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[pointcloud_to_laserscan] publish rate higher than scan_time

asked 2021-01-29 03:58:14 -0500

felixN gravatar image

Hi, I'm having a bit of trouble understanding the "scan_time" parameter from poincloud_to_laserscan package.

I was supposing that it was the publish rate of the output (laser_scan). However, I have it set at 0.333 seconds (so 3Hz), but the publishing frequency of the scan is nearly 10Hz (as the pointcloud topic).

Do I misunderstand the meanning of this parameter?

Thanks a lot in advance

Felix

The launchfile for the laser scan :

<?xml version="1.0"?>
<launch>

    <arg name="camera_name"          default="zed2" />

    <node pkg="pointcloud_to_laserscan" type="pointcloud_to_laserscan_node" name="pointcloud_to_laserscan" >
        <remap from="cloud_in" to="$(arg camera_name)/zed_node/point_cloud/cloud_registered" />
        <remap from="scan" to="$(arg camera_name)/zed_node/scan" />
        <rosparam>
target_frame: base_link
transform_tolerance: 0.01
min_height: -0.7
max_height: 0.3
angle_min: -M_PI/2
angle_max: M_PI/2
angle_increment: M_PI/360/2
scan_time: 0.333
range_min: 0.2
range_max: 40.0
use_inf: true
inf_epsilon: 1.0
concurrency_level: 1
</rosparam>
        </node>
</launch>

Sample output of rostopic hz for the scan:

$ rostopic hz /zed2/zed_node/scan 
subscribed to [/zed2/zed_node/scan]
average rate: 9.285
    min: 0.097s max: 0.121s std dev: 0.00687s window: 9
average rate: 9.358
    min: 0.097s max: 0.121s std dev: 0.00480s window: 18
average rate: 9.354
    min: 0.097s max: 0.121s std dev: 0.00470s window: 27
average rate: 9.360
    min: 0.097s max: 0.121s std dev: 0.00401s window: 37
average rate: 9.338
    min: 0.096s max: 0.121s std dev: 0.00448s window: 46
average rate: 9.330
    min: 0.096s max: 0.121s std dev: 0.00413s window: 55

and for the pointcloud

$ rostopic hz /zed2/zed_node/point_cloud/cloud_registered 
    subscribed to [/zed2/zed_node/point_cloud/cloud_registered]
    average rate: 9.806
        min: 0.065s max: 0.133s std dev: 0.01900s window: 8
    average rate: 9.898
        min: 0.065s max: 0.133s std dev: 0.01296s window: 18
    average rate: 9.891
        min: 0.065s max: 0.133s std dev: 0.01087s window: 28
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answered 2021-01-29 08:18:53 -0500

tryan gravatar image

updated 2021-01-29 08:19:24 -0500

According to the pointcloud_to_laserscan README on GitHub, the scan_time parameter in your launch file has no functional effect; it just adds that value to the outgoing scan message:

scan_time (double, default: 1.0/30.0) - The scan rate in seconds. Used to populate the output laser scan.

Indeed, the source code only uses the parameter once after declaring it--to populate the message field:

scan_msg->scan_time = scan_time_;
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Comments

1

Thanks a lot.

I was unederstanding "used to populate" in the sens that the message was populated (in the sens created and sent) at this frequency (and that this parameter could therefore be used for downsampling).

But the source code seems to confirm your version.

Did it seem clear to you at first reading that it was only used to populate that field? If not, it might be interresting to change the documentation, for exemple replacing it by :

scan_time (double, default: 1.0/30.0) - The scan rate in seconds. Only used to populate the scan_time field of the output laser scan message.
felixN gravatar image felixN  ( 2021-01-29 08:51:40 -0500 )edit

I can definitely see both ways of reading it. I'd say more clarity is better, and it wouldn't take much effort to change the wording. It's even less clear on the wiki:

~scan_time (double, default: 1.0/30.0) The scan rate in seconds.

Can't hurt to express your thoughts to the maintainer(s).

tryan gravatar image tryan  ( 2021-01-29 09:13:39 -0500 )edit

Thanks, I just created an issue on git to propose the change

felixN gravatar image felixN  ( 2021-01-29 10:16:15 -0500 )edit

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Asked: 2021-01-29 03:58:14 -0500

Seen: 106 times

Last updated: Jan 29