java LibGDX 3DSphere在使用鼠标在屏幕上移动时正在更改形状

Question

我使用libGDX在modelBuilder上调用createSphere（）方法，使用纹理创建了一个球体。因为它是3D的，所以我用了透视相机。我正试图用鼠标移动屏幕上的球体。它工作正常，但球体在屏幕上移动时一直在改变形状

我试着改变透视相机的位置和视角。尽管如此，形状正在改变。有人能提出解决办法吗

以下是我的代码部分：

    modelBatch = new ModelBatch();
    ModelBuilder modelBuilder = new ModelBuilder();
    Texture texture = new Texture(Gdx.files.internal("ball.png"));
    Material material = new 
    Material(TextureAttribute.createDiffuse(texture));
    final long attributes = VertexAttributes.Usage.Position | 
    VertexAttributes.Usage.TextureCoordinates;
    sphere = modelBuilder.createSphere(0.5f, 0.5f, 0.5f, 24, 24, material, attributes);
    sphereInstance = new ModelInstance(sphere);

    perCamera = new PerspectiveCamera(67, Gdx.graphics.getWidth(), 
    Gdx.graphics.getHeight());
    perCamera.position.set(0f, 0f, 3f);
    perCamera.lookAt(0f,0f,0f);
    perCamera.near = 1f;
    perCamera.far = 300f;
    perCamera.update();
    [Here I have attached screenshot for the reference, in which the regular sphere changes its shape to ellipse as it is moving away from the camera.][1]