Lenses and sharpening

On 10/7/2014 3:44 AM, Martin Brown wrote:
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)


Thanks,
What you and Dr. Brown say makes sense. From a photo standpoint I will
continue to keep LAB in my workflow, while trying not to overdue it.

There is an interesting discussion on using LAB to control color
saturation at:

http://www.dgrin.com/showthread.php?t=18308

though it may have been posted before.
--
PeterN

In article , Eric Stevens wrote:

Martin Brown:
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?

That wasn't, however, "the question", Eric. You quote Dan saying this:

"RGBLABRGB is damage free"

That is an incorrect statement, which nospam has corrected. That is all.


--
Sandman[.net]

In article , Sandman
wrote:

Martin Brown:
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?

That wasn't, however, "the question", Eric. You quote Dan saying this:

"RGBLABRGB is damage free"

That is an incorrect statement, which nospam has corrected. That is all.

yep.

In article , Eric Stevens
wrote:

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.

then you should shoot entirely in jpeg and keep all your audio in mp3,
because the difference doesn't matter.

there will be 'more damage' from the inaccuracies of the display and
amplifier and speakers or headphones.

In article , Eric Stevens
wrote:

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

once again, i never said it was significant. i said it's lossy and it
is.

stop lying about what i say, but at least you finally agree.

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.

not necessarily.

That's why I think he
is talking nondense when he advocates not using Lab so as to avoid
damage.

there is absolutely no reason whatsoever to use lab because just about
everything you can do in lab can be done without a lab conversion *and*
avoid the losses.

if you apply the same logic, shooting entirely in jpeg is the way to go
because ultimately, the image will be a jpeg anyway.

In article , PeterN
wrote:

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.

that's what *you* like to do.

Even, when as pointed out earlier, it's against
his own argument.

more of your bull****. i have not argued against myself.

In article , Eric Stevens
wrote:

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

and jpegs appear to be identical to the raw. mp3 appears to be
identical to aiff.

however, they aren't, nor is a rgblabrgb conversion.

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

maybe not, but there's damage, which might matter at some future point.

In article , Eric Stevens
wrote:

There is nothing you do in image processing which is not lossless.

straw man.

Not at all. If you are going to criticise conversions to Lab on the
graonds that they incur losses, then you have to acknowledge that
there are other things you can do in Photoshop which are equally
lossy.

the point is that a lab conversion is completely *unnecessary*.

the other things are not.

For
some reason the conversion of RGB -- Lab has been particularly
singled out for criticism in this respect.

it's a bad workflow because what can be done with an rgb-lab-rgb
conversion can be done *without* the conversion and with better
results.

'Bad' is a value judgement.

not always.

Why should your value judgement be taken
any more seriously than anyone elses?

because it's based on facts.

if you can do x with a lossy step or a lossless step, the lossy step
will always be worse than the lossless step. very simple.

In article , PeterN
wrote:

For
some reason the conversion of RGB -- Lab has been particularly
singled out for criticism in this respect.

it's a bad workflow because what can be done with an rgb-lab-rgb
conversion can be done *without* the conversion and with better
results.

Yes it can be done in RGB, but with a lot more effort.

nope. it's less effort and with higher quality results in rgb since you
don't need to make two lossy conversions. it's also quicker.

Take a simple example stock photo and change the color in RGB, and then
make the same color change in LAB.

that's meaningless. change the colour to what?

Or, simply increase color saturation n RGB and make the same change in LAB.

there is absolutely *no* need to go to lab to change saturation.

All yo do is sout questionable theory. Show some real life proof.

there's nothing questionable about it.

read something *other* than the crap marguilis spouts and learn
something new for once.

In article , PeterN
wrote:

You have never noticed the ease of a color change in LAB, compared to
making a similar color change in RGB.

YOu have never brought out color using LAB that could not easily be
brought out in RGB.

nonsense.

you just don't know how to do it in rgb.

Well let's see a FACTUAL comparison.

read the books i've already mentioned.

you won't, because you only want to argue.

Show some proof. As I said earlier, I just want to keep people from
believing your nonsense.

read the books. they go into vastly more detail than i care to.

You have never proven a thing, except yur favorite line.

i don't need to prove 2=2.