[Gimp-user] Re: Re: Altering comment in jpegs

[Guillermo S. Romero / Familia Romero]

[EMAIL PROTECTED] (2001-04-06 at 1217.29 -0400):
> It was easier for me to think that the file was unchanged if all you did
> was look at it.  But this means that all you have to do is to run it
> through the jpeg library and the image data is changed?

No, the problem is if you do the color space conversion. And Gimp does
that when loading, cos it has to show you RGB. Gimp does not know how
to keep data untouched, when saving it tries to put in the file the
RGB data it has, not the original data.

> Even an Image Magick "command line" comment edit would not work?

No if it only touches the comment, which is posible, and I would say
the fastest and best way to do it. If you do not need to change image
info, why do it? It is like renaming files, why do you need to open a
file to rename it? You can, but you do not need.
 
> Is the error measurable?

At least in a per image basis, differencing before and after. And with
some maths, I guess you can do a global study.

> All of these questions, sorry.  One more, however: is there a LUser
> friendly place to read about such things?

It depend in what your background is, I read a FAQ and got a nice idea
(http://www.faqs.org/faqs/compression-faq/). JPEG is based in some
conversions (which, as anything computer based, has loses) and choping
data to reduce size (downsampling components, "forgeting" high level
frequencies... aka big loses of original data).

The trick is that humans do not see the loses, but try recompresing
JPEGs and you will see it. Zoom to a JPEG or save a flat colour image
and you will also see rare things. The magic lays in not losing too
much nor using to simple input images, but using photos and losing a
bit.

GSR
 
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