Having a lot of trouble getting Gamma Correction right...

Having a lot of trouble getting Gamma Correction right...

Post by Matt » Tue, 03 Feb 1998 04:00:00



Hi, I'm having a hell of a time trying to get the gamma correction right
when I scan, partly because there are 3 areas to enter gamma before saving
the image:

1. In the TWAIN32 window under "MONITOR GAMMA"
2. Slider bar in TWAIN32 marked "GAMMA CORRECTION"
3. Photoshop, under "MONITOR GAMMA"

in (1) I have it set at 1.0, (2) is set at 2.35, and (3) is set at 2.35, and
this is the closest I can get to a good scan, but it still looks washed
out...

Any ideas?

Email me at: mfshadow(at)gte.net
if you can help, it is greatly appreciated.

Matt Fetterley

 
 
 

Having a lot of trouble getting Gamma Correction right...

Post by David Wooll » Wed, 04 Feb 1998 04:00:00


You haven't fully specified your hardware and software, but I raised this
as an issue when I registered my Plustek 9630P with the bundled Photo Magic.

It's not a well understood area, and the documentation often oversimplifies
it.

Generally the scanner hardware scans with gamma 1, most PC monitors are
about gamma 2.2 and most PC image files are pre-corrected for gamma 2.2
(whereas the more professional hardware on Macs and some workstations is
best with gamma 1 images).  The pre* of PCs on the web means that
web GIFs and JPEGs should be pre-corrected for something around 2.2 (the
exact spec may be different).

To complicate matters, you should take account of gamma when dithering
images and some software doesn't do this properly (you should do the
dithering calculations after converting quantisation levels and the image
to gamma 1.0).

In particular, with the "lite" version of Photo Magic, if you calibrate the
monitor gamma, it will save images with gamma 1, which is a better gamma
technically, but wrong for common useage and the web.  If you arrange to
store with the web standard gamma, you need to set a monitor gamma of 1,
and all the dithering calculations are wrong.

The 9630P driver also uses a dithered colour patch in the monitor setup, which
is just not going to work, as you can't work out how to dither unless you know
the gamma in the first place!


>Hi, I'm having a hell of a time trying to get the gamma correction right
>when I scan, partly because there are 3 areas to enter gamma before saving
>the image:

>1. In the TWAIN32 window under "MONITOR GAMMA"
>2. Slider bar in TWAIN32 marked "GAMMA CORRECTION"
>3. Photoshop, under "MONITOR GAMMA"

>in (1) I have it set at 1.0, (2) is set at 2.35, and (3) is set at 2.35, and
>this is the closest I can get to a good scan, but it still looks washed
>out...


 
 
 

1. monitor gamma correction / brightness correction

Hello everyone:

I have two development machines one at work ( weekends only, of course
:-) ) and one at home. My software looks great on my monitor at work,
but at home the colors are way too bright.

It is my impression that this problem is due to the intensity with which
each monitor's electron gun strikes the phospher. And this intensity is
called the monitor's "gamma".

If I am correct, how can one compensate for these differences in
monitors ? I tried simply scaling colors, but it doesn't seem to look
quite right. How does monitor gamma correction work ?

If I am incorrect, what is really going on to cause the differences I'm
seeing ?

Thanks very much!

Stu
--
01010011 01110100 01110101 01100001 01110010 01110100
Stuart M. Pomerantz


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