Quantum Comparison of Large Images: If you see them, they are the same; if you don't see them... they are different! :O

[Teknikaz-Unidaz]

Hello everyone.

Disclaimer: I'm probably doing something wrong. If someone could tell me why this is happening, I'd really appreciate it.

Description:
Just like the title, I have two large PNG files to compare. Both contain the same image, one uncompressed and the other losslessly compressed. When I select both files and send them to WinMerge, I get a warning that they are too large to compare. It gives me two options: compare and view the contents of both images, or just compare both files and report the result. I choose the first option, and it tells me they are identical. I choose the second, and it tells me they are different.

This has never happened to me before. Has WinMerge failed?

I normally never leave options permanently checked, but if I had done so in this case, it would always report that these files and perhaps others are different, when in reality they are the same. Does that mean WinMerge performs different types of comparisons, or are these files the exception? I'm very concerned because we use this software intensively to remove duplicate files (and since we blindly trust the program, we never look at the file creation date, and perhaps at some point we've deleted the original and kept the copy because they're "identical," so we haven't bothered with it). If something isn't right, we may be backing up worse or different versions than the ones we deleted from our storage devices.

What could be happening?

I tried to attach both files but there is a 10MB limit, if it is necessary for analysis just let me know and I could share them via Google Drive if necessary.

[sdottaka]

When comparing files larger than 64 MB, WinMerge displays a message box with the message "Show only comparison result (not file contents)?".

If you click Yes, WinMerge compares the files in binary mode by reading them in small chunks to reduce memory usage. Since it does not treat them as images, any differences in metadata will cause the files to be marked as Different.

If you click No, the entire files are loaded into memory. If the files are images, WinMerge compares them pixel by pixel in the image comparison window, ignoring metadata.

This explains why the comparison results may differ.

Currently, image comparison ignores metadata, but we’re considering improving this to notify users when the image data matches but the metadata differs.

[Teknikaz-Unidaz]

Thank you very much for the reply. That clarifies what happened. I also agree that it would be better to notify users that the file comparison will be binary-only for long files, or that the comparison result dialog box not only generically indicates whether they are different or identical in some way, but also clearly indicates what exactly is being referred to: whether at the binary level or at the pixel level, as in this case, or even performing both comparisons at the same time (even if you don't choose to view both images side by side) and displaying the results of both comparisons in the same results dialog box (given that binary comparison isn't apparently very computationally expensive). This way, there would be no confusion about it. It would be great if implemented; it would be very helpful for certain users, as in my use case.

Thank you, Cheers! :D