softstrip.py

#!/usr/bin/env python

import re, struct, sys
from PIL import Image

# Author: @doegox
# Original version was from
# https://www.reddit.com/r/programming/comments/1ye0th/i_found_this_in_a_computer_magazine_from_1986/

# This version expects a perfect BW image, 1 pixel per dot

# Relevant patents:
# https://www.google.com/patents/US4692603
# https://www.google.com/patents/US4728783
# https://www.google.com/patents/US4782221

# very first lines: count number of black lines N -> there are (N+4)/2 nibbles in each data line
# here 10 -> (10+4)/2 = 7 nibbles per line
# dibit/bit/dibit_sync/dibit_P1/dibit_nibbles/dibit_P2/bit_or_dibit/dibit/bit_sync
# P1: parity of the even bits
# P2: parity of the odd bits
# First data line should be 00 00 00 LL HH -> total nr of bytes

image = Image.open("softstrip_bw.png")

w, h = image.size

def getcolor(pixel):
    return pixel^1

def paritycheck(bits):
    a = bits[::2]
    b = bits[1::2]
    if '?' in a:
        a = False
    else:
        a = (sum(a) % 2 == 0)
    if '?' in b:
        b = False
    else:
        b = (sum(b) % 2 == 0)
    return a and b

def getnibbles(bits):
    'return [(nibble, validmask)]'
    ret = []
    par = paritycheck(bits)
    bits = bits[1:-1]  # remove parity bits
    assert len(bits) % 4 == 0
    while len(bits):
        nib, bits = bits[:4], bits[4:]
        n = 0
        invalid = 0
        for b in reversed(nib):
            n <<= 1
            invalid <<= 1
            if b not in (0, 1):
                invalid |= 1
            else:
                n |= b
        if not par:
            invalid = 0b1111
        ret.append((n, invalid ^ 0b1111))
    return ret

def getbytes(nibbles):
    bytes = []
    while len(nibbles) >= 2:
        a, av = nibbles.pop(0)
        b, bv = nibbles.pop(0)
        c = chr((b << 4) | a)
        v = chr((bv << 4) | av)
        bytes.append((c, v))
    return bytes

firstline=[]
nibbles = []
bytes = []
for y in range(h):
    dibits = []
    for x in range(w):
        dibits.append(getcolor(image.getpixel((x,y))))

    bits = []
    for i in range(5, len(dibits) - 5, 2):
        a, b = dibits[i:i+2]
        if a != b:
            bits.append(int(a))
        else:
            bits.append('?')

    print ''.join(' #'[b] for b in dibits), ''.join(map(str, bits)), ''.join("%x" % c for c, v in getnibbles(bits)), ''.join("%x" % v for c, v in getnibbles(bits))

    if nibbles == [] and '?' in bits:
        continue
    if firstline == []:
        firstline = bits
    else:
        if bits != firstline:
            # Start recording data
            nibbles += getnibbles(bits)
bytes += getbytes(nibbles)

raw = ''.join(c for c, v in bytes)
valid = ''.join(v for c, v in bytes)
header = raw[:5]
assert header[:3]=='\0\0\0'
size, = struct.unpack('<H', header[3:])
data = raw[5:]
valid = valid[5:]
assert len(data) >= size, 'data length doesn\'t match size given in header'
if len(data) > size:
    print "Truncating data from %i to %i bytes" % (len(data), size)
    data=data[:size]

open('out.bin', 'w').write(data)
open('out.valid', 'w').write(valid)

fmt = '%-48s %-16s %-32s'

allvalid = bool(re.match(r'\xff+$', valid))

print
print fmt % ('hex', 'ascii', '' if allvalid else 'valid bits')
for i in range(0, len(data), 16):
    d = data[i:i+16]
    v = valid[i:i+16]
    hex = ' '.join('%02x' % ord(c) for c in d)
    ascii = ''.join((c if 32 <= ord(c) <= 126 else '.') for c in d)
    val = '' if allvalid else ''.join('%02x' % ord(c) for c in v)
    print fmt % (hex, ascii, val)