nospam
In article <jvOdnVF7m_WVFs_OnZ2dnUVZ_tadnZ2d@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
Post before and after shots and the subtraction.

nospam
the random image was one i shot a month ago at a dance performance and it's not for public consumption. one performer in the image was holding a guitar and that is where the visible change was. there was also some minor differences on her clothing.

the key is that anyone who claims converting to lab is lossless is mistaken.

Alan Browne
Name each process step from start to finish.

nospam
open image, duplicate (image/duplicate), convert to lab and back to rgb in the second image (image/mode) then subtract (image/calculations). look at the histogram to see the extent of the changes.