pngoffset.py

from apng import PNG, make_chunk, Chunk
import struct, sys, argparse
parser = argparse.ArgumentParser(description='PNG offset update')
parser.add_argument('infile', help='input file')
parser.add_argument('x', type=int, help='x')
parser.add_argument('y', type=int, help='y')
parser.add_argument('-o', '--out', help='output file. default is input file')
parser.add_argument('-t', '--type', help='offset type, caNv or oFFs', choices=['oFFs', 'caNv'], default='oFFs')
args = parser.parse_args()
infile = args.infile
x = args.x
y = args.y
offset_type = args.type
if args.out != None:
    outfile = args.out
else:
    outfile = infile
a = -1
d = -1
im = PNG.open(infile)
for i in range(len(im.chunks)):
    if a < 0 and im.chunks[i].type == 'IDAT':
        a = i
else:
    if d < 0:
        if im.chunks[i].type == offset_type:
            d = i
        if d >= 0:
            if d > a:
                im.chunks[d:d + 1] = []
                d = -1
        if a >= 0:
            if offset_type == 'oFFs':
                im.chunks[a:a] = [
                 Chunk('oFFs', make_chunk('oFFs', struct.pack('!iib', x, y, 0)))]
            else:
                if offset_type == 'caNv':
                    im.chunks[a:a] = [
                     Chunk('caNv', make_chunk('caNv', struct.pack('!IIii', im.width, im.height, x, y)))]
        if d >= 0:
            im.chunks[d:d + 1] = []
    im.save(outfile)