Display devices comprise two varieties, depending on whether the hardware includes a frame buffer or whether a frame buffer must be located on the controlling system.
If the device has no frame buffer then the device driver should be designed
to subclass framebuf.FrameBuffer with a suitably sized buffer on the host. If
the device has its own frame buffer there are two options for the driver. One
is to perform all display operations using the device's own firmware
primitives. This is efficient and avoids the need for a buffer on the host,
however it does involve some code complexity.
The second option is to subclass framebuf.FrameBuffer, provide a buffer on
the host, and copy its contents to the device's buffer when required. This can
result in a very simple device driver at cost of RAM use and update speed. It
also ensures compatibility with additional libraries to simplify display tasks.
If a device subclasses framebuf.FrameBuffer the following libraries enhance
its capability. The Writer class enables it to use multiple
fonts with additional functionality such as word wrap, string metrics and tab
handling. The nano-gui
provides rudimentary GUI displayt capability with minimal RAM use. The
micro-gui library is
more fully featured, providing a means of user input at some cost in size and
complexity.
If a driver relies on a buffer located on the display device, the means of
controlling the text insertion point, performing partial buffer updates and
executing graphics primitives will be device dependent. If the functionality of
the writer or nanogui libraries are required it will need to be
implemented at device driver level. Authors of such code will need to know the
format of the Python font file which is provided in this doc.
Where the buffer is held on the MicroPython host the driver should be
subclassed from the official framebuf.FrameBuffer class. An example of such a
driver is the official SSD1306 driver.
In addition the driver class should have bound variables width and height
containing the size of the display in pixels, plus a show method which copies
the buffer to the physical device.
The device driver defines a buffer of the correct size to hold a full frame of
data and instantiates the framebuf.FrameBuffer superclass to reference it.
Monochrome displays should define the frame buffer format to match the physical
characteristics of the display. In the case of colour displays RAM may be saved
by using framebuf.GS8 8-bit colour. The show method can map this to the
device's colour space if 8-bit mode is not supported.
This design enables the supplied Writer and CWriter classes to be used for
rendering arbitrary fonts to the display. The author of the device driver need
not be concerned with the format of Python font files.
The Writer and CWriter classes require horizontally mapped fonts. This is
regardless of the mapping used in the device driver's FrameBuffer: the
Writer.printstring method deals transparently with any mismatch.
See Supported displays document
for a full list. These drivers are subclassed from framebuf.FrameBuffer and
have been tested with writer, nanogui and microgui. The following are a
few samples:
- The SSD1306 OLED driver
- The Nokia 5110
- The SSD1331 colour OLED
- The HX1230 96x68 LCD
The latter example illustrates a very simple driver.
Authors of such drivers will need to have an understanding of the font file format.
Each font file has a get_ch() function accepting a 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.
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: FreeSans.ttf
# Cmd: ./font_to_py.py -x FreeSans.ttf 17 font10.py
version = '0.28'
def height():
return 17
def max_width():
return 17
def hmap():
return True
def reverse():
return False
def monospaced():
return False
def min_ch():
return 32
def max_ch():
return 126
_font =\
b'\x09\x00\x3c\x00\xc7\x00\xc3\x00\x03\x00\x03\x00\x06\x00\x0c\x00'\
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'\
_index =\
b'\x00\x00\x24\x00\x37\x00\x4a\x00\x5d\x00\x81\x00\xa5\x00\xc9\x00'\
b'\xed\x00\x00\x01\x13\x01\x26\x01\x39\x01\x5d\x01\x70\x01\x83\x01'\
b'\x60\x0b'
_mvfont = memoryview(_font)
def get_ch(ch):
ordch = ord(ch)
if ordch >= 32 and ordch <= 126:
idx_offs = 2 * (ordch - 32 + 1)
else:
idx_offs = 0
offset = int.from_bytes(_index[idx_offs : idx_offs + 2], 'little')
width = int.from_bytes(_font[offset:offset + 2], 'little')
next_offs = offset + 2 + ((width - 1)//8 + 1) * 17
return _mvfont[offset + 2:next_offs], 17, widthheight and width are specified in bits (pixels). See Appendix 1 for extra
code in fonts created with the --iterate arg.
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.
The _font bytearray holds the glyphs corresponding to every character in the
font. Entry 0 is the default glyph, used if an attempt is made to render a
nonexistent character.
The index holds two integers (each occupying 2 bytes) per character. The index has an entry for every character in the specified range, whether or not that character exists.
Index entries are offsets into the _font bytearray representing the start and
end of the glyph. If the font comprises a set of characters which is not
contiguous, missing characters have an index entry which points to the first
glyph in the _font bytearray. This ensures that the default glyph is
rendered.
If a Python font file is created with the -f argument, all characters will
be saved with the width of the widest. In general it is not necessary to
specify this option. The driver can perform fixed pich rendering by rendering
the character as variable pitch, then blanking and advancing the pixel column
by the value returned by font.max_width().
This format is unlikely to find application beyond the e-paper driver. It was designed for micropower applications where the Pyboard has no SD card. Fonts are stored as random access files on power-switched Flash storage or SD card. This method is probably too slow for anything other than e-paper displays.
The format is as follows. Files are binary with a four byte header and 126 fixed length records. The header consists of two file identifiers enabling the file format to be checked, followed by bytes specifying the width and height. The length of each record is (width + 1) bytes.
The file indentifiers depend on the -x and -r arguments specified to font_to_py.py
and are as follows:
hmap reverse byte
-x -r 0 1
0 0 0x3f 0xe7
1 0 0x40 0xe7
0 1 0x41 0xe7
1 1 0x42 0xe7
Each record starts with a width byte specifying the x dimension of the glyph if rendered proportionally spaced, followed by the glyph data. This data includes trailing space ensuring that all records have the size specified in the header.
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].
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, and also to facilitate non-contiguous 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.
Fonts created with the font_to_py utility have been extensively tested with
each of the mapping options. They are used with drivers for SSD1306 OLEDs,
SSD1963 LCD displays, the official LCD160CR and the Digital Artists 2.7 inch
e-paper display.
A guide to writing suitable drivers may be found
here.
These can be very simple as the FrameBuffer base class provides much of the
functionality.
This specialist arg causes extra code to be included in the font file, to provide for iterating over all the glyphs in the file. The following sample of the extra code assumes a font comprising '0123456789:'
def glyphs():
for c in """0123456789:""":
yield c, get_ch(c)Typical usage under CPython 3 (for a font cyrillic.py) is
import cyrillic
res = []
for glyph in cyrillic.glyphs():
res.append(list(glyph)) # Each element is [char, glyph, height, width]