Depth output values

Software ROS2
1

Hi,
I am trying to work to get the depth of the center pixel using the sample tutorial that’s provided by the ZED. I have a doubt about the depth values I am getting as output

protected:
    void depthCallback(const sensor_msgs::msg::Image::SharedPtr msg) {
        // Get a pointer to the depth values casting the data
        // pointer to floating point
        float* depths = (float*)(&msg->data[0]);

        // Image coordinates of the center pixel
        int u = msg->width / 2;
        int v = msg->height / 2;

        // Linear index of the center pixel
        int centerIdx = u + msg->width * v;

        // Output the measure
        RCLCPP_INFO(get_logger(), "Center distance : %g m", depths[centerIdx]);
    }

Above is the code that takes the center pixel and outputs as depth but when I try to echo the depth topic I only see the values from 0-255 which are grayscale values.

image
image432×542 13.5 KB

I am not understanding how did they convert those values to depth or distance in m.
Thanks in advance for your help.

2

The rostopic echo command displays the “data” value of the depth map topic as 8bit unsigned char, not as floating-point values.

Indeed the depthCallback in the example performs a cast conversion:
float* depths = (float*)(&msg->data[0]);