Change rand_image generation algorithm

[woensug-choi]

@crvogt Can you try this out for me?

It's simple change and it builds fine. But I get runtime error saying gzserver: symbol lookup error: /home/woensug/uuv_ws/devel/lib/libnps_multibeam_sonar_ros_plugin.so: undefined symbol: _ZN2cv3Mat6createEiPKii

It crashes at a line which isn't new at all. https://github.com/Field-Robotics-Lab/nps_uw_multibeam_sonar/blob/a9eff3c81df070e14812f202e74d408c63b671a0/src/gazebo_ros_multibeam_sonar.cpp#L382

Maybe my computer problem (running on WSL, or gazebo version...)

Background: The rand_image which was randomized at every sonar calculation should only be calculated whenever the vehicle position is changed. @kerednoslo 's thankful comments

if the medium is static then the scatterers should have the same amplitudes across all frames. If the robot platform moves, then the scatterers will add up in different ways and the speckle pattern will change. But if the geometry is held fixed, the speckle should be the same. This is a real feature of acoustic measurements, and is exploited in a variety of navigation methods (called the redundant phase center, or displaced phase center technique), and even in medical ultrasound (called speckle tracking). ML methods should (eventually) be able to use this property of the acoustic data as well.

[crvogt]

@woensug-choi will do!

[crvogt]

@woensug-choi I tried this gazebo version 9.16.0 and nothing crashed. I could move the sensor around and the sonar image would change, but there was no noise "flickering" (not sure if intentional at the moment)(maybe better described as 'a single random sample of the scene')

[woensug-choi]

@crvogt Yes, they are meant to stop flickering after this update. Thank you for the review!