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Writer and Cwriter classes

These classes facilitate rendering Python font files to displays where the display driver is subclassed from the framebuf class. Basic support is for scrolling text display using multiple fonts. There is a growing list of displays with compatible drivers, see

Supported displays document

Three cross-platform GUI libraries build on this to provide a variety of widgets. These are:

  • nano-gui An extremely lightweight display-only GUI.
  • micro-gui A GUI providing input via either pushbuttons or pushbuttons plus a rotary encoder.
  • micropython-touch Input is provided by touch.

For applications needing only to render text to a display, and optionally to draw graphics using FrameBuffer primitives, the writer module may be used alone.

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

Image
Scrolling text, multiple fonts.

Image
A field containing variable length text with a border.

Image
Right justified text.

Image
Mixed text and graphics.

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Labels and Fields (from nanogui.py).

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The CWriter class (from nanogui): Label objects in two fonts.

Contents

  1. Introduction
    1.1 Release notes
    1.2 Hardware
    1.3 Files
    1.4 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.
         2.2.1 Static Method
         2.2.2 Constructor
         2.2.3 Methods
    2.3 Example color code For most display drivers.
    2.4 Use with 4 bit drivers Color definition uses a different technique.
  3. Icons How to render simple icons.
Main README

1. Introduction

The module provides a Writer class for rendering bitmapped monochrome fonts created by font_to_py.py. The CWriter class extends this to support color rendering. Rendering is to a FrameBuffer instance, e.g. to a display whose driver is subclassed from a FrameBuffer.

The module has the following features:

  • Generality: Writer can work 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. This is rudimentary and "micro".
  • String metrics to enable right or centre justification.
  • Inverse (background color on foreground color) display.

The CWriter class requires a compatible display driver. These are listed in this document. There is no support for RTL languages but a workround is discussed here.

1.1 Release Notes

V0.5.1 Dec 2022__ Add support for 4 bit color display drivers. V0.5.0 Sep 2021
Requires firmware V1.17 or later.

V0.4.3 Aug 2021
Supports fast rendering of glyphs to color displays (PR7682).

V0.4.0 Jan 2021
Improved handling of the col_clip and wrap options. Improved accuracy avoids needless word wrapping. The clip option now displays as much of the last visible glyph as possible: formerly a glyph which would not fit in its entirety was discarded.

1.2 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. Section 2.3 shows how to drive color displays using the CWriter class.

1.3 Files

  1. writer.py Supports Writer and CWriter classes.
  2. ssd1306_setup.py Hardware initialisation for SSD1306. Requires the official SSD1306 driver.
  3. writer_demo.py Demo using a 128*64 SSD1306 OLED display. Import to see usage information.
  4. writer_tests.py Test/demo scripts. Import to see usage information.

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

1.4 Fonts

Python font files should be created using font-to-py.py using horizontal mapping; this is the default. 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. This is highly recommended.

Contents

2. Writer and CWriter classes

The Writer class provides fast rendering to monochrome displays using bit blitting. The CWriter class is a subclass of Writer to support color displays which offers comparable performance.

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.

Contents

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 shapes
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(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(string, invert=False). Renders the string at the current insertion point. Newline and Tab characters are honoured. If invert is True the text is output with foreground and background colors transposed.
  2. height() Returns the font height in pixels.
  3. stringlen(string, oh=False) Returns the length of a string in pixels. Appications can use this for right or centre justification.
    The oh arg is for internal use. If set, the method returns a bool, True if the string would overhang the display edge if rendered at the current insertion point.
  4. set_clip(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(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.
Contents

2.2 The CWriter class

This extends the Writer class by adding support for color displays. A color value is an integer whose interpretation is dependent on the display hardware and device driver. The Python font file uses single bit pixels. On a color screen these are rendered using foreground and background colors. Display drivers provide an rgb classmethod which converts RGB values to an integer suitable for the driver. RGB values are integers in range 0 <= c <= 255 (see example code below).

2.2.1 Static method

The CWriter class has one static method create_color. This is exclusively for use with 4 bit color display drivers. It populates the driver's color lookup table. Args:

  1. ssd The display instance.
  2. idx Color number in range 0 <= idx <= 15. These are arbitrary but by convention 0 is black and 15 white.
  3. r Red value. Values are in range 0 <= red <= 255.
  4. g Green value.
  5. b Blue value.

The return value is the idx value, hence a color can be defined as

GREEN = CWriter.create_color(ssd, 1, 0, 255, 0)

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.

The constructor checks for suitable firmware and also for a compatible device driver: an OSError is raised if these are absent.

2.2.3 Methods

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

  1. setcolor(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).

Contents

2.3 Example color code

The following will not work with 4-bit drivers: see section 2.4.

This demo assumes an SSD1351 OLED connected to a Pyboard D. It will need to be adapted for other hardware. In order to run this, the following files need to be copied to the host's filesystem:

  • writer.py
  • freesans20.py
  • The display driver. This should be installed as per this document to ensure the correct directory structure.
import machine
import gc
import time
from writer import CWriter
import freesans20  # Font to use
from drivers.ssd1351.ssd1351 import SSD1351 as SSD  # Adapt for other hardware

# Needed on my Pyboard D PCB to enable supply to the display
pp = machine.Pin('EN_3V3')
pp(1)
time.sleep(1)

# Adafruit options
# height = 96  # 1.27 inch 96*128 (rows*cols) display
height = 128 # 1.5 inch 128*128 display

pdc = machine.Pin('Y12', machine.Pin.OUT_PP, value=0)
pcs = machine.Pin('W32', machine.Pin.OUT_PP, value=1)
prst = machine.Pin('Y11', machine.Pin.OUT_PP, value=1)
spi = machine.SPI(2, baudrate=20_000_000)
gc.collect()  # Precaution before instantiating framebuf
ssd = SSD(spi, pcs, pdc, prst, height)  # Create a display instance

# Define a few colors (for 4-bit drivers this is done differently)
GREEN = SSD.rgb(0, 255, 0)
RED = SSD.rgb(255,0,0)
BLACK = SSD.rgb(0, 0, 0)

# Demo drawing geometric shapes using underlying framebuf methods
rhs = ssd.width -1
ssd.line(rhs - 20, 0, rhs, 20, GREEN)
square_side = 10
ssd.fill_rect(rhs - square_side, 0, square_side, square_side, GREEN)

# Instantiate a writer for a specific font
wri = CWriter(ssd, freesans20)  # Can set verbose = False to suppress console output
CWriter.set_textpos(ssd, 0, 0)  # In case a previous test has altered this
wri.setcolor(RED, BLACK)  # Colors can be set in constructor or changed dynamically
wri.printstring('Sunday\n12 Aug 2018\n10.30am')
ssd.show()

2.4 Use with 4 bit drivers

Some color display drivers for larger displays use 4-bit colors: this achieves a substantial reduction in the size of the frame buffer at the cost of limiting the number of colors that can be displayed. The driver expands the colors at run time using a lookup table.

This means that colors must be defined using the create_color static method described above.

from machine import SPI, Pin
from writer import CWriter
import freesans20  # Font to use
from drivers.ili93xx.ili9341 import ILI9341 as SSD

spi = SPI(0, sck=Pin(6), mosi=Pin(7), miso=Pin(4), baudrate=30_000_000)
dc = Pin(8, Pin.OUT, value=0)
cs = Pin(10, Pin.OUT, value=1)
rst = Pin(9, Pin.OUT, value=1)
ssd = SSD(spi, cs, dc, rst)

# Define a few colors: populates the lookup table
BLACK = CWriter.create_color(ssd, 0, 0, 0, 0)
GREEN = CWriter.create_color(ssd, 1, 0, 255, 0)
RED = CWriter.create_color(ssd, 2, 255, 0, 0)
YELLOW = CWriter.create_color(ssd, 3, 255, 255, 0)
# Demo drawing geometric shapes using underlying framebuf methods
rhs = ssd.width -1
ssd.line(rhs - 20, 0, rhs, 20, GREEN)
square_side = 10
ssd.fill_rect(rhs - square_side, 0, square_side, square_side, GREEN)

# Instantiate a writer for a specific font
wri = CWriter(ssd, freesans20)  # Can set verbose = False to suppress console output
CWriter.set_textpos(ssd, 0, 0)  # In case a previous test has altered this
wri.setcolor(RED, BLACK)  # Colors can be set in constructor or changed dynamically
wri.printstring('Tuesday\n6th December 2020\n10.30am\n')
wri.setcolor(YELLOW, BLACK)
wri.printstring('Running on a 4-bit driver.')
ssd.show()
Contents

3. Icons

It is possible to create simple icons, for example to create micro-gui pushbuttons with media playback symbols. Take an arbitrary free font and use a font editor to replace the glyphs for 'A', 'B', 'C'... with chosen symbols. Save this modified font under a new name. Then run font_to_py to create a Python font in a chosen size and comprising only those characters (-c ABCDE). Instantiate the buttons with e.g. text="A".

Alternatively icons can be created as bitmaps and converted to Python font files as described here.