Alan Browne
On 2014.04.20, 19:00 , Eric Stevens wrote:

Eric Stevens
On Sun, 20 Apr 2014 13:41:43 -0400, Alan Browne <alan.browne@FreelunchVideotron.ca>wrote:

Alan Browne
On 2014.04.19, 09:59 , nospam wrote:

nospam
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%20copy%20copy.jpg

[3] The difference (substraction - in jpg) (aka: nospam is wrong) https://www.dropbox.com/s/yuum3sfit6e5bp1/HugoBossBeltBuckle_20140323_0002-D.jpg

[4] The difference (with sharpening on the Lab copy, jpg) (aka: test that difference works). https://www.dropbox.com/s/3uwyuwun56nc370/HugoBossBeltBuckle_20140323_0002-SD.jpg

Procedure:

-Image was loaded as raw and duplicated to a 2nd image. -First image was changed to Lab -First image was saved as TIFF (from Lab 'space') ([1] above) -2nd image was saved as TIFF (from RGB 'space') ([2] above) -Both images were re-loaded (they loaded as Lab and RGB - just as they were saved). -Copied the 2nd image and added it as a layer over the first. -Difference would not work when one layer was in Lab and the other in RGB. -Converted the first image back to RGB, then replaced the 2nd image asa layer again.

*Difference was pure black (no differences - [3] above) =======================================================>>> -Sharpened the 2nd image to verify that differences would pop out (they did) and replaced the layer over the 1st image with it. *The sharpening difference showed ([4] above) =======================================================>>> So not only were the differences invisible to the eye they were NOT AT ALL shown by differencing.

Of course you're welcome to show differently.

Eric Stevens
That's an interesting experiment. It may be that the difference shown is not due to RGB vs Lab. I suspect you might get a similar result if you compare RGB-16 bit to RGB-8 bit (to RGB-32 bit) etc. i.e. the difference is an artifact of *any* transform.

Alan Browne
I'm not saying there are no differences - there are no VISIBLE differences - even using the difference or subtract function to pull them out. If there are differences they are at such a small level as to not show. I'd venture that it would take manyconversions to maybe make something show up or drastic colour changes in the image in one mode or the other - and if you're doing that then little changes due to conversion are not worth mention.

The fact that I'm working at 16 bit is important - whatever changes occur (if any) occur in the low order bits - and that will not show on a monitor and definitely not in a print.