Get distance from PointCloud2

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hi,

I have used the following code in order to convert pointcloud into distance.

#!/usr/bin/env python
from roslib import message
import rospy
import sensor_msgs.point_cloud2 as pc2
from sensor_msgs.msg import PointCloud2, PointField

#listener
def listen():
    rospy.init_node('listen', anonymous=True)
    rospy.Subscriber("/velodyne_points", PointCloud2, callback_kinect)
    rospy.spin()

def callback_kinect(data) :
    # pick a height
    height =  int (data.height / 2)
    print("height", height)
    # pick x coords near front and center
    middle_x = int (data.width / 2)
    print("middle_x", middle_x)
    # examine point
    middle = read_depth (middle_x, height, data)
    # do stuff with middle


def read_depth(width, height, data) :
    # read function
    if (height >= data.height) or (width >= data.width) :
        return -1
    data_out = pc2.read_points(data, field_names= ('x','y','z'), skip_nans=True, uvs=[[width, height]])
    #print("data_out", data_out)
    int_data = next(data_out)
    rospy.loginfo("int_data " + str(int_data))
    return int_data


if __name__ == '__main__':
    try:
        listen()
    except rospy.ROSInterruptException:
        pass

Now the issue is that when we have uvs=[[width, height]], the output is as follows (x,y,z):

[INFO] [1659697854.053652]: int_data (-11.6488037109375, -3.0516631603240967, -1.9072492122650146)

and when i consider uvs=[ ], the output is as follows:

[INFO] [1659697107.795461]: int_data (5.123467445373535, 1.491416573524475, -1.0372377634048462)

why the values are different? what is the meaning of uvs? what is the difference between uvs=[[width, height]] and uvs=[ ] ???

thanks