3. Image strings

3.1. Show image

The basic syntax for showing an image is:

display.show(image): Display an image.

3.2. Image strings

The basic syntax for creating an image from a string of 25 brightness values is:

Image(string): Create an image.

string is a string of 25 brightness values with every 5 separated by a colon.

e.g. ‘01234:56789:09090:98765:43210’

The image can be a string made up of a 25 integers where each integer is the brightness from 0 to 9, where 0 if off and 9 is full brightness.
The 25 values are broken up into groups of 5 with a colon between them.
Each group of 5 represents the brightness for a row on the microbit.
e.g. Image('01234:56789:09090:98765:43210')

The code below shows a large bright square.

from microbit import *

display.show(Image('99999:90009:90009:90009:99999'))

The code below shows a vertical brightness gradient of dull to bright from the top to the bottom.

from microbit import *

display.show(Image('11111:33333:55555:77777:99999'))

The code below shows a diagonal brightness gradient (dull to bright) from the top left to the bottom right.

from microbit import *

display.show(Image('12345:23456:34567:45678:56789'))

Tasks

Write code for a large square of brightness 2.
Write code for a vertical brightness gradient of bright to dull from the top down to the bottom.
Write code for a horizontal brightness gradient of dull to bright from the left to right.
Write code for a diagonal brightness gradient of dull to bright from the bottom left to the top right.

3.3. Image strings: Multiplication of a line

Strings can be replicated using multiplication.

string * integer: Returns string + string + string …. integer times.

e.g “Ha” * 3 returns “HaHaHa”

The syntax for creating an Image by replicating a string representing one row is:

Image(line_string * 5): line_string is the first 5 pixel brightness values. e.g. “00000:”

e.g. Image(“00000:” * 5) create an image with all pixels off.

e.g. Image(“99999:” * 5) create an image with all pixels at full brightness.

The code below creates a horizontal gradient by repeating a string of 5 brightness values, 5 times.

from microbit import *

img = Image("13579:" * 5)
display.show(img)

Tasks

Modify the code above to go from dim on the left edge to bright in the middle to dim on the right edge.
Modify the code above to go from bright on the left edge to dim on the right edge.
Write code to cycle between bright on the left edge to bight on the right edge.

3.4. Image strings: line by line

The large square Image(‘99999:90009:90009:90009:99999’) can be rewritten so that the 5 rows are lined up under each other like a 5 by 5 grid. Extra spaces can by used to line up each line.

from microbit import *

large_square = Image('99999:'
                     '90009:'
                     '90009:'
                     '90009:'
                     '99999')
display.show(large_square)

Tasks

Write code for a large square of brightness 3 by lining up the 5 rows of the image under each other.
Write code for a small central square of brightness 9 by lining up the 5 rows of the image under each other.
Write code for 2 symmetrically spaced central horizontal lines of brightness 5 by lining up the 5 rows of the image under each other.
Write code for 2 symmetrically spaced central vertical lines of brightness 5 by lining up the 5 rows of the image under each other.

3.5. Boat sinking animation

Several custom images can be stored in variables. e.g. boat1, boat2, boat3, boat4, boat5, boat6.
Those variables can be put in a list. e.g. sinking_boats
Since display.show can use a list of images, the list of custom images can be shown in sequence, making an animation.

from microbit import *

boat1 = Image('05050:'
              '05050:'
              '05050:'
              '99999:'
              '09990')

boat2 = Image('00000:'
              '05050:'
              '05050:'
              '05050:'
              '99999')

boat3 = Image('00000:'
              '00000:'
              '05050:'
              '05050:'
              '05050')

boat4 = Image('00000:'
              '00000:'
              '00000:'
              '05050:'
              '05050')

boat5 = Image('00000:'
              '00000:'
              '00000:'
              '00000:'
              '05050')

boat6 = Image('00000:'
              '00000:'
              '00000:'
              '00000:'
              '00000')

sinking_boats = [boat1, boat2, boat3, boat4, boat5, boat6]
while True:
    display.show(sinking_boats, delay=500)

Tasks

Write a list variable, rising_boats, that lists the boats in reverse order and animates a rising boat. Rather than manually listing the order, use list(reversed(sinking_boats)).
Combine the 2 animations to show a boat sinking and rising over and over again.

3.6. Rotating line animation

Create 4 custom images. line1, line2, line3, line4.
line1: a horizontal line with y = 2.
line2: a diagonal line from the top left to the bottom right.
line3: a vertical line with x = 2.
line4: a diagonal line from the top right to the bottom left.
Create an animation in which the lines appear to rotate clockwise.
Create an animation in which the lines appear to rotate anticlockwise.

3.7. Image gradients using string definitions

The definition below, horz_grad, takes  2 arguments, width and height.
The for-loop creates a string starting with a brightness of 1 and increasing by one to the width given, which should be 5 for the full width of the 5x5 grid.
The string is then replicated for each line given by height, which is 5 for the full grid.
Image(horz_grad(5, 5)) results in Image(‘12345:12345:12345:12345:12345:’).

from microbit import *


def horz_grad(w, h):
    string = ""
    for i in range(1, w + 1):
        string += str(i)
    return (string + ":") * h


img_horz_grad = Image(horz_grad(5, 5))
display.show(img_horz_grad)

Tasks

Write a vert_grad definition to produce: Image(‘11111:22222:33333:44444:55555:’).
Write a diag_grad definition to produce: Image(‘12345:23456:34567:45678:56789:’).