SplineHelper.java

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.files.FileHandle;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.utils.Array;
import com.badlogic.gdx.utils.XmlReader;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import com.badlogic.gdx.math.CatmullRomSpline;

/** SplineHelper class provides helper methods for populating and traversing CatmullRomSpline objects using XML data generated by 
*   the libGDX Path Editor.
* 
*   Uses snippets from https://github.com/libgdx/libgdx/wiki/Path-interface-&-Splines
*
* @author bc_bytes
*/
public final class SplineHelper
{
    private float screenWidth;
    private float screenHeight;
	
    public SplineHelper(float screenWidth, float screenHeight)
    {
        this.screenWidth = screenWidth;
	this.screenHeight = screenHeight;
    }

    public Vector2[] FlipVerticesHorizontally(Vector2[] original)
    {
        Vector2[] flipped = new Vector2[original.length];
        for (int i = 0; i < original.length; i++)
        {
            flipped[i] = new Vector2(-1 * original[i].x + screenWidth, original[i].y);
        }
        return flipped;
    }

    private Vector2[] FlipVerticesVertically(Vector2[] original)
    {
        Vector2[] flipped = new Vector2[original.length];
        for (int i = 0; i < original.length; i++)
        {
            flipped[i] = new Vector2(original[i].x, -1 * original[i].y + screenHeight);
        }
        return flipped;
    }

    /** Method to load Path Editor XML file. XML data is assumed to be located in a folder named 'splines'.
    * 
    * @param splineFileName String containing the name of the XML file containing the path data.
    * @param reverse        Boolean indicating if the order of the vertices should be reversed once read.
    * @param flip           Set this to true if you're using a coordinate system where 0,0 is top-left.
    * @return               Vector2 array containing vertex data for spline.
    */
    public Vector2[] GetSplineData(String splineFileName, boolean reverse)
    {
        // read XML spline data
        Array<XmlReader.Element> vertices = null;
        List<Vector2> dataset = new ArrayList<Vector2>();
        try
        {
            XmlReader reader = new XmlReader();
            FileHandle xmlFile = Gdx.files.internal("splines/" + splineFileName + ".xml");
            XmlReader.Element root = reader.parse(xmlFile);
            vertices = root.getChildByName("vertices").getChildrenByName("vertex");

            for (XmlReader.Element child : vertices)
            {
                String x = child.getChildByName("x").getText();
                String y = child.getChildByName("y").getText();

                dataset.add(new Vector2(Float.parseFloat(x), Float.parseFloat(y)));
            }
        }
        catch (java.io.IOException e)
        {
            System.out.println("Error reading spline data: " + splineFileName);
        }

        if (reverse)
        {
            // reverses vertices in array
            Collections.reverse(dataset);
        }
		
	// set vertices for spline path
        Vector2[] data = new Vector2[dataset.size()];
        data = dataset.toArray(data);

        if (flip)
	{
	    data = FlipVerticesVertically(data);
	}

        return data;
    }

    /** Method to traverse the vertices in a CatmullRomSpline object. Note that movement speed is constant. Call this method every 
    *   frame to update the traversal.
    * 
    * @param gameObject        A GameObject representing the object you want to traverse the spline. 
    *                          The GameObject should maintain the state of its own spline, so must contain the following members:
    *                          - Vector2 position
                               - float rotation
    *                          - Vector2 splineOut
    *			       - float splineCurrent
    *                          - float splineSpeed
    * @param CatmullRomSpline  CatmullRomSpline object containing valid vertex data.
    * @param boolean           Boolean indicating if the object traversing the spline should point in the direction of travel.
    */
    public void TraverseSpline(GameObject gameObject, CatmullRomSpline spline, boolean pointForward)
    {
        if (pointForward)
        {
            // set gameObject to point in the direction it is currently heading
            spline.derivativeAt(gameObject.splineOut, gameObject.splineCurrent);
            gameObject.rotation = gameObject.splineOut.angle();
        }

        // traverse spline path
        gameObject.splineCurrent += (Gdx.graphics.getDeltaTime() * gameObject.splineSpeed) / gameObject.splineOut.len();
        if (gameObject.splineCurrent >= 1)
        {
            gameObject.splineCurrent -= 1;
        }
        spline.valueAt(gameObject.splineOut, gameObject.splineCurrent);
        gameObject.position.x = gameObject.splineOut.x;
        gameObject.position.y = gameObject.splineOut.y;
    }
}