Been banging my head on writing an apng extraction myself. It somewhat works, but I ru

P4 claims to have done it with their own php<a href="https://github.co

PNG Frame Extract Weirdness about pyapng HOT 3 CLOSED

bannsec commented on June 19, 2024

PNG Frame Extract Weirdness

from pyapng.

Comments (3)

eight04 commented on June 19, 2024

I think this file contains broken frames. Have you successfully extracted all frames within other tools?

from pyapng.

bannsec commented on June 19, 2024

P4 claims to have done it with their own php https://github.com/p4-team/ctf/blob/master/2016-09-09-asis-final/p1ng/README.md

…

On Mon, Feb 18, 2019, 8:55 PM eight ***@***.*** wrote: I think this file contains broken frames. Have you successfully extracted all frames within other tools? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#15 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AHR34KREeBZz8dQt0c9TUJiM3eQUlrzSks5vO1l5gaJpZM4bB85u> .

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eight04 commented on June 19, 2024

The decoding process in your link is not correct. They ignore the image width/height given by the APNG frame.

You can change the width/height of each frame manually by using some tools like tweakpng. After fixing each frame's width/height, you should get an identical result.

Here is a python version that will divide width/height by 10 when they are too large:

import struct
from apng import APNG, make_chunk

im = APNG.open("w.png")
i = 0
first_png = im.frames[0][0]
first_hdr = first_png.chunks[0][1]
for png, ctrl in im.frames:
    # fix header
    width = png.width
    if png.width == 1280:
        width = first_png.width
    elif png.width >= 1000:
        width = png.width // 10
        
    height = png.height
    if png.height == 768:
        height = first_png.height
    elif png.height >= 100:
        height = png.height // 10
        
    chunk_data = make_chunk("IHDR", struct.pack("!II", width, height) + first_hdr[16:-4])
    
    png.chunks[0] = ("IHDR", chunk_data)
    
    png.save("{}.png".format(i))
    i += 1