Responding to Nancy Allison, Ed. wrote:
> I agree with Art that opening and re-saving the file is your best bet. If
> possible, save in another file format than JPG, such as PNG or GIF, as
> you'll lose data as a result of compression. You're essentially "re-jpeging
> a jpeg", which throws out some pixels each time.
That's not quite accurate. JPEG never "throws out" any pixels, because that
would change the dimensions of the image itself. What happens is that it throws
out some of the information (typically color information) *about* some of the
pixels based on its area-based image compression algorithm, which assumes that
the vast majority of individual pixels in a photograph will be pretty close in
color and brightness to the other pixels that immediately surround it. Yes, it
is lossy; and yes, it does cause progressive degradation. But it doesn't do
this by reducing the number of pixels, only by reducing the amount of
information it records about each pixel.
And note that this kind of *image* compression, which is inherently lossy, is a
completely different concept than the lossless *data* compression that is used
to produce zip files (among other things). In data compression, no data is ever
thrown away; it is just stored in a more efficient format.
-Fred Ridder
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