Hwo can I transfrom a specific coordinates from frame_A into frame_B?

asked 2020-04-17 09:16:03 -0600

kane_choigo gravatar image

updated 2020-04-17 09:20:13 -0600

Hello, I'm using ROS melodic with Ubuntu 18.04.

I need to do a frame-transformation of some coordinates, and I also can make the likely function by myself, but I have a feeling that I should utilize tf2_ros module.

As far as I know, tf2_ros has the corresponding method named lookup_transform according to here.

However, the coordinates I'd like to transform do NOT have a frame_id, and because I'm not familiar with checking whether the specific frame is practically being broadcast, I'm not sure those poses are existing with its own frame_id. (Those poses are /gazebo/model_states of gazebo_msgs/ModelStates, actually.)

I also have investigated how to do that, and I think I should broadcast their frame_id before executing transformation.(And I'm still not sure how to do it.)

What should I do to complete my work?

If anyone who's familiar with this kind of works, would you please teach or give me helpful advice?

Thanks in advance. :)

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Comments

Hi @kane_choigo,

The gazebo_msgs/ModelStates mgs are not stamped msgs thus they do not contain any header/frame_id field. What are you trying to achieve? Specifically what information are you trying to extract from that particular topic?. If you want to know the "real" pose of each model in the Gazebo world you can use a Odometry model plugin (or implement one yourself) that publish odometry and tf inforamtion over ROS.

Besides, that partiacular poses are extracted direcly from Gazebo and are assumed to be with respect the internal "world" frame Gazebo has.

Weasfas gravatar image Weasfas  ( 2020-04-17 10:59:24 -0600 )edit

@Weasfas Thanks for your quick reply. Of course, I tried adopting /odom plugin which is a kind of /odom topic. But what I want to know is literally the position of spawned model, and /odom would not be pose information, I think. When a mobile robot is bumped against a wall and the engineer gives /vel_cmd command to the robot, the odometry sensor recognizes that the robot is still moving although it practically is stopped, because the inner odometry sensor counts a motor's resolution resulted from /vel_cmd command. This is the reason that I gave up to use odom topic. What I want to do is to subscribe to the correct position of each model's pose with the frame_id. And as you said, /gazebo/model_states, which includes the correct pose information, doesn't have frame_id, so I thought of that it might be possible to broadcast the new ...(more)

kane_choigo gravatar image kane_choigo  ( 2020-04-18 00:00:16 -0600 )edit

@kane_choigo, well to do what you want you can implement a Gazebo world plugin, query gazebo for all models in the world and convert the multiple model world poses into indivisual odometry msgs with the frame_if of each model. However ,take in mind plugins like the P3D plugin, this is not a standard odometry plugin but a "Ground truth", thus no matter if the robot is colliding with a wall and the motor is sending new cmd_vel commands the pose will remain the same.

Weasfas gravatar image Weasfas  ( 2020-04-18 05:47:03 -0600 )edit
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