Developing
Strategy: * read headers to determine file type * keep a pointer to the start of data * read data into the Palette for all colors present * rewind the file pointer back to start of data * read data into the Bitmap * return a bitmap and palette instance
Shared Bindings
This library uses interfaces from CircuitPython’s displayio module (Bitmap and Palette) to load images from disk into memory.
The Bitmap and Palette objects are related, and together can be used to display the image on a screen for the user.
The Palette is a list of colors present in the image. Its constructor takes a single argument: (int) max_colors, representing how many colors will be populated in the palette.
Palette code example
palette = Palette(4) # 4 represents that we will define four colors in palette
palette[0] = b'\x00\x00\x00\x00' # white
palette[1] = b'\xFF\x00\x00\x00' # red
palette[2] = b'\x00\xFF\x00\x00' # green
palette[3] = b'\x00\x00\xFF\x00' # blue
Bitmap code example
bitmap = Bitmap(3, 2, 4) # 3 pixels wide, two pixels tall, 4 colors
bitmap[0,0] = 0 # palette color 0
bitmap[0,1] = 1 # palette color 1
...
Example of Palette and Image
The example is 4bit.bmp from the examples/images folder:
The Palette object appears like this after loading:
Palette:
[0] b'\x00\x00\x00\x00'
[1] b'\x7f\x00\x00\x00'
[2] b'\xff\x00\x00\x00'
[3] b'w\x00\xb1\x00'
[4] b'\xff\x00\x9d\x00'
[5] b'\x00\x00\xff\x00'
[6] b'\xff\x00\xfe\x00'
[7] b'\xbf\x80\x00\x00'
[8] b'zzz\x00'
[9] b'\xff\x9e\xa5\x00'
[10] b'\x00\x98\xff\x00'
[11] b'\x00\xff\x00\x00'
[12] b'h\xff\x00\x00'
[13] b'\xfb\xff\x9e\x00'
[14] b'\x00\xfb\xff\x00'
[15] b'\xfb\xfb\xfb\x00'
This palette has 16 colors. The value in square brackets [] is the color’s index in the palette. The byte values are the RGB or RGB + padding of each color.
The Bitmap is an grid of which palette color to use in each position of the image.
The corresponding Bitmap to the example above appears like this after loading:
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
11 11 11 5 5 5 15 15 15 5 5 5 2 2 2
11 11 11 5 5 5 15 15 15 5 5 5 2 2 2
6 6 6 5 5 5 1 1 1 5 5 5 10 10 10
6 6 6 5 5 5 1 1 1 5 5 5 10 10 10
6 6 6 5 5 5 1 1 1 5 5 5 10 10 10
14 14 14 5 5 5 9 9 9 5 5 5 8 8 8
14 14 14 5 5 5 9 9 9 5 5 5 8 8 8
14 14 14 5 5 5 9 9 9 5 5 5 8 8 8
3 3 3 5 5 5 0 0 0 5 5 5 13 13 13
3 3 3 5 5 5 0 0 0 5 5 5 13 13 13
4 4 4 5 5 5 12 12 12 5 5 5 7 7 7
4 4 4 5 5 5 12 12 12 5 5 5 7 7 7
4 4 4 5 5 5 12 12 12 5 5 5 7 7 7
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
This grid represents the example image (15 pixels wide and 17 pixels tall).
The coordinates are arranged in a zero indexed grid, starting in the top left at [0,0],
and continuing down and to the right to a final coordinate of [14,16].
The value at each position is an integer, representing an entry in the palette object.
For example, the Bitmap coordinate [0,0] has the value (integer) 5.
This corresponds to the the Palette object’s, [5] which is b'\x00\x00\xff\x00'. This is a byte string that represents a color.
Mypy & type checking
Mypy was tested with version 0.950 and the mypy.ini in project root. Since checks are not currently not passing, it is not installed as a commit hook.
Setup: In your virtual environment, run:
pip3 install mypy==0.050
Developing: To manually run checks, run:
mypy .