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Photos transferred are not exactly the same by CRC32.. Why?

GTang 12 years ago • updated 11 years ago • 4
Hello, I have recently purchased your app so I may transfer my camera rolls from my iPhone 4 to my iPhone 4S and from my iPad to my iPad 2.

I found your app to be simple to use and elegant. The transfer (in Wi-Fi) was seamless. Well done!

A problem I am having is that the transferred photos were NOT exact duplicates of the originals, even across compatible devices. For example, from iPhone 4 to iPhone 4S, I found that the CRC32 of the transferred photos and videos differ from CRC32 of the originals. This is despite full preservation of resolution sizes (in pixels) and even metadata.

Can you explain what alterations (even subtle ones) were made during the transfer? Is it some hidden media data? Are the pictures or photos otherwise "truly" identical?
Photo Transfer App - Android 12 years ago
Hello,

Thanks for posting your question. I'm sorry it took us long to respond to you. This difference in the files may be because we have to read the bytes from the photo file to recreate it. The API that Apple provides does not give direct access to the file but rather to the bytes of the photo file. We read those bytes exactly as they are stored -making an identical copy of the original- but it is likely that this process results on a difference when you run a CRC check even though the contents of then files are the same.

I hope this is helpful. Let us know if you have any other question or comment. We hope our app is useful to you.

Regards,

Enrique
Photo Transfer App
GTang 12 years ago
Hi, Enrique,

If every byte of a file is recreated exactly, how can the CRC be different? Are the meta data somehow subtly altered? as far as I know, CRC should not be altered because the file has been transferred over by different means.
Cody Baker 11 years ago
Could it be when the name of the file is changed after the transfer?
If everything else, in the photo or video, is not changed, the only thing left is the name of the actual file. Is that part of the CRC calculation?

Enrique has stated in other posts, they can't keep the original name of the file because Apple's iOS doesn't support it.
GTang 11 years ago
No, CRC is filename independent. Can Enrique give some theories as to why this is so. This is very important to me as I entirely determine what I can and cannot use your app to trasnfer Photos from my Camera Rolls.

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