Eric Stevens
On Mon, 06 Oct 2014 17:42:50 -0400, Alan Browne <alan.browne@FreelunchVideotron.ca>wrote:

Alan Browne
On 2014.10.06, 17:27 , Eric Stevens wrote:

Eric Stevens
On Sun, 05 Oct 2014 22:37:46 -0400, Alan Browne <alan.browne@FreelunchVideotron.ca>wrote:

Alan Browne
On 2014.10.05, 20:55 , PeterN wrote:

PeterN
On 10/5/2014 6:57 PM, Alan Browne wrote:

Alan Browne
On 2014.10.05, 14:42 , PeterN wrote:

We went through all this some many months ago. I demonstrated clearly that the amount of 'loss' was negligible in practical terms.

PeterN
I would use the terem "color change." anstead of loss.

Alan Browne
Any change is a quality loss. Whether that is colour difference, tone, brightness, sharpness ... whatever, it's a loss.

PeterN
Then you are using a different definition of quality.

Alan Browne
Not at all. A non lossy process would have:

RGB-A -->X-format -->RGB-B

with RGB-A identical to RGB-B

But - the fact is that with Lab

RGB-A -->Lab -->RGB-B

RGB-A =/= RGB-B, therefore there was quality loss.

Eric Stevens
But hang on: we do accept a certain degree of quality loss as part of the normal process of editing. It doesn't take much manipulation to turn a smooth histogram into something like http://pe-images.s3.amazonaws.com/basics/adjustment-layers/fix-white.gif Push things a bit harder and you can get http://www.snoopy.me.uk/misc/365project/histogram/comb3.jpg or even https://aperture64.files.wordpress.com/2014/09/combing.gif

Alan Browne
The 'issue' refers to the questions: "If I take my JPG and throw it into LAB ('cause I want to do something easier done there) and then throw it back, is there a loss? Is it important?"

1. Yes. 2. Negligible.

So in a "normal process of editing" where one goes from a high quality image (raw) to the Adobe "editing space" format and then save as a:

PSD: no loss (other than editing effects) TIFF: no loss (other than editing effects) JPG: lossy

But if one went to Lab space and back along the way, then it will always be lossy even if nothing was done in Lab space.

Eric Stevens
True, but as I found in my experiments (as described again, below) the loss on conversion is close to zero. The argument is not whether or not there is any loss in going through Lab space but whether or not the loss is significant. nospam seems to equate even the smallest loss arising from Lab conversion as significant but he forgets that the fact that he has loaded the image into an editor is going to wreak considerably more damage to the original image. That's why I think he is talking nondense when he advocates not using Lab so as to avoid damage.