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In article <uuou2atgm5l6j5rn9d47jk7mn8s927cpdk@4ax.com>, Eric Stevens <eric.stevens@sum.co.nz>wrote:

not always, since the conversion to lab and back is not lossless.

Eric Stevens
Not strictly correct:

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it is completely correct.

we went through this about six months ago, and apparently will again.

Eric Stevens
https://www.ledet.com/margulis/ACT_postings/ColorCorrection/ACT-LAB-damage. htm

">I have always thought that moving from either CMYK or RGB to Lab

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and back was a damage free process, that is, you would end up with the same color co-ordinates when you arrived back from Lab mode.

Eric Stevens
"RGB>LAB>RGB is damage free, but CMYK>LAB>CMYK is not. The damage isn't all that great, so in many images it pays to come out of CMYK so as to take advantage of LAB's strengths; sharpening, however, is not one of these cases. .... Dan Margulis"

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you clearly don't understand what you're reading, since that link agrees with what i said!

as the other posts in your link clearly show, dan margulis is wrong (as he is about a lot of things).

read the *very* next post, from chris murphy,

Converting to and from Lab has never been a damage free process.

and the one after that,

Eric Stevens
RGB>LAB>RGB is damage free, but CMYK>LAB>CMYK is not.

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I disagree. If you start out with all of the same spaces for RGB and CMYK, and use only those spaces - then convert to and from Lab, you will get some quantization errors with both.

and andrew rodney's post:

Eric Stevens
RGB>LAB>RGB is damage free

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You1re not serious are you Dan? Take an RGB file. Duplicate it. Do an RGB to LAB to RGB conversion and subtract the two. You can turn on or off the 8 bit dither. When you subtract the two and create a new document and look at the Histogram in Levels, you will see there certainly is data loss and a change. Move the sliders of the Levels Histogram over and you1ll see the effects of what differences between the two files you produced. Are you saying this isn1t data loss?

that test is trivial to do. try it yourself.

Eric Stevens
This one continues to bother me. I am still inclined to agree with Dan Margulis.

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don't. he's widely regarded as a quack. he is constantly proven wrong by numerous people, including andrew rodney, bruce fraser, chris murphy and many others.

Eric Stevens
I'm not quite sure what procedure Andrew Rodney is proposing to prove his point so,

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it's very straightforward. take an image, do an lab conversion and back and subtract the two. the result is whatever changed.

Eric Stevens
using Photoshop CC, I have carried out my own test as follows:

1. Find a JPG with a suitable range of colors. This one came from my wife's collection: https://dl.dropboxusercontent.com/u/31088803/Lab%20test%20IMG_2154.jpg I saved a copy as a PSD (see below for the reason).

2. Copy and convert to Lab. I couldn't save to JPG from Lab so I saved to PSD. See https://dl.dropboxusercontent.com/u/31088803/Lab%20test%20IMG_2154-via-Lab.jpg

3. I then loaded the two PSD files into a new file as separate layers. (1) above was the background layer and (2) was the next. I subtracted the 2nd layer from the first with the result shown in https://dl.dropboxusercontent.com/u/31088803/Lab%20test%20Difference.jpg That's right: solid black.

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it may look solid black but it isn't.

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.

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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.

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

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.

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

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).

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