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Blit

A project log for Micro:Boy

Arcade games on the Micro:bit

ꝺeshipuꝺeshipu 01/08/2018 at 13:210 Comments

I'm still waiting for the attiny chips for the button handling, but I realized that my display driver is not really suitable for making games. Sure, it has that cool dirty pages stuff that makes it fast, but it only can draw a pixel at a time. That's not how you are going to get nice smooth animations. We need a blit operation — something that will let me draw a whole sprite in one operation.

The way blit works, it takes whole bytes, shifts them as required, and combines with the current frame buffer using one of the logic operators. For a start I have chosen xor. Why? Because that's the easiest way to have sprites that don't delete the background as they move around — you blit them once to show them, and you blit them a second time to make them disappear. Sure, they don't look pretty when they collide with something, but it's a start. Later I can do similar operations with or and and, so that I can have pretty sprites with a mask. For now I have a bouncing box:

This is done with this code:

---------- more ----------
import microbit


class SH1106:
    _command = 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
        _init = (b'\x00\xae\xd5\x80\xa8\x3f\xd3\x00\x40\x80\x14'
                 b'\x20\x00\xa1\xc8\xa1\xda\x12\x81\xcf\xd9\xf1'
                 b'\xdb\x40\xa4\xa6\xaf')
        microbit.i2c.write(0x3c, _init)
        microbit.i2c.write(0x3c, _init)

    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._command[1] = 0xb0 | page
                self._command[2] = 0x00 | ((2 + dmin) & 0x0f)
                self._command[3] = 0x10 | ((2 + dmin) >> 4) & 0x0f
                microbit.i2c.write(0x3c, self._command)
                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)

    def xor(self, x, y, data):
        if not -8 < x < 128:
            return
        if x < 0:
            data = data[-x:]
            x = 0
        elif x > 120:
            data = data[:120 - x]
        page = y // 8
        shift = y % 8
        if 0 <= page <= 7:
            index = x + page * 128
            for byte in data:
                self.buffer[index] ^= byte << shift
                index += 1
            self.dirty_min[page] = min(self.dirty_min[page], x)
            self.dirty_max[page] = max(self.dirty_max[page], x + 8)
        page += 1
        if 0 <= page <= 7 and shift:
            index = x + page * 128
            shift = 8 - shift
            for byte in data:
                self.buffer[index] ^= byte >> shift
                index += 1
            self.dirty_min[page] = min(self.dirty_min[page], x)
            self.dirty_max[page] = max(self.dirty_max[page], x + 8)


display = SH1106()
box = b'\xff\x81\x81\x81\x81\x81\x81\xff'
x = 0
y = 0
dx = 1
dy = 1
while True:
    display.xor(x, y, box)
    display.show()
    display.xor(x, y, box)
    x += dx
    y += dy
    if not 0 <= x < 120:
        dx = -dx
    if not 0 <= y <= 56:
        dy = - dy
    microbit.sleep(10)

 So I can easily blit 8x8 sprites (and tiles). If I want bigger ones, I can just compose them out of those 8x8 blocks — it's not going to be considerably slower.

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