Gamma correction question

It is my understanding that Gamma correction is for adjusting the color
balance of an image to compensate for the non-linear response of a
monitor. If the purpose of my scanning is to make photo quality prints
do I have to take out the gammma correction when saving the image to the
file that will be used to make the print? Or is the gamma correction
not saved in the file and is only applied by the scanning or processing
software such as Vuescan or GIMP when viewing the image?

"Jack Frillman" <jack@dnaco.net> wrote in message
news:413fb0d5$0$99890$9a6e19ea@news.newshosting.com...
>
> It is my understanding that Gamma correction is for adjusting the color
> balance of an image to compensate for the non-linear response of a
> monitor. If the purpose of my scanning is to make photo quality prints
> do I have to take out the gammma correction when saving the image to the
> file that will be used to make the print? Or is the gamma correction
> not saved in the file and is only applied by the scanning or processing
> software such as Vuescan or GIMP when viewing the image?
It is only applied to the data which is sent to the graphics card.
Jim

In article <413fb0d5$0$99890$9a6e19ea@news.newshosting.com>,
jack@dnaco.net says...
>
>
>
>It is my understanding that Gamma correction is for adjusting the color
>balance of an image to compensate for the non-linear response of a
>monitor. If the purpose of my scanning is to make photo quality prints
>do I have to take out the gammma correction when saving the image to the
>file that will be used to make the print? Or is the gamma correction
>not saved in the file and is only applied by the scanning or processing
>software such as Vuescan or GIMP when viewing the image?


No, gamma is correctly added to all images for all purposes, done at the
source of the image (digital cameras, video cameras, scanners, etc).
Printers are also non-linear and also need gamma, a little less than a CRT
perhaps, but the printer drivers know how to respond correctly to the
standard image gamma. Printers simply understand that all images are
expected to have normal gamma, and you will get very dark images without
it.

--
Wayne
http://www.scantips.com "A few scanning tips"

"Jack Frillman" <jack@dnaco.net> wrote in message
news:413fb0d5$0$99890$9a6e19ea@news.newshosting.com...
>
> It is my understanding that Gamma correction is for adjusting
> the color balance of an image to compensate for the non-linear
> response of a monitor.

It's more a correction of brightness than color (although it can be
used for some non-linear color characteristics of the capture device).
The simple gamma correction wraps several adjustments into a single
parameter. It adjusts for monitor response (typically gamma=2.5),
additionally for eye response (complex response depending on average
local luminance level), and it benefits encoding accuracy in a small
number of bits.

> If the purpose of my scanning is to make photo quality prints
> do I have to take out the gammma correction when saving the
> image to the file that will be used to make the print?

No, the data is (~inverse) gamma adjusted before saving to file.

> Or is the gamma correction not saved in the file and is only
> applied by the scanning or processing software such as
> Vuescan or GIMP when viewing the image?

All devices have a response which requires data that is already
adjusted to a known gamma. The scanner (CCD) essentially collects
linear gamma data, and the quantized film transmission data can be
easily accomodated within the 16-bit/channel file. However, all
subsequent output devices in the imaging chain have their specific
non-linear luminance response, and so does the human eye that views
the output. One can correct those non-linearities with a single
inverse non-linear correction, and it can be reasonably well done with
a simple gamma (power) function. This all will allow to encode the
source data with minimal losses in luminance resolution, and do this
with a minimum of bits, if the data is saved to file after gamma
adjustment.

Bart

In article <413fb0d5$0$99890$9a6e19ea@news.newshosting.com>, Jack
Frillman <jack@dnaco.net> wrote:

> It is my understanding that Gamma correction is for adjusting the color
> balance of an image to compensate for the non-linear response of a
> monitor.

That is incorrect.
Gamma encoding is done to maximize the efficiency of the encoding
(making the best use of the bits).

See http://www.chriscox.org/gamma for details.
(yes, it's a little out of date. But still right.)


> If the purpose of my scanning is to make photo quality prints
> do I have to take out the gammma correction when saving the image to the
> file that will be used to make the print?

No.

> Or is the gamma correction
> not saved in the file and is only applied by the scanning or processing
> software such as Vuescan or GIMP when viewing the image?

The gamma encoding is baked into the bits of the image.
But you convert the image to the printer color space when printing -
and that adjusts the values from the image encoding to what is needed
for the printer.

Chris

Chris Cox <ccox@mindspring.com> wrote:
>In article <413fb0d5$0$99890$9a6e19ea@news.newshosting.com>, Jack
>Frillman <jack@dnaco.net> wrote:
>
>> It is my understanding that Gamma correction is for adjusting the
>>color
>> balance of an image to compensate for the non-linear response of a
>> monitor.
>
>That is incorrect.
>Gamma encoding is done to maximize the efficiency of the encoding
>(making the best use of the bits).
>
>See http://www.chriscox.org/gamma for details.
>(yes, it's a little out of date. But still right.)
>
>
>> If the purpose of my scanning is to make photo quality prints
>> do I have to take out the gammma correction when saving the image
>>to the
>> file that will be used to make the print?
>
>No.
>
>> Or is the gamma correction
>> not saved in the file and is only applied by the scanning or
>>processing
>> software such as Vuescan or GIMP when viewing the image?
>
>The gamma encoding is baked into the bits of the image.
>But you convert the image to the printer color space when printing -
>and that adjusts the values from the image encoding to what is needed
>for the printer.
>
>Chris

Is it better then to let the scanner convert the 16 bits data to
gamma 2.2 and output this, than to have the 'linear' 16 bits output?
Or is there any advantage in processing the 16 bits linear file in
Photoshop? E.g. to get more details out of the highlights or shadows.

--
Regards, Alex

Jack Frillman wrote:
>
> It is my understanding that Gamma correction is for adjusting the color
> balance of an image to compensate for the non-linear response of a
> monitor. If the purpose of my scanning is to make photo quality prints
> do I have to take out the gammma correction when saving the image to the
> file that will be used to make the print? Or is the gamma correction
> not saved in the file and is only applied by the scanning or processing
> software such as Vuescan or GIMP when viewing the image?


Hello

This link gives a pretty good description of what the process is.

http://www.w3.org/TR/PNG-GammaAppendix.html

Mike Engles

In article <iuO%c.17568$tx4.13586@newssvr24.news.prodigy.com>, Jim
<j.n@nospam.com> writes
>
>It is only applied to the data which is sent to the graphics card.

NO! Gamma is normally applied immediately to *all* images irrespective
of their ultimate destination. Try printing raw images which have no
gamma correction or setting the gamma of your camera/scanner to 1.0 and
see where that gets you with your prints.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

In article <110920041745086582%ccox@mindspring.com>, Chris Cox
<ccox@mindspring.com> writes
>In article <413fb0d5$0$99890$9a6e19ea@news.newshosting.com>, Jack
>Frillman <jack@dnaco.net> wrote:
>
>> It is my understanding that Gamma correction is for adjusting the color
>> balance of an image to compensate for the non-linear response of a
>> monitor.
>
>That is incorrect.
>Gamma encoding is done to maximize the efficiency of the encoding
>(making the best use of the bits).
>
>See http://www.chriscox.org/gamma for details.
>(yes, it's a little out of date. But still right.)
>
It also contains a major error in the third paragraph. Human lightness
sensitivity has a gamma of around 0.3-0.4, *not* 2.0-3.0: you seem to
have the inverse of gamma here.

The concept is, as you describe in your document, that the product of
the eye and display gammas should approximate unity so that linear grey
ramp signals sent to the display *appear* as linear ramps with evenly
spaced steps. If the eye gamma were between 2.0 and 3.0 then we would
need to design displays with gammas of 0.3 to 0.5 to make optimum use of
digital data.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

Alex Stols wrote:
>
> Chris Cox <ccox@mindspring.com> wrote:
> >In article <413fb0d5$0$99890$9a6e19ea@news.newshosting.com>, Jack
> >Frillman <jack@dnaco.net> wrote:
> >
> >> It is my understanding that Gamma correction is for adjusting the
> >>color
> >> balance of an image to compensate for the non-linear response of a
> >> monitor.
> >
> >That is incorrect.
> >Gamma encoding is done to maximize the efficiency of the encoding
> >(making the best use of the bits).
> >
> >See http://www.chriscox.org/gamma for details.
> >(yes, it's a little out of date. But still right.)
> >
> >
> >> If the purpose of my scanning is to make photo quality prints
> >> do I have to take out the gammma correction when saving the image
> >>to the
> >> file that will be used to make the print?
> >
> >No.
> >
> >> Or is the gamma correction
> >> not saved in the file and is only applied by the scanning or
> >>processing
> >> software such as Vuescan or GIMP when viewing the image?
> >
> >The gamma encoding is baked into the bits of the image.
> >But you convert the image to the printer color space when printing -
> >and that adjusts the values from the image encoding to what is needed
> >for the printer.
> >
> >Chris
>
> Is it better then to let the scanner convert the 16 bits data to
> gamma 2.2 and output this, than to have the 'linear' 16 bits output?
> Or is there any advantage in processing the 16 bits linear file in
> Photoshop? E.g. to get more details out of the highlights or shadows.
>
> --
> Regards, Alex


Hello

If you want to apply a precise inverse gamma to the image, you will have
to do it in the scanner, as Photoshop curves does not have precise
curves. Photoshop levels will apply a inverse gamma using the middle
box, but in my experience you will end up with darker shadows than if
you used the scanner. In one sense as long as you ALWAYS work in 16 bit
when tweaking a image, it does not matter. The idea is to use all the
tools to make the image look good to you, without creating posterisation
artifacts. You could apply a precise inverse gamma in the scanner, but
any further tweaking using levels or curves or any other tool will give
the image a unknown effective gamma, but it should look good, that is
the point. Don't do much tweaking in 8 bit, it will damage the image.

You do need to calibrate your monitor and set a correct black point.

Mike Engles