Lourens Veen wrote:
> 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.
It appears to be the right spot to store a copyright notice and the alt=
information, especially for the blind.
> > 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.
I will try PNG (I already use ping a lot so PNG should be easy to
Some cameras, like the Nikon Coolpix, feature a lossless mode and some
software has a 100% quality type setting, where I thought it turned off
compression, but did not know there were several types of compression.
In the save as, there is a DCT method that says it is a trade off
between speed and quality. On an 800 MHz AMD chip, speed does not seem
to be a problem so which of the three settings gives the best quality?
> > 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 will try that.
> > 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.
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