JPEG and color management

>So everyone accepts and agrees with the conclusions in (post #53) ?

"Everyone"?

Well, I, for one, am not going back to see what #53 says. Even so, I don't think you've made a single meaningful point so far. :/ Does it even matter?

Thank you :)

An uncompressed version of the ColorChecker is here
on p.10:
http://www.fho-emden.de/~hoffmann/camcal17122006.pdf

Also available on request as sRGB-EPS or Lab-EPS.
This may help to avoid discussions about the accuracy
of the JPG version.

Best regards --Gernot Hoffmann

> One, I've still never seen a profiling target as jpeg - these links are to test images, not profile targets.

It's not as if we wouldn't know the difference. But test images should have accurate colors, too. Anyway, here is a profile target in JPEG format (as you asked for a link, in case one comes my way):

http://www.drycreekphoto.com/images/Charts/MacbethCC-sRGB.jpg

So everyone accepts and agrees with the conclusions in ( post #53) ?

Mark,

You remind me of one of the two bald men fighting over a comb . :)

But I must admit you've held my attention for a lot longer that I would have ever thought I would spend on the subject of JPEGs. :D

Peter Figen must find this amusing. :)

Mark,

Thanks for the feedback. I must have misunderstood what you were saying. Just wanted to make sure we are on the same page. Seems we are.

Lou

Lou Dina, I ran your test and confirm your results.

>So, unless I am missing something, I cannot agree with your conclusion that the colors shift in the gray patches.

I think we misunderstand each other. I can't see where I said in my conclusion, there would be "color shifts in gray patches"

Mark,

My tests agree with what Gernot stated (if I understood him correctly)....that blocks of neutral gray areas remain unchanged, even with high jpg compression levels.

Did you try the test that I ran? Creat a NEW file, sRGB, 200 ppi with a black to white gradient. Posterize it to give you 21 steps of gray. Use this as your "control". Now duplicate this file and save the duplicate as a jpg. Close and reopen the jpg file (just to be sure you are opening jpg compressed data) and look at it under high magnification. Go over it with the eye dropper set to point sample. I don't find ANY portions of ANY gray patch that are not exactly the same as the original file, with the exception of artifacts at or very close to the edges. The grays do NOT shift at all, at least in my experiment.

So, unless I am missing something, I cannot agree with your conclusion that the colors shift in the gray patches.

Lou

Thanks Peter Figen for commenting on this from a practical point of view.

So I guess the bottom line of this thread is, that - independently of any JPEG artefacts near edges or in areas with high local contrasts - there can be very subtle shifts in color, even for very large single-colored areas. However the color shift we could demonstrate was only (0,+1,+2) and a professional confirmed that he has known a single incidence of perceptible color shift for real-world images. A JPEG-Expert has confirmed, that neutral grays are much less likely to be affected by any color shifts.

For me, all this was unexpected though, because I thought that for maximum quality JPEGs, large single-colored areas would always have 100% identical colors.