nospam
In article <oMOdnYZyuclKm8nOnZ2dnUVZ_vadnZ2d@giganews.com>, Alan Browne <alan.browne@FreelunchVideotron.ca>wrote:

the conversions are also not lossless, something which is trivial to prove. make the conversion and subtract from the original. if they're identical, the result will be zero, which it definitely is not, and on an image i randomly picked, it's noticeable without subtracting.

Alan Browne
I just did this on a high key light image. See these 4 images.

[1] Original (now in .jpf (JPEG2000) to save space) (aka: the Lab copy) https://www.dropbox.com/s/esuc08yizhndmvd/HugoBossBeltBuckle_20140323_0002.jpf

[2 Original ( .jpg to save space) (aka: the RGB copy)

https://www.dropbox.com/s/i2ni8bpm738y9ej/HugoBossBeltBuckle_20140323_0002%20c opy%20copy.jpg

[3] The difference (substraction - in jpg) (aka: nospam is wrong)

nospam
there's a faint line where the stitching is.

however, to really see what changed, check the histogram (command-l).

that stuff on the left means there's a difference and looks like it extends to 27 out of 255 levels. slide the white (rightmost) slider to the left and you can maximize just what was actually lost and it's more than just the stitching.

now create a new image filled with pure black and look at its histogram in case you need to see what zero really means.

aka: you are wrong.

after that, go ask anyone on the photoshop team if a lab conversion is destructive. they'll set you straight. i know you don't believe me but maybe you'll believe one of them.