@kawecki

although the file format specs might support whatever comes around, the question at hand is: does the rendering or painting software support it. I welcome more info on this.

TIFF and PNG for geo (heightmaps): of course, we're talking displacement here. Thanks for the addition, it completes the picture.

@haarspalter

I agree with the experiment, and with kaweckis analysis. Sort of.

I'd like to add - as can be seen when switching to Lab mode - that the color hues keep intact all over the file versions while only the luminosity gets dimples at regular intervals. That aside, nothing changes as the pixels at say (50,50), (400,200) and (750,350) keep their exact values over the experiment.

This banding is a well know JPG artifact (at least to me), and the reason why sky-and-clouds shots are better published in PNG or even GIF, than JPG. But I did not know it got worse over time. Thanks for the tip.

So I repeated the experiment, in Paintshop Pro instead. Surprise! No banding. No dimples. Well, it's there though, after a very close investigation (take difference, and increase brightness to +100 and contrast to +60 in an additional adjustment layer). But the dimples are less deep, and about 4 times wider spaced.

So, I hold the theoretical position that - at the pixel level - repeatedly loading and saving JPG will not damage the file, like repeatedly reloading a CD will not change the music or the sound quality. Changing the image and saving however, does.

But the experiments second the practical note that - at the image handling software level - repeatly loading and saving increasingly creates banding artifacts in the luminosity. This is a software flaw, not a JPG feature, and differs per tool used. Like repeatedly loading a CD will cause scratches in real life. Actually, having an end-user photo-oriented tool like PaintshopPro winning over a professionals oriented Photoshop on this, isn't even a surprise, at second thought. It's having JPG as its home market. I guess Photoshop is better in tiff handling. Or might have been improved and options added in the meantime, as BB seems to point out. Doesn't matter. The artifact is in the software, not in the JPG format.

Thats another lesson learned. Thanks for the input.

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