Line Follower I

Review — Treasure Hunt, finished 5 min

• (distance to [mouse-pointer v]) is a number. Small means close.
• <<> and <>> is true only when both sides are true.
• <<> or <>> is true when either side is true.
• <not <>> flips true and false.

Quick-fire

When is this true?

<<(distance to [mouse-pointer v]) < (50)> and <mouse down?>>
            

Today’s Topic 3 min

• A robot you drive: turn, then move forward
• A sensor: <touching color [#23262D]?>
• A sensor is a question asked every frame, inside a forever loop
• Pads on the floor: green is faster, red is back to the start

Learning outcome

By the end of this lesson you will be able to:

1. Use touching colour to find out what is under a sprite.
2. Pick an exact colour with the eyedropper.
3. Make the floor change what the robot does.

Learnt — a floor that talks 28 min

1 · Meet the robot and the course

We look at this robot from above. Its nose points right.

• The floor is pale.
• A thick dark line runs from the start to the finish.
• Green pads sit on the line. Red holes sit beside the corners.
• The chequered square at the end is the finish.

Real robots follow lines like this. Robots in warehouses and factories do it all day.

2 · Driving

You built this in the Car Race. The robot turns, then moves the way its nose points.

when flag clicked
go to x: (-170) y: (-125)
point in direction (90)
forever
  if <key [left arrow v] pressed?> then
    turn ccw (6) degrees
  end
  if <key [right arrow v] pressed?> then
    turn cw (6) degrees
  end
  if <key [up arrow v] pressed?> then
    move (3) steps
  end
end
            

3 · A sensor is a question

A sensor tells a robot something about the world. Our first sensor is aSensing block:

<touching color [#23262D]?>
            

A question asked once is not much use. The robot moves, so we keep asking.

1. From Control, put an if / else block inside the forever loop.
2. Drop <touching color [#23262D]?> into its hexagon.
3. In the top half, put say [On the line!].
4. In the bottom half, put say [Off the line].
5. Drive. The bubble changes the moment you leave the line.

when flag clicked
go to x: (-170) y: (-125)
point in direction (90)
forever
  if <key [left arrow v] pressed?> then
    turn ccw (6) degrees
  end
  if <key [right arrow v] pressed?> then
    turn cw (6) degrees
  end
  if <key [up arrow v] pressed?> then
    move (3) steps
  end
  if <touching color [#23262D]?> then
    say [On the line!]
  else
    say [Off the line]
  end
end
            

4 · Pads that give orders

Now the floor gives orders. We add a second script to the same robot. It asks two more questions.

when flag clicked
go to x: (-170) y: (-125)
point in direction (90)
set [speed v] to (3)
forever
  if <key [left arrow v] pressed?> then
    turn ccw (6) degrees
  end
  if <key [right arrow v] pressed?> then
    turn cw (6) degrees
  end
  if <key [up arrow v] pressed?> then
    move (speed) steps
  end
end
              

when flag clicked
forever
  if <touching color [#22C55E]?> then
    set [speed v] to (6)
  end
  if <touching color [#EF4444]?> then
    go to x: (-170) y: (-125)
    point in direction (90)
    set [speed v] to (3)
  end
end
              

Two scripts run at the same time. One drives. One watches the floor. Neither waits for the other.

5 · The exact colour

Touching colour is fussy. Light green is not green. So we never type a colour. We pick it.

1. Click the colour swatch inside the block.
2. Click the eyedropper at the bottom of the colour menu.
3. Click the pad on the Stage. The swatch turns that exact colour.

Basic — paint the floor, build the pads 24 min

Step 1 — the floor

1. Click the Stage, then the Backdrops tab.
2. Fill the backdrop with a pale colour.
3. Pick the Line tool. Make the outline very thick, about 14.
4. Draw a dark line from the bottom left to the top right, with a few bends.
5. Paint a chequered square at the end of the line.

Step 2 — the pads

1. With the Rectangle tool, paint two green squares on the line.
2. Paint two red squares next to the line, on the inside of two bends.

Step 3 — the robot

1. Paint a new sprite: a rounded rectangle with two wheels.
2. Draw it with its nose pointing right.
3. Make it small enough to sit on the line.

Step 4 — drive it

Build the driving script. Move it to the start of your line. Try all three keys.

Step 5 — the pads script

1. Make a variable called speed, for all sprites.
2. Change the move block to move (speed) steps.
3. Build the pads script. Pick each colour with the eyedropper.
4. Play. Green should speed you up. Red should send you home.

when flag clicked
forever
  if <touching color [#22C55E]?> then
    set [speed v] to (6)
  end
  if <touching color [#EF4444]?> then
    go to x: (-170) y: (-125)
    point in direction (90)
    set [speed v] to (3)
  end
end
            

The finish

Make the chequered square end the game.

• When the robot reaches the finish, it says “Finished!”.
• After that, the arrow keys do nothing.
• The flag starts a new game.

if <touching color [#FFFFFF]?> then
  say [Finished!]
  stop [other scripts in sprite v]
  stop [this script v]
end
            

Count the slips

A good driver stays on the line. Count how often you leave it.

• A variable called slips starts at 0.
• Leaving the line adds exactly one.
• Staying off the line for a long time still counts as one slip.

when flag clicked
set [slips v] to (0)
forever
  wait until <not <touching color [#23262D]?>>
  change [slips v] by (1)
  wait until <touching color [#23262D]?>
end
            

Autopilot, with one sensor

Press space and the robot drives itself.

• It moves forward on its own while it is touching the line.
• It stops the moment no part of it touches the line.
• Then it says “Where did the line go?”.

when [space v] key pressed
repeat until <not <touching color [#23262D]?>>
  move (2) steps
end
say [Where did the line go?] for (2) seconds
            

A boost that wears off

Right now green makes you fast for the rest of the game. Make it last one second.

• A green pad sets speed to 6.
• About one second later, speed goes back to 3 by itself.
• Touching green again starts the second again.

when flag clicked
set [boost v] to (0)
forever
  if <touching color [#22C55E]?> then
    set [boost v] to (30)
  end
  if <(boost) > (0)> then
    set [speed v] to (6)
    change [boost v] by (-1)
  else
    set [speed v] to (3)
  end
end
            

Summary 5 min

• <touching color [#23262D]?> asks: is any part of me on this colour?
• A sensor inside a forever loop is asked every frame, about 30 times a second.
• Two scripts can run at once: one drives, one watches the floor.
• Always pick a colour with the eyedropper. Paint pads in one flat colour.
• One sensor knows on or off. It does not know left from right.

Sensor

Something that tells a robot about the world around it.

Frame

One pass through the forever loop. About 30 every second.

Eyedropper

The tool that copies an exact colour from the Stage.

Pad

A patch of colour on the floor that gives the robot an order.

Hand in

Save your course and your robot. Next lesson the robot gets two sensor dots, and it learns to steer itself.