Quick and Dirty

Published on 2017-10-15 in Micro:Boy.

The code from the previous log does its job, and is not too slow. But we can do much better! The display is organized into 8 “pages” of memory, each containing 8 lines of the display. Instead of updating every page on every refresh, we can keep track of which pages changes, and also the minimum and maximum column of the changed pixels, and only redraw those. The code below does that:

import microbit
 
 
class SH1106:
    move = bytearray(b'\x00\xb0\x02\x10')
 
    def __init__(self):
        self.buffer = bytearray(1024)
        self.dirty_min = bytearray([0] * 8)
        self.dirty_max = bytearray([128] * 8)
        microbit.i2c.init(freq=400000, scl=microbit.pin2, sda=microbit.pin1)
        for i in range(2):
            # First two transaction after init always fail.
            try:
                microbit.i2c.write(0x3c, b'')
            except OSError:
                pass
        microbit.i2c.write(0x3c, b'\x00\xae\xd5\x80\xa8\x3f\xd3\x00\x40\x80\x14'
                b'\x20\x00\xa1\xc0\xa0\xda\x12\x81\xcf\xd9\xf1\xdb\x40\xa4\xa6\xaf')
        
    def show(self):
        index = 0
        buf = self.buffer
        for page in range(8):
            dmin = self.dirty_min[page]
            dmax = self.dirty_max[page]
            if dmax:
                self.move[1] = 0xb0 | page
                self.move[2] = 0x00 | ((2 + dmin) & 0x0f)
                self.move[3] = 0x10 | ((2 + dmin) >> 4) & 0x0f 
                microbit.i2c.write(0x3c, self.move)
                microbit.i2c.write(0x3c, b'\x40' + buf[index + dmin:index + dmax])
            index += 128
            self.dirty_max[page] = 0
            self.dirty_min[page] = 127
    
    def pixel(self, x, y, color=None):
        if not 0 <= x < 128 or not 0 <= y < 64:
            return
        page = y // 8
        index = x + page * 128
        mask = 1 << (y % 8)
        if color is None:
            return bool(self.buffer[index] & mask)
        elif color:
            self.buffer[index] |= mask
        else:
            self.buffer[index] &= ~mask
        self.dirty_max[page] = max(self.dirty_max[page], x + 1)
        self.dirty_min[page] = min(self.dirty_min[page], x)

This is all good, you say, but we had to add 10 lines of code, that now have to be executed. Is it really faster this way, and if so, by how much?

Turns out it’s quite a bit faster — almost an order of magnitude. I ran a simple test, where I light up the same amount of pixels as the display has, except randomly, not filling the whole screen, and I do an update after every pixel. The simple code did this in four minutes fifty seconds. The optimized code took forty five seconds. Of course this is an extreme case, normally the speedup won’t be as large, as nobody does a refresh after a single pixel, right?