Difference between Fine and Normal?

[nossie]

I'm not sure what you mean by 10/12 for a compression setting... However, in GIMP, I opened up a decent size jpeg photo and saved it about 13 times at 100% quality. I can't tell any difference at all. I've never been able to see any degrading in quality from editing JPEGs.

Some people say everytime you save the jpeg, the quality decreases. Others say the opposite. I'm not sure what to believe. All I know is I can't notice a difference.

10-12 is a jpeg/photoshop quality setting when saving. 1 to 12, 1 is the lowest.

When you saved at 100% in GIMP did you; A: make any changes to the file before saving? B: check the file size afterwards?

Everytime you save / resave a file in jpeg it does lose more information. Start out with a small file like 400x300 and you'll see the problem more pronounced.

 

[fatsheep]

10-12 is a jpeg/photoshop quality setting when saving. 1 to 12, 1 is the lowest.

When you saved at 100% in GIMP did you; A: make any changes to the file before saving? B: check the file size afterwards?

Everytime you save / resave a file in jpeg it does lose more information. Start out with a small file like 400x300 and you'll see the problem more pronounced.

A: I didn't think making changes had anything to do with it. I always make copies of the original before doing any editing so this doesn't really affect me as much since I'm not degrading the orignal.
B: No I didn't

I downsized the pic to 800x 536 and hammered control + s for solid 30 seconds or so (probably making about 2-3 saves a second here). Then I closed it out, opened it up and hammered control + s for a while longer. I managed to increased the file size from 367.5 Kb to 370.5 Kb but the images look identical still. Absolutely no noticeable loss in picture quality whatsoever.

What you say about JPEGs discarding information with each save may well be true. However, in the typical photo editing situations I encounter, I don't think it has much effect. I would go to an XCF (gimp) format if I had to do a lot of editing anyway...

 

[yeti]

JPEG uses lossy compression, which is visible pretty much always if you know where to look for it. You can always tell an image has been stored as a JPEG or similar format.

JPEG does not store pixels directly (the way the TIF format does), but rather does some form of Fourier transform on the image and then merely stores the different "frequencies" present. In general you will need to store frequencies with wavelengths twice as high as the distance between the light-sensitive elements of your CCD in order to encode ALL detail you camera captures (Sampling Theorem). Pretty much noone does that, as that is not much different than storing the file in TIF or RAW, but you can throw out some high frequencies without degrading the image in a very perceptible way. In Fine mode you will throw out fewer of them, and thus encode more of the detail within the image, while in Normal you might throw out more and end up with less-detailed image.

Here is an example:

Suppose you are taking a picture of a basketball. Normal mode might not store that the ball has black stripes, as this is hardly visible from the distance you are shooting at, and thus will make your basketball look more like an orange sphere, while Fine will keep more of that detail and you will have an easier time saying "That's a basketball". Looking at the two pictures, however, they will both bear the general shape of a basketball, but one will have slightly more contrast and detail.

The devil is in the details, as they say.

Even if your picture looks acceptable straight out of the camera, you will still see "ringing" effect of areas of high contrast, like edges of things, as well as in places where there are very subtle differences in color, like areas with shadows. These are called compression artifacts, as previously mentioned, and they become more and more pronounced the higher the compression (the less detail you store). To make matters worse, if you post-process your images, you will end up uncompressing your image, altering it in some way, and then recompressing it, meaning that the compression artifacts of the first save start getting compression artifacts of their own. Thus every time you resave a JPEG, you degrade its quality.

I hope this helps you.

 

[nossie]

I hammered control + s for solid 30 seconds or so (probably making about 2-3 saves a second here).

In nearly all applications that work with files there is a flag that is set when you make a change, here's a psuedo example of code
If UserDidSomething = True then FileWasChanged = True
then when saving or closing the file
If FileWasChanged = True Then ask user to save file

So tapping CTRL + S won't do anything unless that flag is set to true, and the flag won't be set to true until you make a change to the file that needs to be saved.
You might notice this when you just open the file and close it straight away, it won't ask to save because you didn't change anything. Make any tiny change and it will ask to save.