Trails and tight spots

A mat that fills up: seeing trails, turning early, keeping to the outside, and turning without stopping.

10.4CompetingRobot club15 min

Do this lesson in the simulator

Print it: worksheet (PDF) · answers (PDF) · as a web page

Some games fill the mat as they go. In Light Cycles every robot leaves a trail, your own included, and touching any of them puts you out. The mat gets smaller every second.

Seeing a trail

A trail on the mat is a line, so the line detector from Module 6 sees it:

from bugbot import *
connect()

set_cv('line')
found = line()
print("line ahead?", bool(found))
if found:
    # line() gives [cx, angle]: where it crosses the picture, and which way it heads
    cx, angle = found
    print("it crosses at", cx, "of 320, heading", angle, "degrees")

Run this in the simulator

In the game itself you also get the whole map: info()['trails'] is every trail as a list of points, your own included. The camera tells you what is 10 to 25 cm ahead; the map tells you what the mat looks like everywhere.

Never drive into a dead end

Two rules will keep you alive longer than any clever plan:

  • Turn early. A robot that turns when a trail is 15 cm away needs space it may not have. Turning at 30 cm is cheap.
  • Keep to the outside. The middle of the mat fills up first. Robots that hug the edges have somewhere to go for longer.

And one that catches everybody: in Light Cycles you may not sit still. Stopping to think is the same as losing. Turn while moving, with drive():

from bugbot import *
connect()

# forward and turning at the same time: no stopping, no reversing
drive(35, 0, 80)
wait(0.6)
stop()
print("still moving while turning")

Run this in the simulator

drive(forward, sideways, turn) sets all three at once. Module 1 used it for sliding; here it is how you turn a corner without stopping.

Task: thread the gap

Two trails lie across the mat, each with a gap at one end, and the gaps are at opposite ends. Drive from the start to the green zone at the top without touching a trail. In this task the trails are low walls, 6 cm high, so the depth sensor sees them and the line detector does not.

from bugbot import *
connect()

while position()[1] < 85:
    forward(45)
    wait(0.1)

stop()

Challenges

  1. Use distance() or scan() to spot the trail ahead and steer for the gap rather than driving a fixed path.
  2. Do it again with drive() only, never stopping.
  3. Count how many times you changed direction. Fewer is usually faster.