Decoding malformed iPhone generated PNG throws 'Bad compression method for ZLIB header: cmf=237' #410

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1. JimBobSquarePants

   self-assigned this

   on Dec 23, 2017
2. JimBobSquarePants

   added

   formats:png

   bug

   on Dec 23, 2017
3. JimBobSquarePants commented on Dec 23, 2017

   

   JimBobSquarePants

   on Dec 23, 2017

   Last edited by JimBobSquarePants

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   Hi @zeus82

   Thanks for the detail and the input image. It turns out that image is not a valid png.

   

   http://iphonedevwiki.net/index.php/CgBI_file_format

   There's quite a few differences between the format

   • extra critical chunk (CgBI)
   • byteswapped (RGBA -> BGRA) pixel data, presumably for high-speed direct blitting to the framebuffer
   • zlib header, footer, and CRC removed from the IDAT chunk
   • premultiplied alpha (color' = color * alpha / 255)

   @antonfirsov @tocsoft @dlemstra

   Theoretically it looks like we could potentially handle these changes with some updates to our decoder based on the following, it looks like a lot of work though for all our different color types:

   http://www.axelbrz.com.ar/?mod=iphone-png-images-normalizer

   I don't know if we should attempt this for V1 though. What do you think?
4. JimBobSquarePants

   removed

   bug

   on Dec 23, 2017
5. tocsoft commented on Dec 23, 2017

   

   tocsoft

   on Dec 23, 2017

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   seeing as chrome doesn't seem to even seem to render it then i'm inclined to think we should defiantly push it past v1 and even then see how much of an issue its causing in the future and make a another call then.

   👍React with 👍1Reacted by James Jackson-South
6. JimBobSquarePants commented on Dec 23, 2017

   

   JimBobSquarePants

   on Dec 23, 2017

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   Ha! Doesn't it! I only have Edge installed at the mo cos I had to get my laptop replaced. That renders something but it ain't pretty. Yeah let's leave it for now then.
7. antonfirsov commented on Dec 23, 2017

   

   antonfirsov

   on Dec 23, 2017

   Last edited by antonfirsov

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   Only Windows + GDI based decoders seem do decode it properly. I consider this issue as low-priority one, definitely not a show-stopper for V 1.0.

   👍React with 👍1Reacted by James Jackson-South
8. JimBobSquarePants

   added this to the Future milestone on Dec 24, 2017
9. JimBobSquarePants commented on Dec 24, 2017

   

   JimBobSquarePants

   on Dec 24, 2017

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   Ok, have moved it to futures. Gonna do a little cleanup in the png decoder around the zlib headers to remove an allocation I saw that we don't need anyway.
10. vpenades commented on Jan 3, 2018

    

    vpenades

    on Jan 3, 2018

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    I would detect this specific format, but just to throw a more specific exception, like "malformed iPhone PNG detected", I think it's Apple's responsability to write proper PNGs, not the responsability of others to read their malformed files.

    👍React with 👍2Reacted by Brian Popow and AraHaan
11. antonfirsov

    changed the title [-]Image.Load<Rgba32> Results in 'Bad compression method for ZLIB header: cmf=237' Exception[/-] [+]Decoding malformed iPhone generated PNG throws 'Bad compression method for ZLIB header: cmf=237'[/+] on May 26, 2018
12. qmfrederik commented on Jun 13, 2018

    

    qmfrederik

    on Jun 13, 2018

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    These files have been processed with an Apple-variation of pngcrush.

    You can use PNGDecrush (source) to decrush the image, which can then be fed to other image libraries.

    E.g.:

    using (Stream input = File.OpenRead("34303888-73d65ca6-e705-11e7-88cf-6c16ad871172.png"))
    using (MemoryStream decrushed = new MemoryStream())
    {
        PNGDecrusher.Decrush(input, decrushed);
        decrushed.Position = 0;
    
        var image = Image.Load(decrushed);
    }

    👍React with 👍4Reacted by Vicente Penades, James Jackson-South, AraHaan and Saulius Menkevičius
13. JimBobSquarePants commented on Jun 13, 2018

    

    JimBobSquarePants

    on Jun 13, 2018

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    @qmfrederik Thanks for the input; that a really useful tool! 👍

    I'm going to get around to natively handling this one day, need to do some cleanup of the decoder first though.
14. brianpopow

    added a commit that references this issue on Apr 23, 2020

    Throw exception when malformed apple png chunk is detected (#410)

    5415bde
15. JimBobSquarePants

    closed this as completedin #3136on May 27, 2026