Mass convert MP3 to Ogg

James Gray james at grayonline.id.au
Mon Jul 10 01:43:12 UTC 2006


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Mark A. Taff wrote:
> On Friday 07 July 2006 04:44, James Gray wrote:
>> Pupeno wrote:
>>> On Friday 07 July 2006 04:46, James Gray wrote:
>>>> [2] Again, not 100% accurate.  You *can* do lossless manipulation of
>>>> JPEG's and PNG's but not lossless conversion between the two as they use
>>>> different compression algorithms.
>>> Isn't PNG's compression algorithm lossless ? I always thought it was.
>> PNG can use either a lossless or lossy compression - it all depends on
>> how the programmer decides to implement compression.  Most
>> implementations of PNG encoding I've seen employ a lossy compression
>> algorithm as the results are "good enough" with significant benefits for
>> file size (same rationale as JPEG, and basically every other lossy
>> encoding).
>>
>> http://www.libpng.org/pub/png/pngintro.html
>>
>> Have a read - it's a good high-level overview of what PNG is and isn't,
>> and how it's implemented. :)
>>
>> Cheers,
>>
>> James
> 
> PNG *is* lossless, and the page you cited says the same thing.  Nowhere did it 
> say that lossy compression algorithms are used.  The only place they even 
> mention lossy is when taking about jpegs.
> 
> If a program were to use a lossy algorithm to create a PNG, that file would in 
> fact *not* be a PNG.
> 
> Maybe you ought to reread that page yourself?

FWIW, I *did* read it, but read the following sentence as "lossy"
instead of "losslessly":

Quote:
'Compression filters are a way of transforming the image data
(losslessly)...'

Despite the rest of the page only referring to lossless compression
algorithms (Zlib, 7-zip and deflate).  Gah. :P

Meh, so shoot me. My point still stands about conversion between lossy
formats which is what I was originally stating.  I misread a document
and sited an correct format (namely PNG) as being lossy, when in fact it
isn't.

To that end, my citation of JPEG still stands - and yes, I *know* you
can do lossless editing of JPEG's but that involves expanding them to a
lossless format as an intermediate step (whether that's done in memory
or on the file system is purely an implementation decision).

Apologies for any confusion.

Cheers,

James
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