Find intrisic camera matrix from gazebo model

asked 2021-04-28 04:16:58 -0500

xavier12358
62 ●9 ●13 ●21

Hello,

I made a my camera model in gazebo and I want to extract the intrinsic matrix to use Opencv matrix. Here is my model:

<gazebo reference="camera">
    <material>Gazebo/Green</material>
    <sensor type="camera" name="camera">
      <update_rate>30.0</update_rate>
      <camera name="head">
        <horizontal_fov>1.3962634</horizontal_fov>
        <image>
          <width>1280</width>
          <height>720</height>
          <format>R8G8B8</format>
        </image>
        <clip>
          <near>0.02</near>
          <far>300</far>
        </clip>
      </camera>
      <plugin name="camera_controller" filename="libgazebo_ros_camera.so">
        <alwaysOn>true</alwaysOn>
        <updateRate>0.0</updateRate>
        <cameraName>mdt/camera1</cameraName>
        <imageTopicName>image_raw</imageTopicName>
        <cameraInfoTopicName>camera_info</cameraInfoTopicName>
        <frameName>camera</frameName>
        <hackBaseline>0.07</hackBaseline>
        <distortionK1>0.0</distortionK1>
        <distortionK2>0.0</distortionK2>
        <distortionK3>0.0</distortionK3>
        <distortionT1>0.0</distortionT1>
        <distortionT2>0.0</distortionT2>
      </plugin>
    </sensor>
  </gazebo>

My intrinsic matrix is define like that : M = [ ax 0 cx; 0 ay cy ; 0 0 1 ] I think that cx = 1280 /2 and cy = 720/2 but what about ax/ay ?

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answered 2021-04-28 23:49:14 -0500

tryan
1421 ●96 ●26

updated 2021-04-29 09:58:11 -0500

You can get the intrinsics by subscribing to the /camera_info topic defined in the plugin:

<cameraInfoTopicName>camera_info</cameraInfoTopicName>

The sensor_msgs/CameraInfo message will contain the camera matrix:

# Intrinsic camera matrix for the raw (distorted) images.
#     [fx  0 cx]
# K = [ 0 fy cy]
#     [ 0  0  1]
# Projects 3D points in the camera coordinate frame to 2D pixel
# coordinates using the focal lengths (fx, fy) and principal point
# (cx, cy).
float64[9]  K # 3x3 row-major matrix

Also, you can explicitly set the intrinsics in Gazebo as indicated in the camera SDF specification:

Update:

Note the ROS plugin and Gazebo sensor use two separate intrinsic parameter sets, so it's on the user to make sure they are correct. The ROS plugin source code shows how it uses the parameters only to fill the camera info. Here's an exampe:

<sensor type="camera" name="camera">
  <update_rate>30.0</update_rate>
  <camera name="head">
    <horizontal_fov>1.3962634</horizontal_fov>
    <image>
      <width>800</width>
      <height>800</height>
      <format>R8G8B8</format>
    </image>
    <clip>
      <near>0.02</near>
      <far>300</far>
    </clip>
    <noise>
      <type>gaussian</type>
      <!-- Noise is sampled independently per pixel on each frame.
           That pixel's noise value is added to each of its color
           channels, which at that point lie in the range [0,1]. -->
      <mean>0.0</mean>
      <stddev>0.007</stddev>
    </noise>
    <lens>
      <intrinsics>
        <fx>100</fx>
        <fy>100</fy>
        <cx>640</cx>
        <cy>360</cy>
        <s>0</s>
      </intrinsics>
    </lens>
  </camera>
  <plugin name="camera_controller" filename="libgazebo_ros_camera.so">
    <alwaysOn>true</alwaysOn>
    <updateRate>0.0</updateRate>
    <cameraName>rrbot/camera1</cameraName>
    <imageTopicName>image_raw</imageTopicName>
    <cameraInfoTopicName>camera_info</cameraInfoTopicName>
    <frameName>camera_link</frameName>
    <Cx>640</Cx>
    <Cy>360</Cy>
    <focalLength>100</focalLength>
  </plugin>
</sensor>

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Indeed I can't get the intrinsic matrix like this. Thank you. About the SDF camera specifications. I tried this:

<?xml version="1.0"?>
<robot>
      <gazebo reference="camera">
        <material>Gazebo/Green</material>
        <sensor type="camera" name="camera">
          <update_rate>30.0</update_rate>
          <camera name="head">
         .....
              <near>0.2</near>
              <far>600</far>
            </clip>
            <lens>
                <intrinsics>
                        <fx>100</fx>
                        <fy>100</fy>
                        <cx>640</cx>
                        <cy>360</cy>
                        <s>0</s>s>
                </intrinsics>
            </lens>
          </camera>

I get this:

Warning [parser.cc:950] XML Element[intrinsics], child of element[lens] not defined in SDF. Ignoring[intrinsics]. You may have an incorrect SDF file, or an sdformat version that doesn't support this element.

xavier12358 ( 2021-04-29 02:22:54 -0500 )edit

Indeed I can't get the intrinsic matrix like this.

Why not?

As for the SDF, I don't get that error. Maybe you have a different Gazebo/SDF version. I updated my answer with an example.

tryan ( 2021-04-29 09:58:20 -0500 )edit

Sorry I made a error when I wrote that. I CAN get the intrinsic file . Thank you.

xavier12358 ( 2021-04-29 11:27:34 -0500 )edit

I try to add the intrinsics parameters like in your example. The problem is the horizontal FOV had to be defined also with the intrinsic parameters, which is not consistent. If I not specify horizontal FOV, it use the default HFOV and I get that warning (and the camera info is not published):

[ WARN] [1619767559.786164054]: The <focal_length>[1067.000000] you have provided for camera_ [camera] is inconsistent with specified image_width [1280] and HFOV [1.047000].   Please double check to see that focal_length = width_ / (2.0 * tan(HFOV/2.0)), the explected focal_lengtth value is [1108.765426], please update your camera_ model description accordingly.

So I just use that formula to set the HFOV and to fill the matrix and not the intrinsic section which is useless...

xavier12358 ( 2021-04-30 02:32:05 -0500 )edit

Thanks for the tip about FOV, but I still cannot solve the problem with the parser not parsing the intrinsics tag and ignoring it. Child of lens tag.

Anvesh Som ( 2022-01-24 02:32:44 -0500 )edit

It seems that your issue is that you have <s>0</s>s> rather than <s>0</s> as one of the children of <intrinsics>

noah ( 2022-06-27 14:05:29 -0500 )edit

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