Helen wrote:
And the comments I get about not really having an image that I
think I have (several people say variations of that) is completely
over my head.  And wonder whether it has a practical significance or just a
semantic one.


It depends what format you're exporting to and what properties of the image you care about (quality, transparency, layers, etc...) It certainly does make a difference for lossy formats such as JPEG.

To make the difference obvious...

In GIMP 2.6:
1. Create an image with some sharp corners (e.g. a solid black square in the centre of a white page).
2. Save it as a JPEG, setting quality really low (e.g. to 5).
3. The image open in GIMP still has nice clean corners. It is NOT showing what the image in the JPEG file looks like (despite the window title showing "....jpg"); it is showing the image you had been working on immediately before saving, ready for you to continue work.
4. Close the image.
5. Open the JPEG file.
6. Notice the artefacts around the edges and particularly the corners of the black square.

Even in GIMP 2.6, the image seen in step 3. was *not* the image contained in the JPEG file.

In GIMP 2.8:
1. Create an image with some hard corners (e.g. a solid black square in the centre of a white page).
2. Export it to JPEG, setting quality really low (e.g. to 5).
3. The image open in GIMP still has nice clean corners. It is NOT showing what the image in the JPEG file looks like, it is showing the image you had been working on immediately before exporting (just like in GIMP 2.6, but that's more obvious now, since the window title doesn't show "....jpg"). 4. Close the image; it warns that you have unsaved data (because you do - that nice clean black square doesn't exist in the JPEG file).
5. Open the JPEG file.
6. As before, notice the artefacts around the edges and corners of the black square. But you were warned that nice clean black square hadn't been saved.

Hopefully that helps clear up the difference between what GIMP shows during the course of working on an image (including immediately after saving/exporting), and what is actually stored in the file.

Mark.
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