Peter wrote:
I found how to change the comment in a JPEG when I save as, and how to
change the default for the save as, but I could not find how to view
the comment in an input file or edit the comment prior to performing a
normal save.
I think this is built in the save routines, I don't think there is a
separate comment attached to an image. It sounds like a useful feature
to me though.
I found how to change the compression level in a JPEG when I save as
but could not find out how to view or change the compression level prior
to a normal save. I think, from what I see of the JPEG format, the
compression level is not stored anywhere but it could be indicated by
displaying something like the colour range (I do not remember the JPEG
term, it is the part that indicates the 24,000,000 colours are squashed
down to an actual range of n colours.
There is no such thing. Indeed, JPEG uses vector quantisation, but it's
a little bit more complicated than just making a palette. The encoder
has a q factor (ranging from 0.0 to 1.0 in Gimp) which determines the
quality, but other than that 0.0 is worst and 1.0 is best, it's
undefined and may vary in meaning from encoder to encoder. I think that
if you Save As with a certain q factor, and the save the same file again
with Save, you get the same q factor again. If you want to save with a
different q factor, use Save As.
I also could not find out how to change the default JPEG quality setting
from .75 to 1 and only discovered the setting when I found a few files
were damaged after minor adjustments.
damaged. You mean they were saved at a lower quality? Please note that
a q factor of 1 does not mean lossless compression. There is indeed a
lossless JPEG mode, which is prediction based and totally different from
the lossy DCT and vector quantisation scheme that is used in Gimp. AFAIK
Gimp cannot save lossless JPG's. If you want to save your files
losslessly then I recommend PNG, it has good compression, is an open
standard and works with web browsers.
Which leads to the next thing. If I start with a JPEG that has zero
compression then edit the file then save as a jpeg with 25% compression,
I see the change image from editing but not after compression. Is it
possible to view the file as saved to disk, without having to close the
file then open the saved file?
Yes, when you save it there is checkbox preview in image window. Make
sure it's checked, and watch the image window. Note that a q factor of
0.75 does not mean 75% filesize or 75% of the quality. It is a
meaningless number.
I am looking forward to replacing some more proprietary software with
open source and Gimp is 98% of what I need for image editing.
Good! Feel free to ask.
Lourens
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