Google| Subject | Re: Is RGB to Lab lossy? - was(Re: Lenses and sharpening) |
| From | nospam |
| Date | 10/04/2014 23:08 (10/04/2014 17:08) |
| Message-ID | <041020141708289880%nospam@nospam.invalid> |
| Client | |
| Newsgroups | rec.photo.digital |
| Follows | Eric Stevens |
| Followups | Eric Stevens (6h & 57m) > nospam |
In article <dam03ahe9ga3n8em89m713beh1t6t7r4c9@4ax.com>, Eric Stevens <eric.stevens@sum.co.nz>wrote:
cmykEric StevensnospamEric StevensEric Stevensnospam
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.
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.
Fir comment. I've just compared the original JPG with a copy -->Lab -->JPG again. JPGs are RGB are they not?
usually but not always
Then what else might they be and under what circumstances?
i didn't say converting to lab was much worse.Eric StevensnospamEric StevensThe 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.
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?
you do realize that adds up, right?
Yes, and it's common to evrything you do. So why does converting to Lab allegedly make it so much worse?
i said that rgb->lab->rgb is not lossless. you may not care about the loss, but it's definitely there.
margulis is wrong.
that's part of it.Eric StevensnospamnospamEric Stevens
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).
What is the difference with rgb-Lab-rgb conversions and what causes them?
read the link and pay attention to andrew rodney.
Do you mean where he says:
"ANY colorspace conversion can cause these quantization errors (RGB to RGB as an example)."
nope.nospamEric Stevens
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.
I bet you are quoting him out of context.
<http://www.brucelindbloom.com/index.html?DanMargulis.html> ...If an example is presented that shows an 8-bit/16-bit difference, a rule is immediately created, on-the-spot, that disqualifies the image. None of Dan's original six conditions would disqualify a ProPhoto image (you can read these conditions below in section I), but it appears as though ProPhoto images are no longer acceptable. If one takes this technique to its logical conclusion, Dan's 16-bit challenge would become "When considering all images showing no 16-bit advantage, 16-bit images show no advantage."
you said you saw black when subtracting them.Eric StevensnospamnospamEric Stevens
do you see people arguing to edit jpegs? of course not.
What exactly do you mean by that?
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.
I didn't say that. Read it all again carefully. I compared an rgb-lab-rgb conversion to the original JPG.
it's *exactly* the same logic.nospamEric Stevens
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.
That is squiffy logic and it's not even a good parody of what I did.
you're position is if you can't see it then there is no difference.
no i haven't at *all*.nospamEric Stevens
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).
You have backed off considerably from your original opinion on this matter.
stop lying about what i say.
09/13/2014-
