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

[Ofnuts <ofnuts gmx com>]

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:

* amount of "detail". This is directly related to the size of the image 
in pixels. If you take a 4000x3000 picture from a camera and scale it 
down to 1000*750, you have lost detail. Given identical compression 
settings, the file size is roughly proportional to the pixel count (so 
the 1000*750 is 16x smaller than the 4000*3000).

* lack of "distortion" due to compression. To compress images, the JPEG 
format drops some of the detail where is is supposed to be not visible, 
this is the JPEG "quality factor". Between 100 and  85, the lost is 
minimal, under 60, compression artifacts cna be a bit too noticeable.

In both cases, quality is related to the amount of information in the 
file, and even with the best compression algorithms, this is related to 
the file size. If you want to keep the quality, you have to keep the 
size. But there are plenty of places where top quality isn't required, 
if the picture is meant to be displayed on a HD screen it doesn't need 
to be more than 1080px high, for instance.