nospam
In article <2gav2atofpmjbalmsv41bkbovlvc6p8s0i@4ax.com>, Eric Stevens <eric.stevens@sum.co.nz>wrote:

Eric Stevens
4. To confirm the point I took a screen shot. See https://dl.dropboxusercontent.com/u/31088803/Lab%20test%20Screen.jpg Note the histogram. All of the pixels appear to be down at the zero end of the scale: that is, jet black.

nospam
notice the differences at the left end of the histogram.

however, this is about round-tripping from rgb to lab and then back. you only did half.

Eric Stevens
Fir comment. I've just compared the original JPG with a copy -->Lab -->JPG again. JPGs are RGB are they not?

nospam
usually but not always

Eric Stevens
Anyway I still got an apparently all-black screen and here is the screen shot showing the histogram: https://dl.dropboxusercontent.com/u/31088803/Lab%20test%20screen%202.jpg

An even tighter all-black bar than previously.

nospam
this is all explained in the link you gave. try reading it.

Eric Stevens
You don't have to be rude. Try reading it yourself and then explain step by step what you think he is proposing.

nospam
i'm not trying to be rude. the answers really are in the link and i've said this many times already.

Eric Stevens
The only conclusion I can reach is that there is no difference between a PSD created from a RGB file and a PSD created from the same image when it has first been converted from RGB to Lab.

nospam
there is. it may not be a huge difference, but there is a difference.

Eric Stevens
As soon as you do anything in Photoshop there is a difference due to rounding errors (quantization) but is this all you are objecting to?

nospam
you do realize that adds up, right?

compare a high quality jpeg with the original and you'll see black as you did above, but there are definitely differences (and actually, less of a difference than the rgb-lab conversion).

Eric Stevens
What is the difference with rgb-Lab-rgb conversions and what causes them?

nospam
read the link and pay attention to andrew rodney.

ignore marguilis, not just in that link but in general. he has claimed that 16 bit editing was a waste, which it absolutely is not. i dunno if he still claims it but he probably does.

do you see people arguing to edit jpegs? of course not.

Eric Stevens
What exactly do you mean by that?

nospam
you say you can't see a difference in an rgb-lab-rgb conversion and you subtracted them and saw all black, therefore, you have deemed them to be equivalent.

if you do the same for jpeg, you will also not see a difference, and if you subtract, you'll also see all black. therefore, a jpeg should be equivalent to an original raw.

the reality is that there *is* a difference. you might not consider the difference to be significant (and indeed it is is very small), but there *is* a difference, therefore it is *not* lossless.

bottom line: rgb->lab->rgb offers no benefit (other than possibly contrived edge cases nobody will ever encounter).