Why the focal_lengths is negative number in intrinsic matrix?

[wenyuqing]

As calculated in this function vision_sensor.py,
The intrinsic is for example,
np.array([[-175.8385604, 0, 64],
[0, -175.8385604, 64],
[0, 0, 1]])
I use the intrinsic and extrinsic to convert the gripper pose 3D positions to pixel location, but cannot get the correct results.
I'm confused about that, could you give me some help?