Re: [Gimp-user] Why does saves as JPG default to quality 85?

[Steve Kinney <admin pilobilus net>]

On 03/23/2012 10:39 AM, Keith Purtell wrote:

> I've noticed that every time I save an image in GIMP as JPG the
> quality slider bar defaults to 85. Even though I keep changing it to
> 75. 

That's odd.  Every version of the GIMP I have ever used defaulted to
the legacy Photoshop value of "75" for JPG save quality.  I usually
use 85%.  Prompted by your comments I went looking through the
config files of my local installation and did not find a variable
that I could tweak to change the default from 75 5o 85.

> If this numeric value is a Photoshop equivalent like other GIMP
> features, then 85 is probably a wasted effort. The research*I'm
> aware of* (note emphasis) says that the quality in JPG images saved
> at a level above 75 is "theoretical." In other words, the increased
> quality is there in technical terms, but imperceptible to the human
> eye. Comments?

The compression artifacts in JPG images saved at quality 100 are
usually not visible in images viewed at 1:1 scale.  At 50, they are
usually very obvious.  Between these values, a lot depends on the
content of the image.  A cloudscape or misty forest scene may look
OK at lower values, but an image with lots of strong, adjacent
contrasts eg sharp edges, shows lots of visible artifacts at higher
compression rates.  Over the years I have settled on 85 as my own
preferred compression setting for most JPG images.

Lossy compression is a trade off between resolution and file size.
Like the default 75 DPI resolution GIMP inherits from Photoshop, the
JPG quality setting of 75 is a legacy of obsolete technology - 75
DPI monitors hooked to computers with dial up network connections
and 500 MB hard drives.  Today the vast majority of monitors have 96
DPI resolution, nearly all network connections are at least 128
kbps, and it's hard to buy a hard drive with less than 100 GB of
storage.  To me it makes sense to adjust media files that live in
this environment accordingly.

75 DPI resolution is "only a suggestion" and does not degrade an
image, but lower JPG quality settings do make a real difference.
Information is lost and artifacts are introduced.  In many instances
the difference between JPG quality of 75 and 85 is visible on modern
monitors.  The savings in file size between JPG quality of 85 and 75
is not large enough, IMO, to justify the small but visible loss of
resolution.  YMMV and lots of people are perfectly happy with the
default setting of 75 for web images.

:o)

Steve