I'm interested in creating a Gazebo simulation environment where the robot can detect metallic rails on the floor to navigate. There is a gradient (i.e. as I move the sensor away from the rails, the reading value reduces linearly) with the analog sensor readings.

I have searched for sensors that I may use with Gazebo but there is nothing available for this purpose.

One possible way is to use a camera pointing downward and use the intensity values of the color, after I replace the rails with a white line with fading intensity on the edges. That may not be the best approach for this. Is there a better way?