JPG - The Lossless Format...How Much?

[Doug West]

Okay, dumb question of the day...

We've all heard that if you save in jpg, you lose data...or whatever
you want to call it.

The question is, how much and how fast?

For example, lets say I have photoa.jpg. Came straight out of my camera.

I work on it and save it and use Quality 12 (Photoshop).

How many times editing that photo before someone says,
"gee, did you use jpg?".

Hope that makes some sense on what I'm trying to ask.

I guess I feel bad for jpg, always getting a bad rep :)

Doug

[Robert O'Toole]

Good Question Doug, this question comes up in workshops all the time!

On the highest setting 12, I dont think you would really ever see a difference in normal circumstances, like viewing on a monitor. Now saving on quality 5, you would see a huge change saving just a single time.

Over the weekend I came across an ad for a Maxtor 1.5 Tb drive for $300 or so and a no name 3Tb drive for $500, I dont think saving files as Tifs and Raw really causes much of cost concern as it has in the past.

Someone out there needs to open a TIFF save on quality 12, and resave 1000 times and see how much smaller the file gets and if there is any difference...

Robert

[Robert O'Toole]

Via Google:

http://www.steves-digicams.com/techc...ober_2004.html

Its an old link but the information is interesting. Looks like on quality 12 there is not any difference in 10 saves of a sample, but on Quality 10, artifacts can be seen in the BG in 10 saves.

Robert

[Doug West]

A 1000 times, eh? Well, I'm off work the rest of the week :)

Doug

[Robert O'Toole]

Oops I meant to write 100, well if you would like to try it for 1000, that would be even more interesting. Sounds good to me:)

I just realized that you could set up an action for 10 saves and make another action to use that action 10 times, then on and on.

Better to do it manually to make sure the save count is accurate :)

Robert

[Doug West]

Well, don't know how scientific this is, but...

I took one of my tiff images and saved it as a jpg, quality 12.
Then I saved it again, with a quality of 5. That gave me a file size of 42.3

Then I ran an action that resaved that jpg file with a quality of 5, for a total
of 500 saves.

The file size was still the same on the 500th save as it was on the first, 42.3.

To these eyes, I don't see any difference between the 2nd save and the 500th.
The artifacts do show up on the first save, but after that, I don't see anything
else changing.

Doug

[William Malacarne]

I was always under the impression that a jpg does not get recompressed if no changes were made to it. So in other words you can open and close it 100's of times and as long as no changes were made then file will remain the same.

Bill M

[Robert O'Toole]

I was always under the impression that a jpg does not get recompressed if no changes were made to it. So in other words you can open and close it 100's of times and as long as no changes were made then file will remain the same.

Bill M

You are right, if you open and do not save you are leaving the file as you opened it.

AFAIK if you open and save, each time the values are rounded off so it will never make an exact copy of the origional even if you save at 12 or 100, whatever the highest quality may be.

Robert

[Robert O'Toole]

Well, don't know how scientific this is, but...

I took one of my tiff images and saved it as a jpg, quality 12.
Then I saved it again, with a quality of 5. That gave me a file size of 42.3

Then I ran an action that resaved that jpg file with a quality of 5, for a total
of 500 saves.

The file size was still the same on the 500th save as it was on the first, 42.3.

To these eyes, I don't see any difference between the 2nd save and the 500th.
The artifacts do show up on the first save, but after that, I don't see anything
else changing.

Doug

That is interesting, you would think it would round off, I wonder if the 42.3 Mb is already rounded off 42.35 actual vs 42.3 rounded, so it will not show the file size change?

Robert

[William Malacarne]

Can you take two identical files and run the test Doug did on one of them and then layer them and do a subtraction and it should show what changes were done to the file.

Bill

[Robert O'Toole]

yes good point Bill, you can add the second as a layer, then change the layer blending mode to difference (as far as I recall). I have seen this procedure done before, wish I had the link.

Robert

[Doug West]

So are you saying, convert one image from tif to jpg and leave that alone.

Then make a copy of that jpg, duplicate the background, change it to difference,
then do a Save As? Then keep doing that a 100 times...or whatever?

What do I do with the difference layer? If I try to save it as a jpg, the difference
layer overwrites the background layer.

Let me know and I'll try it out.

Doug

[Robert O'Toole]

I just tried what I posted previously and it doesnt work, there isnt enough of a change to see the difference. Sorry Doug.

I did come across this interesting link, the site gives you crops from an image saved 10 times at a setting of 8.

http://www.jeremymoore.com/GettingGo...ion/index.html

Robert

[Doug West]

Oh well, it was fun while it lasted :)

Doug