q194046 points out one source of confusion, another is the terms parent/child vs. source/target, and the page above uses 'in'/'relative' ... my approach is to reverse the arguments if I don't get it right on the first try and move on, though that is slow for complex multiple transform situations.

Question

I have a drone where the drone_base is represented in NWU coordinates. Then, I have a sensor on the right side of it, pointing to the right, represented by frame range1. In writing my static_transform_publisher, in order to transform from frame drone_base to child range1, I first translated to the point location of the sensor, then performed the rotation around the Z-axis by -pi/2 to point North to the right. The range message with range1 frame being published to RViz displays properly. Does that sound correct so far? The transform looks like this:

<node pkg="tf2_ros" type="static_transform_publisher" name="drone_base_range1_link_broadcaster" args="0.1 -0.15 0.041 -1.5708 0 0 drone_base range1" />

Here's where I get tripped up. I have four of these range measurements, two facing right and two facing left. I have a node that receives these range measurements as an array, and attempts to place the ranges in a PointCloud2 for re-publishing to RViz. I put the range into the X attribute of a PointStamped, with the same stamp and frame_id. Then I look up the transform and apply it:

auto transform = tfBuffer.lookupTransform("rangeX", "drone_base", ros::Time(0));
tf2::doTransform(inPt, outPt, transform);

(rangeX where X = [0,1,2,3])

The points are in the incorrect positions. If I reverse the frame_ids in the lookup to:

auto transform = tfBuffer.lookupTransform("drone_base", "rangeX", ros::Time(0));

everything works fine. Is this the proper way to accomplish a child -> parent transform lookup? Or, what am I doing wrong?

Asked by seanarm on 2018-05-09 14:02:34 UTC

How should I think about static sensor transforms?

Comments

q194046 points out one source of confusion, another is the terms parent/child vs. source/target, and the page above uses 'in'/'relative' ... my approach is to reverse the arguments if I don't get it right on the first try and move on, though that is slow for complex multiple transform situations.

Answers