Codes
=====

In this chapter we look at how to display different codes. We are looking at

- 16 segment displays
- Pigpen code
- Braille code
- Morse code

.. image:: codes.png

https://scratch.mit.edu/projects/404785655

16-segment code
---------------

A sixteen-segment dispaly is a type of diasplay based on 16 segments which can individually be turned on and off.
A 16-bit binary code encodes and represents all possbile characters.

For example the letter O could be represented with this 16-bit code::

    1110001001000111

The segments are in the order from left to right, and top to down.

.. image:: seg16.png

The following image shows how letters, numbers and special symbols can be represented.

.. image:: seg14_chars.jpg

Scratch cannot draw characters to the screen. There are two ways to print text in scratch:

- use a sprite for each letter
- draw the letter with the pen

Encoding the symbols
--------------------

When drawing each letter with the pen, we need to encode somehow the geometric shape of the letter.
We are going to use a system inspired by the 16 segment display. 
We use the 3x3 points of intersection and encode the number with a path going through these points.

The points are numbered like this::

    6 7 8
    3 4 5
    0 1 2

To store the information about how to draw the symbbols or caracters, we will use 2 lists:

- symbols
- patterns

The function **add** associations a symbol with a pattern and adds them to the two lists.

.. image:: code_add.png

The first letters from *a* to *h* are encoded this way.

.. image:: code_add2.png

This shows all the characters which have been encoded.
More characters could easily be added if needed.

.. image:: text0.png

Draw a character
----------------

When drawing a specific symbol, we must first retrieve its code. 
This is done with the **find in list** block, which returns the index **i** of the pattern.
In a second step we can look up the pattern associated with the symbol.

.. image:: text1.png

We iterate through the sequence of points. The point index **p** starts from zero at the bottom left and goes from left to right.

We can use the two formulas to get the x and y offsets from the point index **p** (0 to 8):

- x + mod(p, 3) * w
- y * floor(p/3) * h

The index 9 is used to lift up the pen, if we want to draw two disconnected lines.

.. image:: text2.png

Draw a text
-----------

We used the index **i** to iterate inside the character function.
To avoid interference we must use a different variable **i** in the **write text** function.

We write each new character next to the previous one. If the right margin (x2) is reached, 
the current character position is reset to the left margin (x1), and the y position is advancen to the next line.

.. image:: text3.png

Settings (local variables)
--------------------------

A number of settings are encoded as local variables.

- the left and right margin (x1, x2)
- the current character position (x, y)
- the half-width and half-height (w, h)
- the pen thickness
- the pen color

.. image:: text4.png

Parsing a command string
------------------------

In order to change these parameters from another sprite, we will use a command string.
The format of the command string is very simple: key=value.
For example to set the left and write margins we would write::

    x1=-100 x2=150

To set size::

    s=5


The **parse** function takes the *limit** character set as an argument.
The index **i** must point to the first character to analyze. 

The **parse** function extracts the next token, which is separated by **=** or space ( ).

- **c** returns the limit character
- **p** returns the found token

.. image:: text5.png


Pigpen cipher
-------------

The pigpen cipher or masonic cipher is a geometric, simple substitution code.
We can easily program it using the 3x3 matrix and a dot.

.. image:: pigpen_cipher.png

The encoding is very similar to the 16 segment encoding. We use the 9 to lift the pen and draw the center dot.

.. image:: pigpen1.png

We set the color to green.

.. image:: pigpen2.png

This is the alphabet printed in pipen cipher.

.. image:: pigpen3.png


Braille code
------------

The Braille code is a code used by blind people by using the sense of touch.
It consist of a pattern of 6 raised dots. 
It is one of the first binary codes. Braille code pattern can be represented with a 6-bit binary code.

.. image:: braille.jpg

Morse code
----------

.. image:: morse_code.png