[extropy-chat] Re: codes in scam letters

[Mike Lorrey]

--- Emlyn <emlynoregan at gmail.com> wrote:

> This assumes that a compressed image I produced by program A will be
> bitwise identical to Compress(Uncompress(I)) using the same program.
> It isn't clear to me that with lossy compression, you can safely say
> I == Compress(Uncompress(I)). DTP professionals will agree I think.
> 
> I just tried this with a commercial program. I opened a bitmap image
> and saved it as a jpeg. I closed the program, then reopened the
> program. I opened the jpeg, did nothing to it, saved it as a new
> file,
> with exactly the same compression settings (which had stayed as
> defaults, so in effect I touched nothing).
> 
> Then I compared the two images. The windows program "comp" told me to
> go away because they were different sizes. It turns out that they
> were
> 24.8KB and 24.7KB respectively (I didn't look any closer than the
> file properties dialog).
> 
> So in this case, I != Compress(Uncompress(I)).

What you did was a double compression. Since the JPG is already
compressed, if you save it again under a new file name with the same
compression settings, it will compress an already lossy picture to
another level of compression. This is a function of the jpg compression
algorithm (i.e. GIF format won't do this, even though it is also a
compressed format.)

Mike Lorrey
Vice-Chair, 2nd District, Libertarian Party of NH
Founder, Constitution Park Foundation:
http://constitutionpark.blogspot.com
Personal/political blog: http://intlib.blogspot.com


		
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