micropython-font-to-py/writer/WRITER.md

Writer and Cwriter classes

These classes facilitate rendering Python font files to displays where the display driver is subclassed from the framebuf class. Examples are:

• The official SSD1306 driver.
• The PCD8544/Nokia 5110.
• The Adafruit 0.96 inch color OLED with this driver.

Basic support is for scrolling text display using multiple fonts. The nanogui module has optional extensions for user interface objects displayed at arbitrary locations on screen.

Example code and images are for 128*64 SSD1306 OLED displays.

Scrolling text, multiple fonts.

A field containing variable length text with a border.

Right justified text.

Mixed text and graphics.

Labels and Fields (from nanogui.py).

Contents

1. Introduction
   1.1 Hardware
   1.2 Files
   1.3 Fonts
2. Writer and CWriter classes
   2.1 The Writer class For monochrome displays.
   2.1.1 Static Method
   2.1.2.Constructor
   2.1.3 Methods
   2.2 The CWriter class For colour displays and for upside-down rendering.
   2.2.1 Static Method
   2.2.2 Constructor
   2.2.3 Methods
3. Notes

Main README

1. Introduction

The original Writer class was a proof of concept intended to demonstrate rendering, on an SSD1306 OLED display, fonts created byfont_to_py.py.

This update for practical applications has the following features:

• Genarality: capable of working with any framebuf derived driver.
• Multiple display operation.
• Text display of fixed and variable pitch fonts with wrapping and vertical scrolling.
• Wrap/clip options: clip, character wrap or word wrap.
• Tab support.
• String metrics to enable right or centre justification.
• Inverse (background color on foreground color) display.
• Inverted display option.

Note that these changes have significantly increased code size. On the ESP8266 it is likely that writer.py will need to be frozen as bytecode. The original very simple version still exists as writer_minimal.py.

1.1 Hardware

Tests and demos assume a 128*64 SSD1306 OLED display connected via I2C or SPI. Wiring is specified in ssd1306_setup.py. Edit this to use a different bus or for a non-Pyboard target. At the time of writing the default of software I2C should be used: the official SSD1306 driver is not compatible with hardware I2C (see Notes).

1.2 Files

1. writer.py Supports Writer and CWriter classes.
2. writer_gui.py Provides optional GUI objects.
3. ssd1306_setup.py Hardware initialisation for SSD1306. Requires the official SSD1306 driver.
4. writer_demo.py Demo using a 128*64 SSD1306 OLED display. Import to see usage information.
5. writer_tests.py Test/demo scripts. Import to see usage information.
6. writer_minimal.py A minimal version for highly resource constrained devices.

Sample fonts:

1. freesans20.py Variable pitch font file.
2. courier20.py Fixed pitch font file.
3. font10.py Smaller variable pitch fonts.
4. font6.py

1.3 Fonts

Python font files should be created using font-to-py.py using horizontal mapping (-x option). The -r option is not required. If RAM is critical fonts may be frozen as bytecode reducing the RAM impact of each font to about 340 bytes.

Contents

2. Writer and CWriter classes

The Writer class provides fast rendering to monochrome displays using bit blitting. Most applications will use this class.

The CWriter class is a subclass of Writer. It can optionally support color displays. It provides additional functionality in the form of an upside-down display option. Owing to limitations in the frmebuf.blit method the CWriter class renders glyphs one pixel at a time; rendering is therefore slower than the Writer class.

Multiple screens are supported. On any screen multiple Writer or CWriter instances may be used, each using a different font. A class variable holds the state of each screen to ensure that the insertion point is managed across multiple instances/fonts.

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2.1 The Writer class

This class facilitates rendering characters from Python font files to a device, assuming the device has a driver subclassed from framebuf. It supports three ways of handling text which would overflow the display: clipping, character wrapping and simple word wrapping.

It handles newline and tab characters, black-on-white inversion, and field blanking to enable variable length contents to be updated at a fixed location.

Typical use with an SSD1306 display and the official driver is as follows:

from ssd1306_setup import WIDTH, HEIGHT, setup
from writer import Writer
import freesans20  # Font to use

use_spi=False  # Tested with a 128*64 I2C connected SSD1306 display
ssd = setup(use_spi)  # Instantiate display: must inherit from framebuf
# Demo drawing geometric shpes
rhs = WIDTH -1
ssd.line(rhs - 20, 0, rhs, 20, 1)  # Demo underlying framebuf methods
square_side = 10
ssd.fill_rect(rhs - square_side, 0, square_side, square_side, 1)
# Instantiate a writer for a specific font
wri = Writer(ssd, freesans20)  # verbose = False to suppress console output
Writer.set_textpos(ssd, 0, 0)  # In case a previous test has altered this
wri.printstring('Sunday\n12 Aug 2018\n10.30am')
ssd.show()

The file writer_demo.py illustrates the use of font files with a 128*64 SSD1306 OLED display and the official SSD1306 driver.

2.1.1 Static Method

The Writer class exposes the following static method:

1. set_textpos Args: device,row=None, col=None. The device is the display instance. This method determines where on screen subsequent text is to be rendered. The initial value is (0, 0) - the top left corner. Arguments are in pixels with positive values representing down and right respectively. The insertion point defines the top left hand corner of the next character to be output.

Where None is passed, the setting is left unchanged.
Return: row, col current settings.

The insertion point applies to all Writer instances having the same device. The insertion point on a given screen is maintained regardless of the font in use.

2.1.2 Constructor

This takes the following args:

1. device The hardware device driver instance for the screen in use.
2. font A Python font instance.
3. verbose=True If True the constructor emits console printout.

2.1.3 Methods

1. printstring Args: string, invert=False. Outputs a text string at the current insertion point. Newline and Tab characters are honoured. If invert is True the text is output as black on white.
2. height No args. Returns the font height in pixels.
3. stringlen Arg: string. Returns the length of a string in pixels. Used for right or centre justification.
4. set_clip Args: row_clip=None, col_clip=None, wrap=None. If row_clip and/or col_clip are True, characters will be clipped if they extend beyond the boundaries of the physical display. If col_clip is False characters will wrap onto the next line. If row_clip is False the display will, where necessary, scroll up to ensure the line is rendered. If wrap is True word-wrapping will be performed, assuming words are separated by spaces.
   If any arg is None, that value will be left unchanged.
   Returns the current values of row_clip, col_clip and wrap.
5. tabsize Arg value=None. If value is an integer sets the tab size. Returns the current tab size (initial default is 4). Tabs only work properly with fixed pitch fonts.

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2.2 The CWriter class

This extends the Writer class by adding support for upside-down and/or color displays. A color value is an integer whose interpretation is dependent on the display hardware and device driver.

2.2.1 Static method

The following static method is added:

1. invert_display Args device, value=True. The device is the display instance. If value is set, causes text to be rendered upside down. The set_textpos method should be called to ensure that text is rendered from the bottom right hand corner (viewing the display in its normal orientation).

If a display is to be run inverted, this method must be called prior to instantiating a Writer for this display.

2.2.2 Constructor

This takes the following args:

1. device The hardware device driver instance for the screen in use.
2. font A Python font instance.
3. fgcolor=None Foreground color. If None a monochrome display is assumed.
4. bgcolor=None Background color. If None a monochrome display is assumed.
5. verbose=True If True the constructor emits console printout.

2.2.3 Methods

All methods of the base class are supported. Additional method:

1. setcolor Args: fgcolor=None, bgcolor=None. Sets the foreground and background colors. If one is None that value is left unchanged. If both are None the constructor defaults are restored. Constructor defaults are 1 and 0 for monochrome displays (Writer). Returns foreground and background color values.

The printstring method works as per the base class except that the string is rendered in foreground color on background color (or reversed if invert is True).

3. Notes

Possible future enhancements:

1. General rendering to a rectangular area. This may be problematic as the framebuf scroll method is only capable of scrolling the entire buffer.
2. Extend word wrapping to cases where words are separated by tabs or hyphens.
3. An asynchronous version.

As stated above the official SSD1306 drriver is incompatible with hardware I2C and this problem cannot efficiently be fixed. PR4020 proposes an enhncement which will facilitate an improved SSD1306 driver capable of using hard or soft I2C.

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