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Re: Binary diffs: real-world differencing?

From: <dberlin_at_dberlin.org>
Date: 2006-02-04 16:47:40 CET

>
>> - An 85kB JPEG file didn't compress at all, even when I saved at 100%
>> quality, opened the copy, saved THAT at 100% quality, and then
>> compared the first and second copies.
>
> either your program did not save at 100% or you used two different JPEG
> implementations for this.

What?

It is likely an artifact that floating point computation is not exact, and
you are performing a lossy transform (quantization) on top of it, so you
continually lose a little bit of precision.

Hint: Loading and saving the same jpeg file multiple times, in the same
program, will generally not give you the same file back.
Try it.

>> - Adding a character to a text file in a ZIP archive compressed
>> badly, with a diff almost as large as the archive itself.
>
> If it was the only file in there: this should be about correct - the
> compression state engine changes at the changed character and yields a
> different stream from there on.

It will general only yield a different stream for the window size of the
zip compression, which is 64 or 128k or something.

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Received on Sat Feb 4 16:49:01 2006

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