PNG Metadata Editor

This library was written for a competition between me and Ethan, who was going to write this same thing in Java. He failed.

You can initialize a PME instance by passing it a filename to an already existing file, or no arguments, like this:

img = new libpme.PME("test.png");
# or
img = new libpme.PME();

If the file is not valid, it will throw an exception

Properties

These are the values that you can access or change. Most are self explanatory. Changing any of these will automatically call recalculate_IHDR. If you really want to suppress this behavior, just back up the IHDR chunk before changing the value. The defaults for an object created with no filename are displayed in parenthesis

width (0)
height (0)
bit_depth (8)
color_type (color_types.RGB_WITH_ALPHA (6))
compression_method (0)
filter_method (0)
interlace_method (0)

Methods

Here are the methods you can call on an image, in no particular order

recalculate_properties

This simply recalculates the seven properties listed above from the data currently in the IHDR chunk.

img.height = 400
img.chunks[0][2] = b'\x00\x00\x00\x00\x00\x00\x00\x00\x08\x06\x00\x00\x00'
# img.height remains unchanged at 400
img.recalculate_properties()
# img.height is now set to 0

recalculate_IHDR

This takes the seven properties listed above and destructively replaces the first chunk's data and crc with updated fields

img.width = 0
# img.chunks[0][2] is now set to b'\x00\x00\x00\x00\x00\x00\x00\x00\x08\x06\x00\x00\x00'
img.width = 400
img.recalculate_IHDR()
# img.chunks[0][2] is now set to b'\x00\x00\x01\x90\x00\x00\x00\x00\x08\x06\x00\x00\x00'

recalculate_crc

This takes an index of a chunk (see below) and overwrites the crc in that chunk with a newly calculated crc based on that chunk's label and data

img.color_type = libpme.color_types.RGB
img.width = img.height = 1
img.chunks[1][2] = b'\x00\xFF\x00\x00' # a 1x1 red image, assuming that the second chunk is the only IDAT chunk
img.recalculate_crc(1) # img.chunks[1][3] is now set to b'T\xbb\xd3\xea'

recalculate_length

This takes an index of a chunk (see below) and updates the length of that chunk with a newly calculated length based on that chunk's data

img.color_type = libpme.color_types.RGB
img.width = img.height = 1
img.chunks[1][2] = img.compress(b'\x00\xFF\x00\x00') # a 1x1 red image, assuming that the second chunk is the only IDAT chunk
img.recalculate_length(1) # img.chunks[1][0] is now set to b'\x00\x00\x00\x04'

save

This saves the image to the disk, overwriting a file if it was already there. If the object was created from an existing file and no arguments are passed, it will use the original file.

img.save("red1.png")
img.save() # Only if the object was created from an existing file.

get_concatenated_idat_data

Concatenates the (still compressed) data of each IDAT chunk, then returns it.

img.width = 2
img.chunks.insert(2, [b'\x00\x00\x00\x03', b'IDAT', img.compress(b'\x00\xFF\x00'), b'j\xee\xb3\xd0']) # making it a 2x1 image with a red and a green pixel
img.decompress(img.get_concatenated_idat_data()) # returns '\x00\xFF\x00\x00\x00\xFF\x00';

write_raw_idat_data

Deletes all IDAT chunks except the first one, then sets that chunk's data to the argument it was passed, and recalculates its crc and length.

img.write_raw_idat_data(img.compress(b'\x00\x00\x00\xFF\x00\x00\xFF')) # a 2x1 image with all blue pixels

Indexes

Any function that takes an index can be passed either the numerical index of the chunk (so 0 for IDAT, 1 for the second chunk, -1 for the last chunk, etc..), or a list that exists in img.chunks, or a 4-length bytes object that is equal to the label of one of the chunks in the image. If a chunk with that label appears more than once, the first one will be used

Color types

The following color types are defined

GREYSCALE = 0
RGB = 2
PALETTE = 3
GREYSCALE_WITH_ALPHA = 4
RGB_WITH_ALPHA = 6

Some examples

In this example, we will write draw a red circle in a new 100x100 file

img = libpme.PME();
img.width = img.height = 100
img.color_type = libpme.color_types.RGB
newdata = b''
for y in range(100):
	newdata += b'\x00' # to indicate that we are writing raw pixel data, not differences
	for x in range(100):
		if (x-50)**2 + (y-50)**2 < 50**2: # If we're within 50 pixels of the center of the image
			newdata += b'\xFF\x00\x00' # red
		else:
			newdata += b'\xFF\xFF\xFF' # white
img.write_raw_idat_data(img.compress(newdata))
img.save("output.png")

The output should look something like this:

In this example, we will draw a red sine wave on a new 100x100 file

import math
img = libpme.PME();
img.width = img.height = 100
img.color_type = libpme.color_types.RGB
newdata = b''
for y in range(100):
	newdata += b'\x00'
	for x in range(100):
		if abs(math.sin(float(x) / 5) * 50 + 50 - y) < 5:
			newdata += b'\xFF\x00\x00' # red
		else:
			newdata += b'\xFF\xFF\xFF' # white
img.write_raw_idat_data(img.compress(newdata))
img.save("output2.png")

The output should look something like this:

Changelog

1.2

You can now do libpme.compress instead of having to write libpme.PME.compress

1.1

Added the damaged argument to the constructor. If damaged is set to true, it will allow you to open potentially damaged png files, which maybe don't have IDAT as the first chunk, or have invalid CRCs