Gamma & linearity - how to use them

I use ufraw (Linux) for raw conversion. In the "color" section of ufraw
there are controls for gamma (default value 0.45) & linearity (default
value 0.10). I tried to read about them and found a lot of mathematical
explanation for each but unfortunately all those went well over my head.
What I noticed that changing them change the contrast but in subtly
different ways. Linearity also appears to change saturation to some
extent. Could somebody please explain their practical use (without the
maths please) - firstly whether I should bother about them at all and
secondly if I should, when to use them.Can the same effects be achieved
after conversion to tiff/jpeg by using level or saturation controls ?

Thanks

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Gautam Majumdar

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In article ,
Gautam Majumdar wrote:

I use ufraw (Linux) for raw conversion. In the "color" section of ufraw
there are controls for gamma (default value 0.45) & linearity (default
value 0.10). I tried to read about them and found a lot of mathematical
explanation for each but unfortunately all those went well over my head.
What I noticed that changing them change the contrast but in subtly
different ways. Linearity also appears to change saturation to some
extent. Could somebody please explain their practical use (without the
maths please) - firstly whether I should bother about them at all and
secondly if I should, when to use them.Can the same effects be achieved
after conversion to tiff/jpeg by using level or saturation controls ?

Thanks

Gamma is an exponential function applied to a brightness value in the
range of 0 to 1. What you see is the midtones becoming brighter or
darker as the gamma is adjusted.

Gamma correction was originally meant to correct for CRTs and printers
having a non-linear response. CRT efficiency changes with the
brightness while printers have dots that overlap each other. Even LCD
panels are non-linear. Gamma correction is needed to match the camera
with the display.

Gamma correction is still used in digital images because the sensitivity
of the human eye is not linear either. A linear 8-bit per channel image
can show banding in highlights. If it's a JPEG, the compression loss
will be more visible in the bright areas too. A higher gamma takes
detail from the shadows and gives it to the highlights. The resulting
image usually has banding below the threshold of visibility at all
brightness levels. 24 bit sRGB and most 8 bit grayscale images have a
gamma of 2.2.

On Tue, 21 Nov 2006 06:08:17 +0000, Kevin McMurtrie wrote:

[Good explanation snipped]

Thanks Kevin. So I should change gamma to adjust the midtones.

--

Gautam Majumdar

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Gautam Majumdar wrote:
I use ufraw (Linux) for raw conversion. In the "color" section of ufraw
there are controls for gamma (default value 0.45) & linearity (default
value 0.10).

Those settings amount to the same as the often mentioned "gamma 2.2"
used by most computer displays.

I tried to read about them and found a lot of mathematical
explanation for each but unfortunately all those went well over my head.
What I noticed that changing them change the contrast but in subtly
different ways. Linearity also appears to change saturation to some
extent. Could somebody please explain their practical use (without the
maths please) - firstly whether I should bother about them at all and
secondly if I should, when to use them.Can the same effects be achieved
after conversion to tiff/jpeg by using level or saturation controls ?

Turn on the "Overexposed ... indicate" option down in the lower
left part of the screen. Then click on "Color" and set the
input and output profiles.

If you have a custom profile for your camera, you might use that
for the input profile, otherwise you will almost certainly want
either sRGB or perhaps aRGB if your camera uses it. The output
should probably be sRGB too. The "intent" should be
"perceptual". Also set "AHD Interpolation".

If you want to use custom profiles, the input and output
profiles can be virtually anywhere you want, but since other
programs look in /usr/share/color/icc, that seemed to me like
the right place. Click on the folder to the right of the
input/output profile option to get a dialog box to configure
those options.

Those are basic defaults you won't likely change from one image
to another.

For each image... Start by setting the white balance for best
results. Then adjust "exposure" to just below where highlights
with desired detail are over exposed. Then adjust gamma and
linearity to get the contrast range you want, using linearity
with the midlevel tones where you want them (I tend to like 0.10
to 0.14, but occassionally want to pull something out by using
smaller values). Both gamma and linearity will affect
"exposure", as the three controls interact signficantly, hence
it requires adjusting all three of them repeatedly to obtain
best results. It is also very difficult to see much difference
between reducing gamma and reducing linearity, so just shoot for
what looks best to you.

That amounts to a "basic" adjustment. There are of course other
things you might want to do too. Color saturation adjustment,
use of the "color matrix" or not, or use of the custom curves
for irregular gamma/linearity, are all possible things to
change, though probably not needed for most average images.

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

In article ,
Kevin McMurtrie wrote:
Gamma correction is still used in digital images because the sensitivity
of the human eye is not linear either. A linear 8-bit per channel image
can show banding in highlights. If it's a JPEG, the compression loss
will be more visible in the bright areas too. A higher gamma takes
detail from the shadows and gives it to the highlights. The resulting
image usually has banding below the threshold of visibility at all
brightness levels. 24 bit sRGB and most 8 bit grayscale images have a
gamma of 2.2.

It is the other way around, there are more code points for dark colors than
for bright ones.

Without gamma correction 127/255 is 50% grey, with a gamma of 2.2, you
get 22% grey.


--
That was it. Done. The faulty Monk was turned out into the desert where it
could believe what it liked, including the idea that it had been hard done
by. It was allowed to keep its horse, since horses were so cheap to make.
-- Douglas Adams in Dirk Gently's Holistic Detective Agency

On Tue, 21 Nov 2006 08:53:16 +0000, Floyd L. Davidson wrote:

[Good guidance for use snipped]

Thanks very much Floyd.

--

Gautam Majumdar

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