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Difference Between File Extensions in Gimp

Jan 3, 2012, 16:54
Post: #1
Difference Between File Extensions in Gimp
I wanted to know what the difference between .JPEG and .JPG were. I found this link:

http://wiki.answers.com/Q/Is_there_any_d...27.jpeg%27

Basically they are the same extension and on older computers they could not hold more than a 3 character abbreviated file extension.

But, I want know, what other extensions are relative to Gimp, or what and why is .xcf, .png, .gif, .jpeg, .jpg, the extensions always used? Are there others? What is the relevance to .xcf, and the plug ins are all saved as .scm, .gbr, etc. That part makes sense, but I have a plug in for G'MIC with a strange extension (.par2) and I cannot install it. I figure if I understood this it may help.

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Jan 3, 2012, 18:12
Post: #2
RE: Difference Between File Extensions in Gimp
The wikipedia entry is as good as any.

http://en.wikipedia.org/wiki/Image_file_formats

The 3 letter suffix is from MSDOS days when a file name was 8.3 characters and has stuck. Linux does not care, so you might also get .jpeg and .tiff

But taking tiff as an example, these files can be subdivided by other factors such as compression type, none lossless, lossy same as jpeg.

For Gimp use its native format xcf for all work in progress, you will save all layer, mask, path...etc info

When finished then think about a smaller (size) format. PNG is good because it saves transparency and is lossless compression. Jpeg will give great compression but don't try and re-edit and re-save or the image will degrade.

Gimp scripts end in scm, plugins are either binary or python (.py) the other things you mentioned are Gimp brushes etc except for your par2 file.

This is a parity file used to recover lost parts of an archive. How did you get it associated with G'mic. Go to their site and download a proper version. http://gmic.sourceforge.net/
also see: http://www.quickpar.org.uk/ thats the windows version, there is similar for linux.

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Jan 3, 2012, 21:18
Post: #3
RE: Difference Between File Extensions in Gimp
(Jan 3, 2012 16:54)Pl3th0r4x Wrote:  I wanted to know what the difference between .JPEG and .JPG were. I found this link:

http://wiki.answers.com/Q/Is_there_any_d...27.jpeg%27

Basically they are the same extension and on older computers they could not hold more than a 3 character abbreviated file extension.

But, I want know, what other extensions are relative to Gimp, or what and why is .xcf, .png, .gif, .jpeg, .jpg, the extensions always used? Are there others? What is the relevance to .xcf, and the plug ins are all saved as .scm, .gbr, etc. That part makes sense, but I have a plug in for G'MIC with a strange extension (.par2) and I cannot install it. I figure if I understood this it may help.
For the various image formats in a Gimp perspective, see http://gimpforums.com/thread-pros-and-co...ge-formats

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Jan 7, 2012, 09:15
Post: #4
RE: Difference Between File Extensions in Gimp
OK, but why when I save it as jpg it asks if I want it as 85% ? Wouldn't the quality be lowered of my pic and shouldn't the default be set to 100% ? I am trying to remember if I should be more specific, but you know what I mean right? I first must export the new file extension and then a window asks me what quality I want to save it as....

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Jan 7, 2012, 13:27
Post: #5
RE: Difference Between File Extensions in Gimp
Well, jpeg is a lossy compressed format.

If the image is from your camera you might be able to determine the quality using ImageMagick command
identify -verbose <filename>.jpg
Somewhere in the data there might be the quality setting but depending on camera, how it is reported varies. My old Samsung saves at 98% - very little compression. My even older Olympus has a value of 6, whatever that is.

Taking the Samsung, if I open in Gimp and save-as, Gimp will want to use 98% same as the original.

file sizes for the same image are this
98% 3.435 MB
85% 1.072 MB
75% 0.792 MB
10% 0.200 MB

Inspect the image and you will not see much difference between 98% and 75% after that colours tend to become 'blocky'

Do some tests yourself, the correlation between quality and size is a curve, so below about 75% not much gain in file size and quality drops off. Try that 10% mark and see the result.

Bottom line is 85% is a good compromise between quality and file size. What you do not do is, edit jpeg -> save -> edit -> save, because you have lost imformation, then some more, then some more. Use one of the lossless formats, png, tiff (but check the options) or Gimp xcf which saves with a lossless compression. Then if you want to, save as a jpeg and use whatever compression you want.

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Jan 7, 2012, 13:50
Post: #6
RE: Difference Between File Extensions in Gimp
In Gimp if you load a JPEG for editing it will by default save it with the same quality settings.

You can also set the default for new images (check box in JPEG save options dialog).

JPEG is, by design, a format that lets you make a compromise between size and quality on "natural" images (photos and some digital painting). You can use quality level 100, but the file is about the same size as a PNG of the same image. At quality level 95 you are at 50% of the level 100 size (2:1 compression) and you may never notice the difference. At quality level 80 you are at 20% of the size (5:2) and you need a very sharp eye to notice compression artifacts on some photos.

So the right compression level is whatever is enough for the intended purpose. If I do significant work under Gimp on an image, I archive it as XCF anyways, the JPEG is only a byproduct, and using quality 89 or 70 for the web is OK.

Of course images from my camera are at level 97 and I archive them that way.

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Jan 7, 2012, 14:02
Post: #7
RE: Difference Between File Extensions in Gimp
(Jan 7, 2012 13:27)rich2005 Wrote:  What you do not do is, edit jpeg -> save -> edit -> save, because you have lost imformation, then some more, then some more.
No so true, actually. This is only true of you do global editing (color/contrast), or crop. Otherwise the algorithm used is stable, and if you do local editing (add a signature, or minor cleanup) unchanged areas end up re-encoded with the very same values and there is zero loss.

And to pick nits, this of course applies to load->edit->save->close->load->edit->save->close cycles. If you do load->edit->save->edit->save then there is not loss since you are always saving from the "perfect' version in memory.

(*) unless your crop is at a multiple of eight from the previous borders due to the way the algorithm works.

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