[thelist] compression / uncompression

Lee Kowalkowski lee.kowalkowski at googlemail.com
Fri Jan 18 10:03:59 CST 2008

On 18/01/2008, Jason Handby <jason.handby at corestar.co.uk> wrote:
> Image file formats like JPEG use "lossy" compression; if you "compress"
> an image by saving it out as a JPEG then yes, data is lost in the
> process.
> In programs like Photoshop you can choose the quality setting when you
> save an image as a JPEG, and this determines how much data gets thrown
> away and how big the resulting JPEG file is.

Well, JPEGs always lose data, they approximate the image a bit like
you could a do 'line of best fit' on a plotted graph using a Bézier
curve.  The quality setting for JPEG just does a more detailed 'line
of best fit', which may or may not be a perfect fit, but perfection is
never guaranteed.  You normally wouldn't notice though unless extreme
settings are used.

PNG does loss-less compression just like ZIP does, but they cannot
compete with JPEG on compressed file size, they may still be too big
to email (as maximum attachment/email sizes depend on so many things).
 Although there would be no point converting from JPEG to PNG.

You should be able to cram a single good quality JPEG into a 1MB email
though (much less I would have thought), and a single 1MB email should
not be a problem for most systems.  Other techniques would be better
though as Jason says.


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