我在使用bspline路径时遇到了使用derivativeat（）方法的问题，因为它没有返回正确的值，但是derivativeat（）方法对catmullromspline路径有效，因此这是spline特有的。wiki和api没有提到样条线需要与其他路径区别对待……样条线是否有我遗漏的地方，或者这是libgdx的一个bug？
我做了一个简单的程序来说明这个问题。它使用DerivativeEat（）方法绘制控制点（黄点）、路径本身（浅灰色线）和切线（绿线），该方法应指向与路径相同的方向。
这是它应该做的。catmulromspline工作正常，因为绿色切线指向与路径相同的方向：

下面是bspline的作用。切线指向不同的方向，这些方向不是路径的方向：

这是我的密码。任何帮助使这个正常工作将不胜感激。

public class Test extends Game {
    private int width, height;
    private Stage stage;
    private Table mainTable;
    private Texture texture;
    private Image image;
    private Pixmap pixmap;  
    private Path<Vector2> path;
    private Vector2 point1=new Vector2(), point2=new Vector2();
    private Vector2 derivative=new Vector2(), normal=new Vector2();

    private float pathSegments=10000, derivativeSegments=40;
    private int lineLength = 20;

    private Vector2[] points = { new Vector2(400,700), new Vector2(100,600),
            new Vector2(100,250), new Vector2(400,100), new Vector2(700,250),
            new Vector2(700,600), new Vector2(400,700)
    };

    // TRUE  = use BSpline
    // FALSE = use CatmullRomSpline
    private boolean USE_BSPLINE = true;

    @Override
    public void create () {
        stage = new Stage();
        stage.setViewport(new ScreenViewport(stage.getViewport().getCamera()));
        Gdx.input.setInputProcessor(stage);
        mainTable = new Table();
        mainTable.setFillParent(true);
        stage.addActor(mainTable);
        width = Gdx.graphics.getWidth();
        height = Gdx.graphics.getHeight();
        pixmap = new Pixmap(width, height, Pixmap.Format.RGBA8888);

        if (USE_BSPLINE){
            path = new BSpline<Vector2>();
            ((BSpline<Vector2>)path).set(points, 3, false);
        } else {
            path = new CatmullRomSpline<Vector2>();
            ((CatmullRomSpline<Vector2>)path).set(points, false);
        }

        drawControlPoints();
        drawPath();
        drawDerivative();
        texture = new Texture(pixmap);
        image = new Image(texture);

        mainTable.add(image).size(image.getWidth(), image.getHeight());
    }

    private void drawControlPoints(){
        pixmap.setColor(Color.YELLOW);
        for (int i=0; i<points.length; i++){
            pixmap.fillCircle((int)points[i].x, (int)points[i].y, 6);
        }
    }
    private void drawPath(){
        pixmap.setColor(Color.LIGHT_GRAY);
        float t;
        for(int i=0; i<=pathSegments; i++){
            t = i / pathSegments;
            path.valueAt(point1, t);
            pixmap.drawPixel((int)point1.x, (int)point1.y);
        }
    }
    // << ERROR OCCURS HERE WHEN USING BSPLINE >>
    private void drawDerivative(){
        pixmap.setColor(Color.GREEN);
        float t;
        for(int i=0; i<=derivativeSegments; i++){
            t = i / derivativeSegments;
            path.valueAt(point1, t);
            path.derivativeAt(derivative, t);
            normal.set(derivative);
            normal.nor();
            point2.set((point1.x + normal.x*lineLength), (point1.y + normal.y*lineLength));
            pixmap.drawLine((int)point1.x, (int)point1.y, (int)point2.x, (int)point2.y);
        }
    }

    @Override
    public void render () {
        Gdx.gl.glClearColor(0.0f, 0.0f, 0.08f, 1f);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
        stage.act(Gdx.graphics.getDeltaTime());
        stage.draw();
    }
    @Override
    public void resize(int width, int height){
        stage.getViewport().update(width, height, true);
    }
    @Override
    public void dispose(){
        pixmap.dispose();
        texture.dispose();
        stage.dispose();
        super.dispose();
    }
}