Frustrated with Vuescan profiling

[Yan]

Hi all
I am trying to set up a color managed workflow so that scans match
real slides in color. I use Vuescan (7.6.78) and IT8 targets from Wolf
Faust to calibrate my Canon FS4000. After spending for about a week
reading the forum and trying, I still can't get the color match. I am
really frustrated. I would appreciate any of you experts' comments and
suggestions.

This is what I have done:
I shot mostly Velvia and Provia and have used corresponding IT8
targets to generate scanner profiles for each individual film. I
followed the instruction in Vuescan and used Input|Media type|Image,
48 bit RGB at 4000 dpi. After generating the scanner profiles, I
rescan the IT8 target slide and output the scan as a raw file. Then I
used the same exposure (by lock exposure) to scan my own slides and
output them as raw files. My intention of doing this is to save the
raw files as archives.

To edit the scans, I first rescan the IT8 target raw file in Vuesan.
My setting is

Input|Scan from|file
Input|Media type| Image
Color|Scanner color space|ICC profile
Color|Scanner ICC profile| I use the profile generated from Profile
Scanner
Color|Color Balance| Manual
Color|Black point(%)|0
Color|white point(%)|0.1
Color|Brightness| 1
Color|Brightness red|1
Color|Brightness green|1
Color|Brightness blue|1
Color|Output color space|Adobe RGB
Color|Monitor color space|ICC profile
Color|Monitor ICC profile| I use a profile generated by Spyder
OptiCAL.

I set the color balance by right click square GS0 of the IT8 target.
For the Provia target, after doing this, the RGB reading of GS0 is
about 247,247,247, and the RGB reading of GS11 is about 102,102,105. I
got the readings from Vuescan preview. The readings are average of
four points picked from the square. Velvia target gave similar result.

Then I locked image color and used the setting to open the raw files
of my own slides. The color was close but never perfectly matched the
slide view from a light box lighted with 5000K fluorescent tubes.(my
monitor was calibrated to 5000K too). Some slides are better, some are
worse. In general, slides shot in day light will be scanned more
faithfully than those shot in twilight. The strong blue cast of the
film shot in twilight is the worst to reproduce.

I have tried using Input|Media type|Slide film. The results were
identical to what I got from using Input|Media type|Image. I did
notice that by adjusting Color|White Point % will improve the result
sometime.

Am I missing anything here? Do I need to get film profiles shot under
different lighting condition to get perfect match?

I have also tried to open the raw files in Photoshop and then assign
to the file the ICC scanner profile generated by Vuescan. The color
was way off. After assigning the ICC profile, the RGB of GS0 reads
190, 136,155. I have read in the forum that people seem to have
success doing this. What did I do wrong?

 

[Erik Krause]

Hello, Yan
you wrote...

I am trying to set up a color managed workflow so that scans match
real slides in color. I use Vuescan (7.6.78) and IT8 targets from Wolf
Faust to calibrate my Canon FS4000. After spending for about a week
reading the forum and trying, I still can't get the color match.

I hope your monitor is profiled as well. Profiling your scanner is of
no use, if your monitor colors and gamma are way off.

Furthermore profiling is of very limited use, if color balance is other
than 'None' or 'Neutral' (or for profiles generated with Vuescan:
'Manual');

I am
really frustrated. I would appreciate any of you experts' comments and
suggestions.

Vuescan IT8 profiling is not very sophisticated. If you look at the
profiles you'll see that the curves are simple gamma curves and they
all start and end in one point. Ed Hamrick confirmed, that the profiles
are of the matrix type and mainly do a gamma correction on the three
color channels in order to keep them CCD exposure independent.

If you want a better profiling, go to
http://www.littlecms.com/profilers.htm and get the lcms profilers. They
produce very sophisticated profiles. But be warned: These profiles
apply to one CCD exposure value only and of course you have to keep all
color and brightness relevant settings the same as you did when you
scanned the target.

Vuescan can not read these profiles (although it does not complain).
You have to apply them in photoshop or any other ICC aware image
editor. lcms supplies two command line utilites, that can apply
profiles to Tiffs or jpegs.

This is what I have done:
I shot mostly Velvia and Provia and have used corresponding IT8
targets to generate scanner profiles for each individual film. I
followed the instruction in Vuescan and used Input|Media type|Image,
48 bit RGB at 4000 dpi. After generating the scanner profiles, I
rescan the IT8 target slide and output the scan as a raw file. Then I
used the same exposure (by lock exposure) to scan my own slides and
output them as raw files. My intention of doing this is to save the
raw files as archives.

Raw files are not color corrected, so if you scan them with the same
CCD exposure as you initially scanned the target, you are preperad for
any type of profiling. BTW.: you can enter the exposure values manuall
once you locked exposure. There is no need to rescan the target.

 

[Yan]

Thank you for your help, Erik. I will give a try on littlecms.

Vuescan IT8 profiling is not very sophisticated. If you look at the
profiles you'll see that the curves are simple gamma curves and they
all start and end in one point. Ed Hamrick confirmed, that the profiles
are of the matrix type and mainly do a gamma correction on the three
color channels in order to keep them CCD exposure independent.

Where do you look at the profiles generated by Vuescan?

Yan

 

[Erik Krause]

Hello, Yan
you wrote...

Thank you for your help, Erik. I will give a try on littlecms.

Good luck! It is scarcely documented but works great. Her you can find
a comparison between several profilers:
http://www.tkupfer.de/imaging/Scan_Profiling.html

Where do you look at the profiles generated by Vuescan?

Either with the lcms profile checker, or with the ICC Profile Inspector
from www.color.org/membersonly/profileinspector.html or with
Curvemeister profile plotter photoshop plugin:
http://home.pacbell.net/geigy/downloads/profileplotter/index.htm
(main page: http://curvemeister.com/ )