pbm.py

from bitarray import bitarray, bits2bytes

class PBM: # Portable Bitmap
    def __init__(self, w=0, h=0):
        self.size = (w, h)
        self.update()
        self.data = bitarray(self.bits, endian='big')

    def update(self):
        w, h = self.size
        self.bytes_per_row = bits2bytes(w)
        self.bits_per_row = 8 * self.bytes_per_row
        self.bytes = self.bytes_per_row * h
        self.bits = 8 * self.bytes

    def info(self):
        print('size: %s x %s' % self.size)
        print('bytes per row: %s' % self.bytes_per_row)
        print('bits per row: %s' % self.bits_per_row)
        print('bitarray: ' + repr(self.data.buffer_info()))

    def clear(self):
        self.data.setall(0)

    def save(self, filename):
        fo = open(filename, 'wb')
        fo.write(b'P4\n')
        fo.write(b'# This is a partable bitmap (pbm) file.\n')
        fo.write(('%i %i\n' % (self.size)).encode())
        self.data.tofile(fo)
        fo.close()

    def load(self, filename):
        fi = open(filename, 'rb')
        assert fi.readline().strip() == b'P4'
        while True:
            line = fi.readline()
            if not line.startswith(b'#'):
                self.size = tuple(map(int, line.split()))
                break
        self.update()
        self.data = bitarray(endian='big')
        self.data.fromfile(fi)
        fi.close()
        assert self.data.buffer_info()[1] == self.bytes

    def address(self, x, y):
        return x + self.bits_per_row * y

    def __getitem__(self, s):
        x, y = s
        return self.data[self.address(x, y)]

    def __setitem__(self, s, val):
        x, y = s
        self.data[self.address(x, y)] = val


if __name__ == '__main__':
    # draw picture with straight line from (10, 10) to (390, 390)
    a = PBM(500, 400)
    a.info()
    a.clear()
    for x in range(10, 391):
        a[x, x] = True
    a.save('pic1.ppm')

    # copy the picture
    b = PBM()
    b.load('pic1.ppm')
    b.save('pic2.ppm')

    # draw a straight line from (490, 10) to (110, 390) on top
    for i in range(381):
        b[490-i, 10+i] = 1
    b.save('pic3.ppm')