PSD or jPEG= confused??

[MattUK]

Originally Posted by donoreo

That sounds like a bad programming choice in the software. I believe Chris's point is that according to the JPEG standard it should not have to.

I'll tell that to Mr Adobe

[Cathus]

OK,

I just did a quick test with an 800pixel jpg saved at 100% (12) quality in PS5.



I closed the file then reopened it, saved it again with a numerically different filename at exactly the same settings, no changes, just opened & saved again. I did this incrementally, 40 times.



It's not so obvious in the wasp itself but check out the background, it has definately degraded with prominent jpg artifacting. Not only that but the file size has crept up with every save for some reason. It started as 534k & ended up as 557k

Is this what you meant Chris - in terms of saving again & again with exactly the same settings - or have I not understood what you were saying?

[MattUK]

That was the test I did (though not 40 times!!)

I agree with the principle Chris is conveying, however in practice, as demonstrated by Cathus, I believe modern applications still compress irrespective of change being made.

[chris-p]

Originally Posted by Cathus

Chris, can you try not to talk in riddles - MCUs, DCTs, quantisation tables???
I for one don't have a clue what you're talking about.

Sorry! Frankly, it doesn't matter. It's all nonsense and I wouldn't get bogged down in how JPEG actually works - there are much more interesting things in life. Like watching paint dry, for example...

Originally Posted by Cathus

Is this what you meant Chris - in terms of saving again & again with exactly the same settings - or have I not understood what you were saying?

Sort of. You changed the file name, which will cause it to re-compress. You also didn't create the JPEG in Photoshop.

I've not been very clear. I did say that there was a massive caveat. Perhaps I should have said MASSIVE caveat.

If you create a JPEG in a program, from scratch, and do nothing other than saving in that same program, assuming that the software has a proper implementation of the JPEG2000 standard (not all do) you shouldn't get any compression.

In effect, this is rarely the case because we're talking about photos which are, regardless of the file format used in camera, not produced by the software that you're using to re-save.

It might be easier to go back and retract my original comment. I tried to suggest that JPEG doesn't always throw information away without going into the technical details (which are mind-numbing).

I should have just kept quiet. Or, at least, explained that it's not really relevant to photographers... Sorry!

[MattUK]

Originally Posted by chris-p

Or, at least, explained that it's not really relevant to photographers... Sorry!

That was my earlier point

[Cathus]

Originally Posted by chris-p

Sort of. You changed the file name, which will cause it to re-compress. You also didn't create the JPEG in Photoshop

don't know if it's relevant or not but I did create the jpg with Photoshop in that the original was a PSD file which I created the first jpg from then used that to open & save the next one.

But anyway, I think the confusion has now been removed

[gtirman]

Well....thanks for all the input peoples.......unfortunately i am none the wiser to the original question....is a yes or no a suitable answer ?? LOL

[GeoffWessex]

Yes and No, I think.

[OldBoy]

Originally Posted by Cathus

don't know if it's relevant or not but I did create the jpg with Photoshop in that the original was a PSD file which I created the first jpg from then used that to open & save the next one.

But anyway, I think the confusion has now been removed

I did as you did in saving a Jpeg image about 12 times, but after the third save the file size didn't increase.

[OldBoy]

Originally Posted by gtirman

Well....thanks for all the input peoples.......unfortunately i am none the wiser to the original question....is a yes or no a suitable answer ?? LOL

The answer is no, if you save it less that twelve times and yes if you save it more than this.