On 2 Jul 2005 at 12:47, Brenda Dencer wrote:

> I don't have an answer but I do have a question for the group.  I heard that
> each time .jpg files are opened and then closed, the quality degrades
> slightly.  I don't remember what the best choice was, but I think .bmp was
> what this person said.  Is that correct?
>
> Brenda

  It's not the closing, but the re-writing.  To "open" the file, a program
has to undo the compression to get the correct pixel values to show on the
screen.  But because the compression process can discard some detail or
colour fidelity, this uncompressed bitmap is very slightly different from
the original.  So if you SAVE it as a JPEG, the compression gets re-done,
starting from a slightly degraded image, and so the result will almost
certainly be a bit further degraded.

  IF you have a camera which provides your shots in JPEG format, you
absolutely do not want to save a viewed image back over top of the file from
the camera!

  BMP is the Windows image format which closely corrersponds to what most
digital camera makers call "RAW" -- the uncompressed actual pixel values.
Not all cameras offer this choice, but if you can afford one that does,
that's a very good choice.
  The one other format to consider is TIFF, also offered by some high end
cameras.  I believe it does some compression to save storage space, but
doesn't degrade the image the way JPEG can.

David Gillett

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