Green markers: how a Rescue Line robot decides where to turn

On a RoboCup Junior Rescue Line course, intersections are marked with green squares. They are the robot's turn signals, and reading them correctly is one of the classic make-or-break skills.

The rules in short

At an intersection, the green markers tell the robot which way to follow the line:

• Green on the left only: turn left.
• Green on the right only: turn right.
• Green on both sides: the path is a dead end, turn around (180 degrees).
• No green: go straight through.

Simple to state, harder to do. The robot has a fraction of a second, while moving, to see the green, decide, and commit to the turn before it overshoots the intersection.

Why it is tricky

• Timing: by the time the marker is under the robot it is often too late. The green has to be detected early enough to leave room to react.
• Color vs shadow: dark green can read as black under bad lighting, and a reflection can wash it out. Reliable green detection needs good color discrimination, not just a brightness threshold.
• False positives: a stray green pixel should not trigger a turn. The marker has to be big enough and in the right place relative to the line.
• Double green: a dead-end turnaround has to be distinguished from a normal turn, and executed without losing the line on the way back.

Practice it before the field

You can drill green-marker logic with no hardware in our free Rescue Line Simulator: place T and cross junctions, cycle their green markers, and watch your code react. The example robot already handles single and double green, so you can read it, break it, and improve it.

More Rescue Line guides on our blog.