Perhaps a rolling average or weighted spline of point distances to come up with your velocity ? Or if the velocity should be kept current, you would not need to do anything other than follow the curve.

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### #4Richard Goodrum  Members

Posted 14 March 2012 - 10:38 AM

Sorry I think I was a bit unclear about my intentions. My plane has a velX and velY value that is added to the position to move it. When a path is created I'm having an issue moving the plane along to the next point. I tried :

double theta = Math.atan2(pathPoints.firstElement().y-posY, pathPoints.firstElement().x - posX);

velX=(float) (magnitude * Math.sin(theta));
velY=(float) (magnitude * Math.cos(theta));

but this results in the plane moving in a straight line away from the point, in the opposite direction. I also tried doing posY-velY but this resulted in the plane flying in a circle. What am I doing wrong?

### #5Zael  Members

Posted 15 March 2012 - 11:59 AM


double theta = Math.atan2(nextY - posY, nextX - posX); //calculate the angle of the next coordinate relative to the current position

velX = magnitude*Math.cos(theta); //calculate the velocity of X given using the angle of theta and magnitude of the total velocity

velY = magnitude*Math.sin(theta); //calculate the velocity of Y given using the angle of theta and magnitude of the total velocity

currX+=velX*(timeElapsed); //add the velocity to the currentX

currY+=velY*(timeElapsed); //add the velocity to the currentY



### #6clb  Members

Posted 15 March 2012 - 01:53 PM

Sorry I think I was a bit unclear about my intentions. My plane has a velX and velY value that is added to the position to move it. When a path is created I'm having an issue moving the plane along to the next point. I tried :

double theta = Math.atan2(pathPoints.firstElement().y-posY, pathPoints.firstElement().x - posX);

velX=(float) (magnitude * Math.sin(theta));
velY=(float) (magnitude * Math.cos(theta));

but this results in the plane moving in a straight line away from the point, in the opposite direction. I also tried doing posY-velY but this resulted in the plane flying in a circle. What am I doing wrong?

With this code snippet, you are doing a redundant euclidean -> polar -> euclidean conversion. It is simpler and faster to just work with 2D vectors as follows:
// Unnormalized direction vector to move to.
velX = pathPoints.firstElement().x - posX;
velY = pathPoints.firstElement().y - posY;
float length = velX*velX + velY*velY;
float desiredVelocity = x;
if (length <= desiredVelocity*desiredVelocity)
{
// distance to target is shorter than our velocity. We have arrived to our target, what to do?
// Immediately snap to target, or take next waypoint instead?
return;
}

// Scale the velocity to the desired length.

length = 1.0f / Sqrt(length);
velX *= length * desiredVelocity;
velY *= length * desiredVelocity;


However, if I understand correctly, you are moving your plane through a set of waypoints. In that case, just going linearly from waypoint to waypoint will look jaggy, and instead, you will be looking to use a smooth curve with arc length parametrization. At least, that's what the flight control games on Android and iPad are doing.
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