3d - libgdx draw text via project draws twice?

Question

I try to draw 2D texts for my 3D world objects with libgdx's camera.project function but have a weird problem.

See the pics below:

As you can see in the pictures, all is well in picture 1 but when I turn around 180 degrees the ball's name (codename 1) is be drawing the blank space also (picture 2). I couldn't get what the problem is and after hours of thinking decided to ask here. Please help me.

My code is:

public static void drawNames(){
    TheGame.spriteBatch.begin();
    for(int i = 0; i < TheGame.playerMap.size; i++){
        Player ply = TheGame.playerMap.getValueAt(i);
        if(!ply.isAlive)
            continue;
        TheGame.tmpVec.set(ply.getPos().x, ply.getPos().y, ply.getPos().z);
        TheGame.cam.project(TheGame.tmpVec);
        TheGame.fontArialM.draw(TheGame.spriteBatch, ply.name, TheGame.tmpVec.x, TheGame.tmpVec.y, 0, Align.center, false);
    }
    TheGame.spriteBatch.end();
}

Answer 1

This is because if you project something that is behind the camera it still gets valid screen coordinates from the project method. Consider the following that prints the screen coordinates of two world coordinates

        PerspectiveCamera camera = new PerspectiveCamera(60, 800, 600);
        camera.position.set(0, 0, -10);
        camera.lookAt(0, 0, 0);
        camera.update();

        Vector3 temp = new Vector3();
        Vector3 p1 = new Vector3(1, 0, 10); // This is in front of the camera, slightly to the right
        Vector3 p2 = new Vector3(0, 0, -100); // This is behind of the camera

        camera.project(temp.set(p1));
        System.out.println("p1 is at screen " + temp);
        if (camera.frustum.pointInFrustum(p1))
            System.out.println("p1 is visible to the camera");
        else
            System.out.println("p1 is not visible to the camera");

        camera.project(temp.set(p2));
        System.out.println("p2 is at screen " + temp);
        if (camera.frustum.pointInFrustum(p2))
            System.out.println("p2 is visible to the camera");
        else
            System.out.println("p2 is not visible to the camera");

In your code, before rendering the text you need to check if the ply.getPos() vector is visible to the camera, and only render the text if it is.

if (TheGame.cam.frustum.pointInFrustum(ply.getPos()) {
    TheGame.tmpVec.set(ply.getPos().x, ply.getPos().y, ply.getPos().z);
    TheGame.cam.project(TheGame.tmpVec);
    TheGame.fontArialM.draw(TheGame.spriteBatch, ply.name, TheGame.tmpVec.x, TheGame.tmpVec.y, 0, Align.center, false);
}

Note that there are other ways to cull things behind the camera that might be more efficient for you.