code/prirowpng

#!/usr/bin/env python

# https://docs.python.org/3.5/library/itertools.html
import itertools
import sys

import png

Description = """Join PNG images in a row left-to-right."""


class FormatError(Exception):
    """
    Some problem with the image format.
    """


def join_row(out, l):
    """
    Concatenate the list of images.
    All input images must be same height and
    have the same number of channels.
    They are concatenated left-to-right.
    `out` is the (open file) destination for the output image.
    `l` should be a list of open files (the input image files).
    """

    l = [png.Reader(file=f) for f in l]

    # Ewgh, side effects.
    for r in l:
        r.preamble()

    # The reference height; from the first image.
    height = l[0].height
    # The total target width
    width = 0
    for i,r in enumerate(l):
        if r.height != height:
            raise FormatError('Image %d, height %d, does not match %d.' %
              (i, r.height, height))
        width += r.width

    # Various bugs here because different numbers of channels and depths go wrong.
    pixel, info = zip(*[r.asDirect()[2:4] for r in l])
    tinfo = dict(info[0])
    del tinfo['size']
    w = png.Writer(width, height, **tinfo)

    def iter_all_rows():
        for row in zip(*pixel):
            # `row` is a sequence that has one row from each input image.
            # list() is required here to hasten the lazy row building;
            # not sure if that's a bug in PyPNG or not.
            yield list(itertools.chain(*row))
    w.write(out, iter_all_rows())

def main(argv):
    import argparse

    parser = argparse.ArgumentParser(description=Description)
    parser.add_argument(
        "input", nargs="*", default="-", type=png.cli_open, metavar="PNG"
    )

    args = parser.parse_args()

    return join_row(png.binary_stdout(), args.input)

if __name__ == '__main__':
    main(sys.argv)