micropython-font-to-py/DRIVERS.md

DRIVERS.md

This document comprises two sections, the first for users of an existing device driver and the second for writers of device drivers.

User Documentation: the Writer class

This class facilitates rendering characters from Python font files to a device, assuming the device has a driver conforming to the specification below. Typical use is as follows:

from writer import Writer
from device_driver import Display
import freeserif
import freesans20
display = Display(args_required_by_driver)
wri_serif = Writer(display, freeserif)
wri_sans = Writer(display, freesans20)
Writer.set_clip(True, True)
wri_serif.printstring('Tuesday\n')
wri_sans.printstring('8 Nov 2016\n')
wri_sans.printstring('10.30am')

display.show()  # Display the result

The file driver_test.py illustrates the use of font files with an SSD1306 display and a complete example of an SSD1306 driver may be found here.

Class Methods

The Writer class exposes the following class methods:

1. set_textpos Args: row, col. This determines where on screen any 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. They reference the top left hand corner of the first character to be output.
2. set_clip Args: boolean row_clip, col_clip. If these 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.
3. mapping Arg: an integer. This defines the mapping of bytes in the buffer onto pixels. The module exposes three constants for use here: VERT HORIZ and WEIRD, the latter being specific to the official SSD1306 driver. VERT is for true vertically mapped displays. HORIZ, for horizontally mapped devices, is currently unsupported. By default the mapping is for SSD1306 devices using the official driver.

As class methods these settings apply to all font objects. The insertion point of characters is maintained regardless of the font in use.

Method

1. printstring Arg: a text string. Outputs a text string at the current insertion point. Newline characters are honoured.

Note on the Writer class

This is more a proof of concept than a final implementation. Obvious enhancements include rendering to a rectangular area, support for proper word wrap and support for format control characters such as tabs.

Device Driver Implementation

Display devices comprise two varieties, depending on whether the framebuffer is located on the controlling system or on the physical display device. In the former case the Writer class simplifies the design of the driver. It merely has to expose certin attributes and methods with Writer instances taking care of text rendering. It is strongly recommended that such device drivers use the oficial framebuf module, as per the official SSD1306 driver which exposes the required components.

Where the buffer is located on the display device the means of controlling the text insertion point will be device dependent. The driver will need to implement the functionality of the Writer class itself.

Drivers with local buffers

The writer of a device driver need not be concerned with the structure of a Python font file so long as the driver exposes certain attributes and methods required by the Writer class. These are as follows:

Attributes:

1. buffer The underlying bytearray instance holding the display buffer.
2. height The screen height in pixels.
3. width The screen width in pixels.

Methods:

1. show Display the current buffer contents.
2. scroll Arguments x, y amount to scroll horizontal and vertical.
3. fill Argument col colour 1 == fill 0 == clear.

An example of such a driver, using the official framebuf module, is the SSD1306 driver (drivers/display/ssd1306.py in the source tree).

The driver documentation should specify the arguments for font_to_py.py to ensure users create font files with a layout corresponding to that of the buffer/device.

Drivers for remote buffers

Specifying the font file

Each font file has a get_ch() function accepting an ASCII character as its argument. It returns a memoryview instance providing access to a bytearray corresponding to the individual glyph. The layout of this data is determined by the command line arguments presented to the font_to_py.py utility. It is the responsibility of the driver to copy that data to the physical device.

The purpose of the font_to_py.py command line arguments specified to the user is to ensure that the data layout is optimised for the device so that this copy operation is a fast bytewise copy or SPI/I2C transfer. The driver documentation should therefore specify these arguments to ensure the layout is optimal. Mapping may be horizontal or vertical, and the bit order of individual bytes may be defined. These are detailed below.

In the case of devices with their own frame buffer the Writer class will need to be re-written or adapted to match the hardware's method of tracking such things as the text insertion point. Consideration should be given to employing the same interface as the Writer class to simplify the porting of user code between displays with differing hardware.

Font files

Assume the user has run the utility to produce a file myfont.py This then has the following outline definition (in practice the bytes objects are large):

 # Code generated by font-to-py.py.
 # Font: FreeSerif.ttf
version = '0.1'

def height():
    return 21

def max_width():
    return 22

def hmap():
    return False

def reverse():
    return False

def monospaced():
    return False

_font =\
b'\x06\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'\
b'\x00\x00\x00\x00\x08\x00\xfe\xc7\x00\x7e\xc0\x00\x00\x00\x00\x00'\

_index =\
b'\x00\x00\x14\x00\x2e\x00\x4b\x00\x71\x00\x97\x00\xd2\x00\x0a\x01'\
b'\x1b\x01\x35\x01\x4f\x01\x75\x01\x9e\x01\xb2\x01\xcc\x01\xe0\x01'\

    # Boilerplate code omitted

def get_ch(ch):
    # validate ch, if out of range use '?'
    # get offsets into _font and retrieve char width
    # Return: memoryview of bitmap, height and width
    return memoryview(_font[offset + 2, next_offset]), height, width

height and width are specified in bits (pixels).

In the case of monospaced fonts the max_width function returns the width of every character. For variable pitch fonts it returns the width of the widest character. Device drivers can use this to rapidly determine whether a string will fit the available space. If it will fit on the assumption that all chars are maximum width, it can be rendered rapidly without doing a character by character check.

get_ch() returns a memoryview of an individual glyph with its dimensions and contains all the bytes required to render the character including trailing space.

Mapping

A character occupies a space where (0, 0) represents the coordinates of the top left hand corner of the bitmap. It comprises a set of pixels where increasing x values represent locations to the right of the origin and increasing y values represent downward positions. Mapping defines the relationship between this abstract two dimensional array of bits and the physical linear sequence of bytes.

Vertical mapping means that the LSB of first byte is pixel (0,0), MSB of first byte is (0, 7). The second byte (assuming the height is greater than 8 pixels) is (0, 8) to (0, 15). Once the column is complete the next byte represents (1, 0) to (1, 7).

Horizontal mapping means that the MSB of byte 0 is pixel (0,0) with LSB at (7,0), with the second byte covering (8, 0) to (15, 0) if the width is greater than 8.

Bit reversal provides for the case where the bit order of each byte is reversed i.e. a byte comprising bits [b7b6b5b4b3b2b1b0] becomes [b0b1b2b3b4b5b6b7].

Specification and Project Notes

The design aims primarily to minimise RAM usage. Minimising the size of the bytecode is a secondary aim. Indexed addressing is used to reduce this in the case of proportional fonts, at a small cost in performance. The size of the Python source file is a lesser consideration, with readability being prioritised over size. Hence they are "pretty formatted" with the large bytes objects split over multiple lines for readability.

The approach has been tested on SSD1306 devices using both the pseudo-horizontal and true vertical mapping.

The font_to_py utility has been extensively tested with each of the mapping options.