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Lenses and sharpening



 
 
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  #721  
Old October 6th 14, 02:19 PM posted to rec.photo.digital
PeterN[_5_]
external usenet poster
 
Posts: 741
Default Is RGB to Lab lossy? - was( Lenses and sharpening)

On 10/5/2014 10:37 PM, Alan Browne wrote:
On 2014.10.05, 20:55 , PeterN wrote:
On 10/5/2014 6:57 PM, Alan Browne wrote:
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.


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

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


Then you are using a different definition of quality.


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.


It seems to me that the assumption in that logic is:
the quality of RGB-A quality of RGB-B.
LAB has a larger color gamut than RGB. If there is no processing in LAB
I would think that there would be no need for interpolation on the
return trip.

Do the round trip x + 10 times without processing and one might see a
difference. It is doubtful that there will be a noticable difference
from 10 round trips. Meanwhile there are color modification processes
that are easier to perform in LAB than RGB. I would think that if the
changes made in LAB created color outside the RGB gamut there would have
to be some interpolation. The interpolation coud mae a better image, or
it could make the changed image horrific.

In another area, I have found images to be fine with a color cast, but
when I remove the cast, to my eye the image looks horrific.

--
PeterN
  #722  
Old October 6th 14, 02:22 PM posted to rec.photo.digital
PeterN[_5_]
external usenet poster
 
Posts: 741
Default Is RGB to Lab lossy? - was( Lenses and sharpening)

On 10/6/2014 4:31 AM, Eric Stevens wrote:
On Sun, 05 Oct 2014 21:05:19 -0400, nospam
wrote:

In article , PeterN
wrote:


I gaae him some common uses. He typically uses "edge case' to give him
wriggle room.

wrong again.

what i call an edge case is an edge case and what you're calling common
can be done *without* lab more easily and with better quality results.

in other words, you're blaming others for your own lack of knowledge
and unwillingness to learn.

Well then explain with facts and detail.
Warning. I have Dan's book and will use it as a reference.


that's your problem. dan is wrong and reading his books has led you
astray.

if you read other books, you'll see that they consistently prove just
how much of an idiot dan really is. i've mentioned two such books in
this thread and other books in other threads.


You seem to have a thing about Lab color. Your comments are quite
unbalanced. You don't seem to have a good comment about *any* aspect
of it. What happened to you?


He just likes to argue. Even, when as pointed out earlier, it's against
his own argument.

--
PeterN
  #723  
Old October 6th 14, 02:55 PM posted to rec.photo.digital
Alan Browne
external usenet poster
 
Posts: 12,640
Default Is RGB to Lab lossy? - was( Lenses and sharpening)

On 2014.10.06, 09:19 , PeterN wrote:
On 10/5/2014 10:37 PM, Alan Browne wrote:
On 2014.10.05, 20:55 , PeterN wrote:
On 10/5/2014 6:57 PM, Alan Browne wrote:
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.


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

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


Then you are using a different definition of quality.


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.


It seems to me that the assumption in that logic is:
the quality of RGB-A quality of RGB-B.
LAB has a larger color gamut than RGB. If there is no processing in LAB
I would think that there would be no need for interpolation on the
return trip.


It's not about colour gamut since the "larger gamut" of LAB can't
inherit anything better from the lesser gamut in the RGB image. If it
was missing in the RGB it won't magically appear in the LAB. You'll
just have more room to maneuver in the LAB version when you edit there.

It's not about "need for interpolation" - it's about what changed.
Any change is loss. Whether it occurred in step RGB- Lab or Lab-RGB
is not relevant to the end product.

If the end product changed then there was loss.

Any deviation from the original, whether more or less colour, lighter or
darker tone, etc. is loss of information (quality) from the original.
Period. There is no interpretation to do. It's a strictly technical
thing. From a quality standpoint it is loss.

*to re-iterate: this is all quite pedantic as the actual amount of loss
(change) is not visible in practical terms, edge cases aside.

--
Among Broad Outlines, conception is far more pleasurable
than “carrying [the children] to fruition.”
Sadly, “there’s a high infant mortality rate among
Broad Outlines—they often fall prey to Nonstarters.”
"Bestiary of Intelligence Writing" - CIA

  #724  
Old October 6th 14, 04:33 PM posted to rec.photo.digital
Martin Brown
external usenet poster
 
Posts: 821
Default Is RGB to Lab lossy? - was( Lenses and sharpening)

On 06/10/2014 14:19, PeterN wrote:
On 10/5/2014 10:37 PM, Alan Browne wrote:
On 2014.10.05, 20:55 , PeterN wrote:
On 10/5/2014 6:57 PM, Alan Browne wrote:
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.


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

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


Then you are using a different definition of quality.


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.


It seems to me that the assumption in that logic is:
the quality of RGB-A quality of RGB-B.
LAB has a larger color gamut than RGB. If there is no processing in LAB
I would think that there would be no need for interpolation on the
return trip.


The problem is not one of interpolation but that there are unavoidable
minor rounding errors in the nonlinear transform from RGB to CIELAB and
also on the way back due to the finite representation of the results. See:

http://en.wikipedia.org/wiki/Lab_color_space

There is no assumption here beyond the very definition of a lossless
transform. Applying a lossless transform and then its inverse to the
result you must be able to get to back your original image *EXACTLY*.

Do the round trip x + 10 times without processing and one might see a
difference. It is doubtful that there will be a noticable difference
from 10 round trips. Meanwhile there are color modification processes
that are easier to perform in LAB than RGB. I would think that if the
changes made in LAB created color outside the RGB gamut there would have
to be some interpolation. The interpolation coud mae a better image, or
it could make the changed image horrific.


It isn't interpolation - if anything it is a quantisation effect with
very different steps in CIELAB than in RGB. See for example:

http://en.wikipedia.org/wiki/Lab_color_space

Some pixel values may converge to an attractor on the first or second
pass around the loop and after that do not change at all. A few unlucky
ones may drift away from the original colour (but probably not by much).

Out of gamut values pretty much end up clipped against the nearest RGB
boundary on a minimise the absolute perceived colour error basis.

In another area, I have found images to be fine with a color cast, but
when I remove the cast, to my eye the image looks horrific.


CIELAB does a better job of managing just noticeable visual differences
and allows better adjustment of visual saturation and lightness.


--
Regards,
Martin Brown
  #725  
Old October 6th 14, 06:03 PM posted to rec.photo.digital
Alan Browne
external usenet poster
 
Posts: 12,640
Default Is RGB to Lab lossy? - was( Lenses and sharpening)

On 2014.10.06, 11:33 , Martin Brown wrote:
On 06/10/2014 14:19, PeterN wrote:
On 10/5/2014 10:37 PM, Alan Browne wrote:
On 2014.10.05, 20:55 , PeterN wrote:
On 10/5/2014 6:57 PM, Alan Browne wrote:
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.


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

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


Then you are using a different definition of quality.

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.


It seems to me that the assumption in that logic is:
the quality of RGB-A quality of RGB-B.
LAB has a larger color gamut than RGB. If there is no processing in LAB
I would think that there would be no need for interpolation on the
return trip.


The problem is not one of interpolation but that there are unavoidable
minor rounding errors in the nonlinear transform from RGB to CIELAB and
also on the way back due to the finite representation of the results. See:



Excellent point.


--
Among Broad Outlines, conception is far more pleasurable
than “carrying [the children] to fruition.”
Sadly, “there’s a high infant mortality rate among
Broad Outlines—they often fall prey to Nonstarters.”
"Bestiary of Intelligence Writing" - CIA

  #726  
Old October 6th 14, 10:27 PM posted to rec.photo.digital
Eric Stevens
external usenet poster
 
Posts: 13,611
Default Is RGB to Lab lossy? - was( Lenses and sharpening)

On Sun, 05 Oct 2014 22:37:46 -0400, Alan Browne
wrote:

On 2014.10.05, 20:55 , PeterN wrote:
On 10/5/2014 6:57 PM, Alan Browne wrote:
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.


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

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


Then you are using a different definition of quality.


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.



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/ba.../fix-white.gif
Push things a bit harder and you can get
http://www.snoopy.me.uk/misc/365proj...gram/comb3.jpg
or even https://aperture64.files.wordpress.c...09/combing.gif

The production of histograms like the first one is common and
generally acceptable. The second histogram is worse but even then may
be acceptable. Only the last one is so bad that it will nearly always
be unacceptable. The point of all this is that some degree of quality
loss is virtually inevitable as soon as you start to manipulate an
image.

In the context of the present discussion, the question is, does the
conversion to Lab colour incur any more damage than one can expect in
the course of ordinary editing? My understanding of nospam's claim is
that it does. My (admittedly limited) experience with it suggests that
conversion to Lab causes no significant damage; certainly less than I
am going to inflict on the image by the changes I want to make.

As to the extent of the damage, I can only refer to my original
experiment described in Message-ID:


------------------------------------------

This one continues to bother me. I am still inclined to agree with Dan
Margulis. I'm not quite sure what procedure Andrew Rodney is proposing
to prove his point so, 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/...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/...54-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/...Difference.jpg
That's right: solid black.

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

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.

I'm not wedded to the perfection of the method I have used and I would
be interested to hear from anyone who has a meaningful criticism.

--------------------------------

and my later comments in Message-ID:


========================

I've just compared the original JPG with a copy -- Lab
-- JPG again. JPGs are RGB are they not? Anyway I still got an
apparently all-black screen and here is the screen shot showing the
histogram:
https://dl.dropboxusercontent.com/u/...screen%202.jpg

An even tighter all-black bar than previously.

=============================

The histogram of the JPG(RBG) --- Lab --- PSD when compared with
JPG(RBG) --- PSD shows only very slight evidence of differences
between the two.

However JPG(RBG) --- Lab --- JPG(RBG) appears to be identical to the
original JPG(RBG).

So far I don't think I have found any evidence of damage worth
worrying about.
--

Regards,

Eric Stevens
  #727  
Old October 6th 14, 10:42 PM posted to rec.photo.digital
Alan Browne
external usenet poster
 
Posts: 12,640
Default Is RGB to Lab lossy? - was( Lenses and sharpening)

On 2014.10.06, 17:27 , Eric Stevens wrote:
On Sun, 05 Oct 2014 22:37:46 -0400, Alan Browne
wrote:

On 2014.10.05, 20:55 , PeterN wrote:
On 10/5/2014 6:57 PM, Alan Browne wrote:
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.


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

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


Then you are using a different definition of quality.


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.



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/ba.../fix-white.gif
Push things a bit harder and you can get
http://www.snoopy.me.uk/misc/365proj...gram/comb3.jpg
or even https://aperture64.files.wordpress.c...09/combing.gif


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.


The production of histograms like the first one is common and
generally acceptable. The second histogram is worse but even then may
be acceptable. Only the last one is so bad that it will nearly always
be unacceptable. The point of all this is that some degree of quality
loss is virtually inevitable as soon as you start to manipulate an
image.

In the context of the present discussion, the question is, does the
conversion to Lab colour incur any more damage than one can expect in
the course of ordinary editing? My understanding of nospam's claim is
that it does. My (admittedly limited) experience with it suggests that
conversion to Lab causes no significant damage; certainly less than I
am going to inflict on the image by the changes I want to make.

As to the extent of the damage, I can only refer to my original
experiment described in Message-ID:


------------------------------------------

This one continues to bother me. I am still inclined to agree with Dan
Margulis. I'm not quite sure what procedure Andrew Rodney is proposing
to prove his point so, 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/...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/...54-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/...Difference.jpg
That's right: solid black.

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

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.

I'm not wedded to the perfection of the method I have used and I would
be interested to hear from anyone who has a meaningful criticism.


I'll try your method above when I have a chance. I've gone through this
exercise in the past and the difference (by subtraction) was visible
(faint, but unmistakable).

https://www.dropbox.com/s/3uwyuwun56...02-SD.jpg?dl=0

The only issues a

1. Lossy formats. Repeatedly opening and then saving a JPG at a lower
quality level will increase loss. This will be visible at some point.

2. Format conversion. For all the mentioned reasons, including the
quantization error the Martin Brown pointed out, there is a change in
the image and therefore it is quality loss. There is no other term.

=============================

The histogram of the JPG(RBG) --- Lab --- PSD when compared with
JPG(RBG) --- PSD shows only very slight evidence of differences
between the two.


The better way to see is to do image subtraction. While a histo may
bear witness to change, an imaage subtraction will always bear witness.


However JPG(RBG) --- Lab --- JPG(RBG) appears to be identical to the
original JPG(RBG).

So far I don't think I have found any evidence of damage worth
worrying about.


Never said different.

--
Among Broad Outlines, conception is far more pleasurable
than “carrying [the children] to fruition.”
Sadly, “there’s a high infant mortality rate among
Broad Outlines—they often fall prey to Nonstarters.”
"Bestiary of Intelligence Writing" - CIA

  #728  
Old October 7th 14, 12:44 AM posted to rec.photo.digital
Eric Stevens
external usenet poster
 
Posts: 13,611
Default Is RGB to Lab lossy? - was( Lenses and sharpening)

On Mon, 06 Oct 2014 17:42:50 -0400, Alan Browne
wrote:

On 2014.10.06, 17:27 , Eric Stevens wrote:
On Sun, 05 Oct 2014 22:37:46 -0400, Alan Browne
wrote:

On 2014.10.05, 20:55 , PeterN wrote:
On 10/5/2014 6:57 PM, Alan Browne wrote:
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.


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

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


Then you are using a different definition of quality.

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.



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/ba.../fix-white.gif
Push things a bit harder and you can get
http://www.snoopy.me.uk/misc/365proj...gram/comb3.jpg
or even https://aperture64.files.wordpress.c...09/combing.gif


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.


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.


The production of histograms like the first one is common and
generally acceptable. The second histogram is worse but even then may
be acceptable. Only the last one is so bad that it will nearly always
be unacceptable. The point of all this is that some degree of quality
loss is virtually inevitable as soon as you start to manipulate an
image.

In the context of the present discussion, the question is, does the
conversion to Lab colour incur any more damage than one can expect in
the course of ordinary editing? My understanding of nospam's claim is
that it does. My (admittedly limited) experience with it suggests that
conversion to Lab causes no significant damage; certainly less than I
am going to inflict on the image by the changes I want to make.

As to the extent of the damage, I can only refer to my original
experiment described in Message-ID:


------------------------------------------

This one continues to bother me. I am still inclined to agree with Dan
Margulis. I'm not quite sure what procedure Andrew Rodney is proposing
to prove his point so, 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/...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/...54-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/...Difference.jpg
That's right: solid black.

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

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.

I'm not wedded to the perfection of the method I have used and I would
be interested to hear from anyone who has a meaningful criticism.


I'll try your method above when I have a chance. I've gone through this
exercise in the past and the difference (by subtraction) was visible
(faint, but unmistakable).

https://www.dropbox.com/s/3uwyuwun56...02-SD.jpg?dl=0

The only issues a

1. Lossy formats. Repeatedly opening and then saving a JPG at a lower
quality level will increase loss. This will be visible at some point.

2. Format conversion. For all the mentioned reasons, including the
quantization error the Martin Brown pointed out, there is a change in
the image and therefore it is quality loss. There is no other term.

=============================

The histogram of the JPG(RBG) --- Lab --- PSD when compared with
JPG(RBG) --- PSD shows only very slight evidence of differences
between the two.


The better way to see is to do image subtraction. While a histo may
bear witness to change, an imaage subtraction will always bear witness.


However JPG(RBG) --- Lab --- JPG(RBG) appears to be identical to the
original JPG(RBG).

So far I don't think I have found any evidence of damage worth
worrying about.


Never said different.

--

Regards,

Eric Stevens
  #729  
Old October 7th 14, 08:44 AM posted to rec.photo.digital
Martin Brown
external usenet poster
 
Posts: 821
Default Is RGB to Lab lossy? - was( Lenses and sharpening)

On 06/10/2014 22:27, Eric Stevens wrote:
On Sun, 05 Oct 2014 22:37:46 -0400, Alan Browne
wrote:

On 2014.10.05, 20:55 , PeterN wrote:
On 10/5/2014 6:57 PM, Alan Browne wrote:
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.


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

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


Then you are using a different definition of quality.


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.



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/ba.../fix-white.gif
Push things a bit harder and you can get
http://www.snoopy.me.uk/misc/365proj...gram/comb3.jpg
or even https://aperture64.files.wordpress.c...09/combing.gif

The production of histograms like the first one is common and
generally acceptable. The second histogram is worse but even then may
be acceptable. Only the last one is so bad that it will nearly always
be unacceptable. The point of all this is that some degree of quality
loss is virtually inevitable as soon as you start to manipulate an
image.

In the context of the present discussion, the question is, does the
conversion to Lab colour incur any more damage than one can expect in
the course of ordinary editing? My understanding of nospam's claim is
that it does. My (admittedly limited) experience with it suggests that
conversion to Lab causes no significant damage; certainly less than I
am going to inflict on the image by the changes I want to make.


Since CIELAB is a colour space intended to manage just noticeably colour
differences more optimally than the naive RGB colour space it isn't too
surprising that you cannot *see* a difference in the final JPG taken
from RGB or via CIELAB. But they are very slightly different.

As to the extent of the damage, I can only refer to my original
experiment described in Message-ID:


------------------------------------------

This one continues to bother me. I am still inclined to agree with Dan
Margulis. I'm not quite sure what procedure Andrew Rodney is proposing
to prove his point so, 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/...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/...54-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/...Difference.jpg
That's right: solid black.


It is only solid black so long as you don't use the histogram tool to
look in detail at the noise floor (also I am not sure how you did the
differencing - you may be missing half the differences if you did a
simple subtraction which clipped to zero as opposed to an absolute
difference where any discrepancy is rendered as a positive difference).

Use the histogram tool and you will see that the images do differ in the
luminance least significant bit. This would be undetectable in practice
but it is non-the-less a difference (ie not lossless).


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

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.


In practice you will not be able to see the difference and without pixel
peeping you can't see the difference on a simple difference image but it
is still there - just below your visual threshold.

I'm not wedded to the perfection of the method I have used and I would
be interested to hear from anyone who has a meaningful criticism.


The only thing that did surprise me was that the resulting errors are
entirely in luminance there is no chroma noise introduced at all.
(this might be an artefact of how you did the differencing)

--
Regards,
Martin Brown
  #730  
Old October 7th 14, 09:39 AM posted to rec.photo.digital
Eric Stevens
external usenet poster
 
Posts: 13,611
Default Is RGB to Lab lossy? - was( Lenses and sharpening)

On Tue, 07 Oct 2014 08:44:45 +0100, Martin Brown
wrote:

--- snip ---

In the context of the present discussion, the question is, does the
conversion to Lab colour incur any more damage than one can expect in
the course of ordinary editing? My understanding of nospam's claim is
that it does. My (admittedly limited) experience with it suggests that
conversion to Lab causes no significant damage; certainly less than I
am going to inflict on the image by the changes I want to make.


Since CIELAB is a colour space intended to manage just noticeably colour
differences more optimally than the naive RGB colour space it isn't too
surprising that you cannot *see* a difference in the final JPG taken
from RGB or via CIELAB. But they are very slightly different.


Agreed, but the question is, does the difference matter? In particular
does it matter enough to earn the reputation that nospam is trying to
assign to it? I would generally answer 'no' to both of thos questions.

As to the extent of the damage, I can only refer to my original
experiment described in Message-ID:


------------------------------------------

This one continues to bother me. I am still inclined to agree with Dan
Margulis. I'm not quite sure what procedure Andrew Rodney is proposing
to prove his point so, 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/...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/...54-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/...Difference.jpg
That's right: solid black.


It is only solid black so long as you don't use the histogram tool to
look in detail at the noise floor (also I am not sure how you did the
differencing - you may be missing half the differences if you did a
simple subtraction which clipped to zero as opposed to an absolute
difference where any discrepancy is rendered as a positive difference).

Use the histogram tool and you will see that the images do differ in the
luminance least significant bit. This would be undetectable in practice
but it is non-the-less a difference (ie not lossless).


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

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.


In practice you will not be able to see the difference and without pixel
peeping you can't see the difference on a simple difference image but it
is still there - just below your visual threshold.


The histogram of a subtraction will give you the answer.

I'm not wedded to the perfection of the method I have used and I would
be interested to hear from anyone who has a meaningful criticism.


The only thing that did surprise me was that the resulting errors are
entirely in luminance there is no chroma noise introduced at all.
(this might be an artefact of how you did the differencing)


I did nothing fancy at all.
--

Regards,

Eric Stevens
 




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