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

[Steve Kinney <admin pilobilus net>]

On 07/29/2018 05:45 AM, Ofnuts wrote:
> On 07/26/18 08:59, Rodolfo Medina via gimp-user-list wrote:
> > Hi all Gimp users.
> >
> > I'm new to Gimp.  I need rescaling and resizing a jpeg picture.  But,
> > after
> > rescaling and resizing it with Gimp, and after exporting it as jpeg
> > format, the
> > size of the output file is much smaller: the original 12MB become 300K...
> > Is it possible, and how, to preserve the original quality?
> >
> > Thanks for any help.
>
> "Quality" is two things:

[ etc ]

Here's some more pointers...

In the present context, "quality" has two factors, resolution and
compression.

Resolution:

Resolution is measured in DPI or "dots per inch" which means pixels per
inch (or other unit of measurement) in your image editor or on-screen
display.

72 DPI is a common default resolution for exported digital images. I
would call that a "legacy" setting, and a lower limit for most uses.

96 DPI images look good on computer screens, and most website
maintainers avoid higher resolutions (= larger file sizes) for most
images, to save bandwidth.  For quite a while 96 DPI was the native
resolution of most monitors.

Most word processor documents will be viewed on computer monitors and/or
printed with office laser printers.  150 DPI usually looks good in both
of these contexts.  Since word processor documents are often transmitted
via e-mail, and stored in bulk on desktop computers, higher resolution
images may be undesirable, again due to file size issues affecting
bandwidth and storage space.

For color printing on glossy paper, 300 DPI resolution is a well
established convention.  Resolutions higher than 300 DPI will produce
larger file sizes, but usually not result in a visible difference in the
finished product.  The user will be aware of higher resolution needs on
any occasion where they turn up.

Compression:

The JPG format uses lossy compression; the compression algorithm
'guesses' what information can be discarded without a major impact on
the appearance of the image, resulting in very substantial reductions in
file size.  Note that a JPG compression setting of "100%" quality still
uses lossy compression, producing larger file sizes with little or no
visible difference from smaller files compressed at, for instance, 85%
quality.

Another factor to consider:  When a JPG file is opened, edited, and
re-saved as JPG, another round of lossy compression 'on top of' the
already lossy compression from the original image file results in
further loss of detail.  This is one of the reasons why programs like
the GIMP "save" images in the editor's native lossless format, and
"export" images to other formats including JPG.

Where higher quality matters more than lower file size, for instance
images intended for color printing on glossy stock, the lossless PNG
format will usually be more appropriate than JPG.  The output files are
substantially larger, but with PNG what you save is what you get, with
no loss of detail except due to scaling the image up or down in size
while editing.

:o)