The Brick display

The Brick display

The Brick’s screen is a small black-and-white whiteboard. You can wipe it, draw one of the built-in pictures on it, or write your own words at a spot you choose. It is how the robot tells a person what it is doing.

Blocks reference

clear display :: display

display [Eyes / Neutral v] :: display

display [Eyes / Neutral v] for (2) seconds :: display

write [Hello] at line (1) :: display

write [Hello] at x: (10) y: (40) with font [normal black v] :: display

Clear it, or write on top of what is there

The screen has no memory of whose writing is whose. It keeps every mark until something wipes it — including the marks left by the last run. Watch the same program with and without its clear display block.

Both Bricks are told exactly the same thing. The left one wipes the screen first, so line 7 holds one message and reads correctly. The right one never wipes, so each new message is drawn over the last and the words turn into a smudge. This is why a program that seems to display nonsense is usually displaying the truth — several times over.

Where the writing goes

There are two blocks for writing text, and they describe where in two completely different ways. Watch one message move around the screen under both of them.

Writing on a line

write [EV3] at line (1) is the simple one. You give it a line number and it puts the text there, starting hard against the left edge — you do not choose how far across, only how far down.

• Line 1 is the top. The numbers count downwards, so line 2 is below line 1, not above it.
• There are eight of them, each about ten pixels lower than the last. That means all eight sit in the upper part of the screen and the line block cannot reach the bottom at all.
• One message per line. Two messages sent to the same line land on top of each other and turn into a smudge — put them on different lines, or clear the screen in between.

For most robots this is all you need: a status word on line 1, a reading on line 3, a warning on line 5. Pick your lines at the start and keep each one for one job, the way you would keep one colour of status light for one meaning.

Writing at x and y

write [EV3] at x: () y: () with font [] treats the screen as a grid of pixels — 178 across and 128 down — and lets you put the text anywhere on it.

• x is how far across, from the left edge. x: 0 is the far left; x: 177 is the far right. Bigger x moves right, which is what everybody expects.
• y is how far down, from the top edge. y: 0 is the very top; y: 127 is the bottom. Bigger y means lower — this is the one that catches people out, because on a graph at school a bigger y means higher.
• The corner is 0, 0. x: 0 y: 0 is the top-left, and it puts the text in exactly the same place as at line 1 — the two blocks are describing the same screen.
• The pair of numbers is the top-left corner of the text, not its middle. The letters go down and to the right from the point you name.

The block ends with a font dropdown, and normal black is the usual choice. A larger font makes the writing easier to read from across the room but takes more room across the screen, so fewer characters fit before the text runs off the edge.

Which one to use

• Use lines for ordinary status messages — quick to write, impossible to get wrong, and eight of them is plenty for one robot.
• Use x and y when the position itself matters: putting a label and its number side by side, lining a heading up over a reading, filling the lower part of the screen, or moving something across the screen as a value changes.

Putting a number on the screen

The text slot of either block will take a reporter, so a sensor value can be shown directly: drop (4 distance in cm) into the slot and the screen shows the reading. That is the EV3’s version of a print statement, and it is the fastest way to find out what a robot actually thinks it is seeing.

A bare number on its own is hard to read, though. To show DIST: 23 rather than 23, use the operator block join [DIST: ] () to glue the label to the value, and put the join into the write block’s text slot.

when program starts :: events hat
clear display :: display
forever
  write (join [DIST: ] ([4 v] distance in [cm v] :: sensors)) at line (1) :: display
end

Note the forever: a value written once is a value from the start of the program. To watch a reading change, the write block has to be inside a loop.

Advanced — a dashboard, not a number

Once one value is on the screen the rest follows, and the useful trick throughout is join: it glues two pieces of text together and hands the result to the write block. Anything that reports a value can go in either slot — a variable, a sensor, or another join.

A variable with a label on it

Counting is the classic case. A variable counts how many times the Touch Sensor has been pressed, and a bare 5 on the screen tells nobody anything — PRESSED: 5 tells them everything:

when program starts :: events hat
clear display :: display
set [count v] to (0) :: variables
forever
  wait until <[1 v] is pressed? :: sensors>
  change [count v] by (1) :: variables
  write (join [PRESSED: ] (count)) at line (1) :: display
  wait until <not <[1 v] is pressed? :: sensors>>
end

The two wait until blocks are what make it count presses rather than counting as fast as the loop runs while your finger is down. That is the Touch Sensor’s own lesson, but it shows up here because a counter on the screen is where you first notice it going wrong.

Several sensors, one line each

Give every reading its own line and keep it. A line that changes meaning halfway through a program is unreadable at a glance, which is the only speed a screen on a moving robot gets read at.

when program starts :: events hat
clear display :: display
forever
  write (join [DIST: ] ([4 v] distance in [cm v] :: sensors)) at line (2) :: display
  write (join [DEG: ] ([A v] degrees counted :: sensors)) at line (5) :: display
end

degrees counted is worth putting on the screen the first time you use it — it is the fastest way to find out whether a motor is turning as far as you think it is, and it is the block behind every “why did it stop early” question.

Turning a number into a word

Sensors report numbers, and people read words. The Colour Sensor reports 5; the person watching wants RED. A chain of if … then … else blocks does the translation, and it is worth doing once into a variable rather than in every write block:

if <([3 v] color :: sensors) = (5)> then
  set [name v] to [RED] :: variables
else
  if <([3 v] color :: sensors) = (4)> then
    set [name v] to [YELLOW] :: variables
  else
    set [name v] to [OTHER] :: variables
  end
end
write (join [COLOR: ] (name)) at line (4) :: display

The same shape turns a distance into a warning. Here the screen carries the number and what the number means, which is what lets somebody across the room tell whether the robot is about to hit something:

if <([4 v] distance in [cm v] :: sensors) < (10)> then
  set [status v] to [TOO CLOSE!] :: variables
else
  if <([4 v] distance in [cm v] :: sensors) < (30)> then
    set [status v] to [OBSTACLE FOUND] :: variables
  else
    set [status v] to [NOTHING FOUND] :: variables
  end
end
write (status) at line (3) :: display

Drive all four of those at once below. The screen is the real thing — five write blocks, five lines, and every one of them a join.

Advanced — stopping the screen from flickering

The moment a reading goes inside a loop, one of two things goes wrong, and which one depends on where the clear display block went.

Almost everybody tries clear inside the loop first. It gives the right text — and a screen that blinks several times a second, because between the clear and the write there is genuinely nothing on the screen and the loop goes round faster than your eye.

So the clear gets moved to the start of the program, the blinking stops, and the words go wrong. Writing text paints only the characters it has and leaves everything past them exactly where it was. Write RED over YELLOW and the LOW is still there: the screen says REDLOW. Go the other way — yellow first, then red — and it looks fine, which is why this bug takes so long to pin down.

when program starts :: events hat
clear display :: display
forever
  write (name) at line (3) :: display
end

The fix is to make the new text at least as long as the old one, and join already does that: pad it with spaces. RED plus three spaces is six characters — exactly enough to paint over YELLOW. Nothing is left behind, and nothing is ever blank.

write (join (name) [   ]) at line (3) :: display

Pad to the length of the longest thing that line can ever show. Six characters covers RED, BLUE and GREEN but not NOTHING FOUND — count the longest message, and add spaces to match. The same applies to numbers: DIST: 5 after DIST: 40 leaves a stray 0 on the end, so a line that shows a two-digit reading needs a trailing space.

Use clear-inside-the-loop when the whole screen changes at once and a flicker does not matter; use the padding trick whenever a value is being watched.

Advanced — a whole list on the screen

Once several values need showing, the line number becomes a counter and the values come out of a list: write item 1 at line 1, item 2 at line 2, and so on inside a repeat. That needs the list blocks rather than the display blocks, so it lives with them — Lists covers building one and reading it back item by item, and the loop that walks it is the same loop that fills the screen.

Why it matters

Machines in the real world tell you what they are doing: a microwave counts down, a lift shows its floor, a car dashboard warns you. A robot that shows WAITING and then OPEN is far easier to understand — and far easier to debug — than one that moves silently.