Hi @jvj00:
What you have here is a message of type" sensor_msgs/PointCloud2.msg:
Reference: http://docs.ros.org/en/noetic/api/sensor_msgs/html/msg/PointCloud2.html
# This message holds a collection of N-dimensional points, which may
# contain additional information such as normals, intensity, etc. The
# point data is stored as a binary blob, its layout described by the
# contents of the "fields" array.
# The point cloud data may be organized 2d (image-like) or 1d
# (unordered). Point clouds organized as 2d images may be produced by
# camera depth sensors such as stereo or time-of-flight.
# Time of sensor data acquisition, and the coordinate frame ID (for 3d
# points).
Header header
# 2D structure of the point cloud. If the cloud is unordered, height is
# 1 and width is the length of the point cloud.
uint32 height
uint32 width
# Describes the channels and their layout in the binary data blob.
PointField[] fields
bool is_bigendian # Is this data bigendian?
uint32 point_step # Length of a point in bytes
uint32 row_step # Length of a row in bytes
uint8[] data # Actual point data, size is (row_step*height)
bool is_dense # True if there are no invalid points
Let's break it a little further:
The header message is of type std_msgs/Header.msg
uint32 seq
time stamp
string frame_id
In your case: header: seq: 19 stamp: secs: 124 nsecs: 110000000
uint32 height
uint32 width
frame_id: "camera_depth_optical_frame" height: 480 width: 640
Then comes
PointField[] fields
PointField[] fields is of type sensor_msgs/PointField.msg
The sensor_msgs/PointField.msg is composed of:
# This message holds the description of one point entry in the
# PointCloud2 message format.
uint8 INT8 = 1
uint8 UINT8 = 2
uint8 INT16 = 3
uint8 UINT16 = 4
uint8 INT32 = 5
uint8 UINT32 = 6
uint8 FLOAT32 = 7
uint8 FLOAT64 = 8
string name # Name of field
uint32 offset # Offset from start of point struct
uint8 datatype # Datatype enumeration, see above
uint32 count # How many elements in the field
In your case: fields: - name: "x" offset: 0 datatype: 7 count: 1 - name: "y" offset: 4 datatype: 7 count: 1 - name: "z" offset: 8 datatype: 7 count: 1
is_bigendian, point_step and row_step and is_dense are straight forward to understand.
Finally,
uint8[] data
data contains the Actual point data, size is (row_step*height)
In your case: data: [0, 0, 192, 127, 0, 0, 192, 127, 0, 0, 192, 127, 0, 0, 0, 0, 178, 178, 178, 0, 0, 0, 0, 0, 0, 0, 0, ...]
If you like to learn how to read this point with code, refer to this tutorial: http://wiki.ros.org/pcl/Tutorials
Originally posted by osilva with karma: 1650 on 2022-01-20
This answer was ACCEPTED on the original site
Post score: 0
Original comments
Comment by jvj00 on 2022-01-21:
Thanks a lot! I'll try to write a program with this information that you give to me
Comment by osilva on 2022-01-21:
Glad it helped. Kindly accept the answer by clicking on the check mark. Thank you.
Comment by Mike Scheutzow on 2022-01-22:
@jvj00 You do not need to convert this PointCloud2 msg to "useful" form yourself, there is already standard code in pcl_ros to do it (although I feel it is documented poorly.) See Section 3.1 on this page: http://wiki.ros.org/pcl_ros?distro=noetic