Re: [Gimp-user] How to export jpeg without loss of quality?

[Steve Kinney <admin pilobilus net>]

> Thanks all who kindly replied to my request of help...  I understand there's a
> loss of the original size because of my resizing and scaling under Gimp..  But
> here we're talking of a 12MB file that becomes 380KB...!  Isn't it too much
> also with those considerations?  

Depends what you mean by "too much."  A very large original source file
would usually be a Good Thing, indicating a high resolution (high DPI)
image.  Squeeze 12MB of data into a 300KB container, and a LOT of
information gets lost no matter what:  More so, however, if you are
using a lossy format for the output file.

> As for the suggestion of using the png format,
> I have the original image in jpeg format...  Are you suggesting of converting
> jpg to png before modifing it with Gimp?  Or to save it, within Gimp, from jpg
> to png?  The final use of this job is not for the web, but for printing, so
> preserving the original quality is important...

Normal procedure is to start with the largest available source image
(usually the only one in the case at hand), open it with the GIMP, and
save it as an XCF file.  XCF uses lossless compression, and also saves
all image layers, text layers, masks, etc. present in images in process.
 If you have to quit editing and resume later, the XCF file will still
have all your work in progress.

Sometimes editing large images may be a slow-ish process, more or less
so as the computer used has more or less system memory and a slower or
faster CPU.  Some operations such as rotating images, scaling images, or
applying complex filters to whole images may take a while with an image
that started as a 12MB JPG.

If possible, it's best to avoid scaling the original image down until
you consider it print-ready.  Then, save the XCF file one last time
/before/ scaling down, just in case you have to go back and change
something, or want to re-use all of part of the image in other projects.
 That done, calculate the X and Y dimensions of your print image in
pixels, based on 300 per inch on the printed page (or other units per
locale), and scale it down.  Export the scaled image as PNG, and it's
ready to import into a desktop publishing tool like Scribus or etc.

Your desktop publishing application will enable you to control the page
margins, where the image (or images) appears on the page, and add lines,
boxes, text or etc. as needed.  Once your page is configured for print,
save the desktop publisher's source file in its own native format
(again, so you can change or reuse it later), then export the file to
PDF, making sure the PDF document is set to 300 DPI.  (Or other, if you
are using higher or lower resolution images.)

PDF a.k.a. Portable Document Format, could also be called Printable
Document Format, because that's what Adobe, working with printer
manufacturers, designed the PDF format for all those years ago.

The first copy printed out will be your "proof."  Examine it closely,
with special attention to color:  Printers and monitors have gotten a
lot more uniform in recent years, and with any luck your image will look
just right when printed.  But if not, open up your XCF file again, save
it with a new name (maybe add a version number to the original file
name), and work on the new XCF file.  Precise color adjustment methods
are WAY out of scope for beginners, and for advice delivered here:
Years of reading tutorials and watching videos on this subject have
given me /some/ clues but I know I have miles left to go.

As a starting point, do Layers > New from visible, and work on the new
layer rather than the "original" layers; that makes starting over as
needed way easier.  Or maybe your color problem only affects parts of
the image on one layer; if so, duplicate that layer, and work on the
duplicate.  You may find the Hue-Saturation tool, the Levels and Curves
tools useful; note that you can separately adjust the Red, Blue and
Green channels with the Curves tool.

One gotcha:  If your printed page has any text added with the desktop
publishing application (not in the GIMP image or images), there will be
print errors if the computer the printer is hooked to does not have the
fonts you used installed on it:  Typically, the program that's sending
the job to the printer will substitute a generic font for the one you
wanted.

:o)