code/pdsimgtopng

#!/usr/bin/env python
# $URL$
# $Rev$
# PDS Image to PNG

import re
import struct

import png

class FormatError(Exception):
    pass

def pdskey(s, k):
    """Lookup key `k` in string `s`.  Returns value (as a string), or
    raises exception if not found.
    """

    assert re.match(r' *\^?[:\w]+$', k)
    safere = '^' + re.escape(k) +r' *= *(\w+)'
    m = re.search(safere, s, re.MULTILINE)
    if not m:
        raise FormatError("Can't find %s." % k)
    return m.group(1)

def img(inp):
    """Open the PDS IMG file `inp` and return (*pixels*, *info*).
    *pixels* is an iterator over the rows, *info* is the information
    dictionary.
    """

    err = __import__('sys').stderr

    consumed = 1024

    s = inp.read(consumed)
    record_type = pdskey(s, 'RECORD_TYPE')
    if record_type != 'FIXED_LENGTH':
        raise FormatError(
          "Can only deal with FIXED_LENGTH record type (found %s)" %
            record_type)
    record_bytes = int(pdskey(s,'RECORD_BYTES'))
    file_records = int(pdskey(s, 'FILE_RECORDS'))
    label_records = int(pdskey(s, 'LABEL_RECORDS'))
    remaining = label_records * record_bytes - consumed
    s += inp.read(remaining)
    consumed += remaining

    image_pointer = int(pdskey(s, '^IMAGE'))
    # "^IMAGE" locates a record.  Records are numbered starting from 1.
    image_index = image_pointer - 1
    image_offset = image_index * record_bytes
    gap = image_offset - consumed
    assert gap >= 0
    if gap:
        inp.read(gap)
    # This assumes there is only one OBJECT in the file, and it is the
    # IMAGE.
    height = int(pdskey(s, '  LINES'))
    width = int(pdskey(s, '  LINE_SAMPLES'))
    sample_type = pdskey(s, '  SAMPLE_TYPE')
    sample_bits = int(pdskey(s, '  SAMPLE_BITS'))
    # For Messenger MDIS, SAMPLE_BITS is reported as 16, but only values
    # from 0 ot 4095 are used.
    bitdepth = 12
    if sample_type == 'MSB_UNSIGNED_INTEGER':
        fmt = '>H'
    else:
        raise 'Unknown sample type: %s.' % sample_type
    sample_bytes = (1,2)[bitdepth > 8]
    row_bytes = sample_bytes * width
    fmt = fmt[:1] + str(width) + fmt[1:]
    def rowiter():
        for y in range(height):
            yield struct.unpack(fmt, inp.read(row_bytes))
    info = dict(greyscale=True, alpha=False, bitdepth=bitdepth,
      size=(width,height), gamma=1.0)
    return rowiter(), info


def main(argv=None):
    import sys

    if argv is None:
        argv = sys.argv
    argv = argv[1:]
    arg = argv
    if len(arg) >= 1:
        f = open(arg[0], 'rb')
    else:
        f = sys.stdin
    pixels,info = img(f)
    w = png.Writer(**info)
    w.write(sys.stdout, pixels)

if __name__ == '__main__':
    main()