How a depth camera is modelled?

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What do the values given by a depth camera represent? I am using a depth camera simulation in gazebo and I do not know which distance each pixel represents. Distance to the center of the image, distance along x axis...

  <gazebo reference="kinect_depth_frame">
    <sensor type="depth" name="fotonic">
      <update_rate>30.0</update_rate>
      <camera name="fotonic">
        <horizontal_fov>1.221730476</horizontal_fov>  <!-- 70 deg -->
        <image>
          <width>800</width>  <!-- 160 -->
          <height>600</height> <!-- 120 -->
          <format>R8G8B8</format>
        </image>
        <clip>
          <near>0.05</near>
          <far>7.0</far>         <!-- outdoor down to 3m depending on luminosity -->
        </clip>
        <noise>
          <type>gaussian</type>
          <mean>0.0</mean>
          <stddev>0.007</stddev>
        </noise>
      </camera>

      <plugin name="kinect_depth_optical_frame_controller" filename="libgazebo_ros_openni_kinect.so">
        <baseline>0.2</baseline>
        <alwaysOn>true</alwaysOn>
        <updateRate>30.0</updateRate>
        <cameraName>fotonic_3dcamera</cameraName>
        <imageTopicName>/fotonic_3dcamera/image_raw</imageTopicName>
        <cameraInfoTopicName>/fotonic/depth/camera_info</cameraInfoTopicName>
        <depthImageTopicName>/fotonic/depth/image_raw</depthImageTopicName>
        <depthImageInfoTopicName>/fotonic_3dcamera/depth/camera_info</depthImageInfoTopicName>
        <depthImageCameraInfoTopicName>/fotonic_3dcamera/camera_info</depthImageCameraInfoTopicName>
        <pointCloudTopicName>/fotonic_3dcamera/depth/points</pointCloudTopicName>
        <frameName>kinect_depth_optical_frame</frameName>
        <pointCloudCutoff>0.5</pointCloudCutoff>
        <distortionK1>0</distortionK1>
        <distortionK2>0</distortionK2>
        <distortionK3>0</distortionK3>
        <distortionT1>0</distortionT1>
        <distortionT2>0</distortionT2>
        <CxPrime>0</CxPrime>
        <Cx>0</Cx>
        <Cy>0</Cy>
        <focalLength>0</focalLength>
        <hackBaseline>0</hackBaseline>
      </plugin>      
    </sensor>
  </gazebo>

Do you know which is the implementation?

Thank you