Function pack_rows

_plotly_utils/png.py:963–994

Yield packed rows that are a byte array. Each byte is packed with the values from several pixels.

(rows, bitdepth)

def pack_rows(rows, bitdepth):
    """Yield packed rows that are a byte array.
    Each byte is packed with the values from several pixels.
    """

    assert bitdepth < 8
    assert 8 % bitdepth == 0

    # samples per byte
    spb = int(8 / bitdepth)

    def make_byte(block):
        """Take a block of (2, 4, or 8) values,
        and pack them into a single byte.
        """

        res = 0
        for v in block:
            res = (res << bitdepth) + v
        return res

    for row in rows:
        a = bytearray(row)
        # Adding padding bytes so we can group into a whole
        # number of spb-tuples.
        n = float(len(a))
        extra = math.ceil(n / spb) * spb - n
        a.extend([0] * int(extra))
        # Pack into bytes.
        # Each block is the samples for one byte.
        blocks = group(a, spb)
        yield bytearray(make_byte(block) for block in blocks)


def unpack_rows(rows):