[Gimp-developer] jpeg quality factor.

[gg at catking.net]

On Sat, 07 Jul 2007 07:19:57 +0200, Guillermo Espertino  
<gespertino at gmail.com> wrote:

> Se only opened the image from the camera, adjusted the curves, and
> scaled it down (BTW, the downscale code should do oversamplig by
> default. It always breaks a little the edges). Until she saved, the
> image quality was good.
> Then she saved with CTRL+S, without changing the "quality" factor, and
> the picture turned out like that. Heavily compressed.

Maybe you should try adjusting the compression level on the camera, it  
maybe a simple A,B or C choice and is probably not presented as "jpeg  
quality".

Jpeg is a sensible choice for a memory limited device like a camera but  
you have to make a chioce in using it. Once the information is lost it is  
gone for ever. If you wish to readjust you colours and downscale you are  
completely altering the form of the image. This is probalby about the  
worst thing to do between two jpeg compressions. It's really not a case of  
"she only did..." . I dont think there is anything anomolous about what  
you describe.

Now getting back main point of the thread, gimp vs. PS "quality" parameter.

Thanks for the detail of the comparison. The jpeg quaility parameter is  
defined in the jpeg standard and its meaning is clear. It is possible that  
since the useful range of values tends to be 60 - 85 (and note this is not  
a percentage ) PS may be mapping this in someway in presenting it to the  
user to give a more intuitive idea of quality which is in any case rather  
subjective and difficult caracterise in a simple two digit number.

I dont use PS but my guess is that there is no real difference in the  
implementation , simply in the way this parameter is presented to the user.

Other programs give a "compression" parameter which in effect adjusts the  
jpeg compression.

Does that tie in with your experience?

Thx.