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How to get the camera matrix of a camera sensor in Ignition Gazebo

asked 2021-03-17 10:45:17 -0500

FirefoxMetzger gravatar image

updated 2021-03-17 10:46:45 -0500

I have a world in Ignition Gazebo that contains a camera and a robot arm (panda). The camera has the robot in view. I would like to compute the pixel position of the robot's end-effector within the recorded camera image.

What I would like to know is how/where to find the camera matrix (and information about distortion if applicable) so that I can compute the projection.

I can find the projected position of the end-effector in the camera's frame of reference, but to convert the location into pixels I need the camera's intrinsic parameters. I thought they would be published on the /camera_info topic, but the simulator doesn't appear to publish any messages on this topic. My next thought was to check the camera sensor's source code, but I can't seem to figure out where it is located.

Any pointers where to find this information are highly appreciated :)

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answered 2021-03-19 09:59:55 -0500

FirefoxMetzger gravatar image

updated 2021-03-24 08:17:45 -0500

It is indeed possible to get the camera matrix from the /camera_info topic. It also directly provides a projection matrix to map between pixel coordinates and meters.

However, there is a gotcha that is not so obvious: Gazebo doesn't follow the typical convention of defining the camera's coordinate system with the z-axis pointing forward. Instead, it follows the default robotics joint convention and cameras have the x-axis pointing forward (z-axis pointing up). The projection matrix, however, is given in a format that assumes the standard camera coordinate frame (z-axis pointing forward, y-axis pointing down). As a result, an additional transformation (rotation) is needed to convert the position, before applying the projection.

Here is the critical piece of code in python to (hopefully) make things more clear:

def world2px_cam_info(pos_world):
    # camera info msg based projection

    ## pos in world coords -> pos in camera coords
    # gazebo doesn't use conventional camera coordinates
    # camera points in x-direction instead of z-direction
    tf = transform(np.array((0,0,0, np.pi/2, np.pi/2, 0)))
    pos_cam_gazebo = homogenize(world2cam(pos_world))
    pos_cam = np.matmul(tf, pos_cam_gazebo)

    projection = np.array(cam_info.projection.p).reshape((3, 4))
    pos_px = cartesianize(np.matmul(projection, pos_cam))

    #image indexing uses y-x order instead of x-y
    pos_px = pos_px[::1]

    return pos_px

I've added a full code example below (using gym-ignition and ropy). It produces the following image:


Edit: The .sdf was causing problems because it contained absolute paths for my local file system due to the traffic cone. I updated the .sdf, moved the code into a gist for syntax highlighting, and added ropy which takes care of the additional rotation and makes the code cleaner overall.

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Great that you made it work, good job! To have camera with "standard camera coordinate frame", you can also set the link or sensor pose (relative_to requires SDF 1.7).

<model name="camera_model">
  <pose>0 0 10 0 3.14 0</pose>
  <link name="sensor_link">
    <sensor name="camera_sensor" type="camera">
      <pose relative_to="camera_model::sensor_link">0 0 0 -1.57 -1.57 -1.57</pose>
      <!-- ... -->
    <!-- ... -->
AndrejOrsula gravatar imageAndrejOrsula ( 2021-03-19 14:42:08 -0500 )edit

@AndrejOrsula Thanks! I would upvote your comment because it is really useful; unfortunately, it requires 3 rep to do so, meaning I will have to wait with that until somebody upvotes my question or answer :)

FirefoxMetzger gravatar imageFirefoxMetzger ( 2021-03-20 01:21:10 -0500 )edit

answered 2021-03-17 15:49:11 -0500

AndrejOrsula gravatar image

Your intuition about /camera_info topic is correct. Can you try the following example to see if that works for you?

ign gazebo camera_sensor.sdf -r
ign topic -e -t /camera_info

You should see ign_msgs.CameraInfo messages printed to stdout. In order to see more information about these messages, you can use ign msg -i ign_msgs.CameraInfo.

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Thanks for the reply! Unfortunately, I have a blank output when I run the two lines you mentioned, i.e., echo is waiting for messages to arrive. However, after I click on the button "Refresh list of topics publishing images" in the simulator GUI I start to see messages printed by the echo subscriber. Is this intended? If so, is it possible to get a camera to publish /camera_info immediately after the simulation starts?

FirefoxMetzger gravatar imageFirefoxMetzger ( 2021-03-18 10:50:44 -0500 )edit

I do not believe this behaviour is intended, as sensors should publish data whenever simulation is running. Have you tried subscribing to the topic programatically?

What's interesting is that when I run the example locally, I get the messages immediately. However, I am able to reproduce your issue when I run the same commands using one of my Docker images. The main difference is that locally I build Ignition from source but install Debian package inside Docker.

AndrejOrsula gravatar imageAndrejOrsula ( 2021-03-18 13:12:04 -0500 )edit

As a workaround, you can add <topic>camera</topic> under the ImageDisplay GUI plugin inside SDF and that seems to make it publish immediately.

Don't ask my why, I have no logical explanation. :D Maybe someone who worked on it has an idea.

AndrejOrsula gravatar imageAndrejOrsula ( 2021-03-18 13:32:04 -0500 )edit

Sounds a bit like a bug that has been fixed on master, but didn't propagate to the package archive yet. I've noticed this same behavior when subscribing to topics from python [tracking issue]. There is something funny about tracking subscribers in ign-transport. Another workaround is to additional subscribe to /camera via ign -e -t /camera, which then triggers publication of /camera_info as well. . .

FirefoxMetzger gravatar imageFirefoxMetzger ( 2021-03-18 16:01:07 -0500 )edit

Thanks @AndrejOrdula, I was facing the same issue with my rover world. I was trying to retrieve the image taken by the sensor.

Adwait gravatar imageAdwait ( 2021-04-18 08:11:05 -0500 )edit
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Asked: 2021-03-17 10:45:17 -0500

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Last updated: Mar 24