Lines

The line detector: cx and angle, and the empty list.

6.1Seeing moreRobot club15 min

Do this lesson in the simulator

A dark line taped to the mat is the oldest guide a robot has. Factories still use them. The camera looks down at the mat just in front of the robot, and the line detector on the vision processor tells you where the line crosses the picture. This module starts there and ends with the robot reading tags on other robots and staying out of their way.

The line detector

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

# camera: the line detector
set_cv("line")
print(line())

Run this in the simulator

Two numbers: [cx, angle]. set_cv("line") picks the detector, the same way set_cv("apriltag") picked tags in Module 4.

  • cx is where the line crosses a scan row about 10 cm ahead of the robot, in pixels across the 320 wide picture. 160 is dead ahead. The line here is 4 cm to the right, so cx is about 270.
  • angle is which way the line is heading relative to you, in degrees, positive to the right. A line running straight ahead gives 0.

Move and look again

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

# camera: the line detector
set_cv("line")
print("start:", line())
# slide right at 50 for 4 cm, then stop
right(50, distance=4)
print("over the line:", line())
# turn 20 degrees clockwise, then stop
turn_right(30, angle=20)
print("turned right 20:", line())

Run this in the simulator

Sliding right by the 4 cm puts the line under the nose: cx near 160. Turning right makes the line appear to swing left, with a negative angle: it is you that turned.

When there is no line

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

# camera: the line detector
set_cv("line")
# turn 90 degrees clockwise, then stop
turn_right(30, angle=90)
print("looking away:", line())

Run this in the simulator

An empty list, like an empty tag list. The detector only sees about 12 cm across the strip 10 cm ahead, so a line more than 6 cm off to the side is out of view. Every line program has to cope with [].

Where the numbers come from

The detector looks for a dark band on the light mat along a row of the image and reports its centre. On the real robot that is a contour detector on the lower part of the frame; in the simulator it is the same geometry. Either way, what reaches your program is the two numbers, not the picture, so the code you write here runs unchanged on a BugBot.

Task: see the line

Print line at cx <cx>, angle <angle> from the detector. Do not drive.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

print(line())

Challenges

  1. Print cx as a distance in centimetres. Ten centimetres ahead, one pixel is about 0.036 cm.
  2. Turn left in steps of 5 degrees and print the angle each time. When does the line leave the view?
  3. Drive forward 30 cm and print the line reading every 5 cm.