[Digikam-users] Color Managed View - How?

Hal V. Engel hvengel at astound.net
Sat Sep 1 20:52:12 BST 2007


Some very good questions.  I will try to provide some answers in line.

On Saturday 01 September 2007 04:57:47 Arnd Baecker wrote:
> Hi Hal,
> thanks a lot for this very helpful overview!
> So far I have not started using color management, because
> I don't have a device for profiling my screen.
> Unfortunately, it seems that there are not many
> which work under linux. For example the spyder hack
> http://www.yan-ke.com/spyder/
> to support ColorVisions Spyder colorimeter
> did not make any progress in the last months.
> (And the DTP94 Optix listed at
> http://www.argyllcms.com/doc/ArgyllDoc.html
> is difficult to get and not that cheap)

LProf CVS now supports the EyeOne Display line (original, 2 and LT) as well a 
the EyeOne Pro meters.  It also supports a number of older discontinued  
meters like the DTP94, DTP92 and Spectrolino.  EyeOne Display LTs are fairly 
inexpensive at least in the US with street prices in the $120 to $130 range 
which also includes Windows and Mac calibration and profiling software - 
which you can't use on your Linux workstation.   This is still not "cheap" 
but DTP94's were seeling for $199 with no software when they were 
discontinued and only two years ago these were closer to $300.  The ArgyllCMS 
0.70 beta releases also support these meters.  

I also have an OEM agreement with X-Rite for the EyeOne meters and if there 
were sufficient interest I might be able to sell these at a slightly lower 
price then the normal street prices.  But I have to order these in larger 
quantities, I can't order just 2 or 5 units, so this means I would need firm 
commitments for enough buyers to cover an order from the vendor.

> But maybe this is not really necessary and
> setting the monitor to sRGB (as I can do on my Eizo)
> and adjust gamma with lprof
> (http://sourceforge.net/project/screenshots.php?group_id=146038&ssid=21740)
> is already better than nothing?

Yes better than nothing but you will be surprised at how much better your 
results are if you do a measurement based calibration and profiling of your 
display.  The LProf rough profiler gamma screen uses the Norman Koren gamma 
chart (with permission) and allows you to "measure" the gamma of the 
individual display channels.  This visual gamma chart is the best one 
available and is much better than those used in other products such as 
AdobeGamma.  Monica also uses this gamma chart.

> Still I think that to ease the use of color management
> in the context of digikam there should be a brief list of steps

If only it were that simple.

> (like a recipe ;-) which one should follow to get
> everything setup properly.
> The handbook already provides a description of the concepts,
> http://docs.kde.org/development/en/extragear-graphics/digikam/using-iccprof
>ile.html but I am looking more for something like 
> 1) set screen to sRGB

Only some displays have a setting for this type of thing - most will not.

> 2) adjust gamma and brightness of the screen properly

How do you do this without a measurement device?  

Actually if your display allows you to adjust the gamma curves of the 
individual channels (most do not but the Eizo display may) then you could use 
the LProf rough profiler gamma screen to adjust this.  It will not be as 
close as using a measurement device and it will not correct for 
non-linearities but you should be able to get the overall gamma within 0.1 of 
your target value.  

A typical measurement based calibration will get the gamma of the three 
channels to with in 0.03 of your target and will also correct for non-linear 
display response.   It also allows you to precisely set the white point, 
luminosity (brightness control on an LCD and contrast control on a CRT), 
black point (if the display supports this adjustment - most do not) and black 
level (brightness control on a CRT).  I do not know of any way to do these 
setting accurately without a meter.

> 3) enable color management and use the right ICC profile for you camera

With LProf or ArgyllCMS and an IT8 target you can create your own custom 
camera and scanner profiles.  LProf documentation has a tutorial on how to do 
this for use in UFRAW.   You should be able it extrapolate this for use with 
other RAW work flows.  LProf CVS now supports other targets including the 
ColorChecker series and Hutch Color targets which were added as part of the 
Google Summer of Code project.  Some of this code (ColorChecker DC and SG 
support) went in just last night and I have not been able to review it yet 
and it is mostly untested at this point.   

> 4) do all manipulation in a "large" color space
>    (e.g. ProPhoto RGB ?,
>    http://www.luminous-landscape.com/tutorials/prophoto-rgb.shtml)

IMO ProPhotoRGB, at about 94% of Lab space, is too large and it must be used 
with great care.  It should not be used for 8 bit/channel workflows ever.  In 
fact you should avoid 8 bit/channel workflows in general no matter what color 
space you are using.  A better choice would be something like BetaRGB (69% of 
Lab space) which is designed to be large enough for almost any photographic 
content but is shaped in such a way that almost all of it's gamut is usable 
(much of the ProPhotoRGB gamut is not even visible to humans).  Stay away 
from sRGB (35% of Lab space) and only use it for web output, emails and 
things like that (consider it an output only profile).  In addition AdobeRGB 
(50% of Lab space) is also somewhat on the small side but is better than sRGB 
as an editing color space.  

> 4) (?) use the right output profile for your printer or one provided by
>    the lab to print your images (of course you need a good lab for this)

Many labs now know how to handle CM work flows even some fairly inexpensive 
ones.  As an example Costco, a large US "big box" chain store, publishes the 
profiles for their printers on-line for each store (IE. these are custom 
profiles).   Costco has garnered a good reputation both for the high quality 
of prints created using these profiles and the low cost of the service.  This 
is just one example and there are likely other low cost printing services 
available that provide ICC profiles for their printers or color management 
work flow information for their service.  

Profiling your own printers is more of an issue.  For Windows and Mac many 
paper vendors will supply free paper/printer specific profiles that are 
almost as good as custom profiles.  But this is not the case for other 
platforms such as Linux and you can not use a profile intended for use on a 
Mac or Windows.  This means we must create our own profiles or go to a 
profiling service.  The equipment to create printer profiles is not cheap 
starting at about $650.  Fortunately there are a number of services that will 
produce custom printer profiles for around $25 and for most inkjet printers 
this only needs to be done one time for each paper you use for the life of 
the printer.   One example is located here


So this is not expensive. 

> Note the previous points are in complete ignorance of how
> it should be done correctly!
> While all the concepts behind color-management and color spaces
> are not trivial, somehow I have the feeling that
> it still should be simple to use in the end
> (presumably it is already, but I haven't grasped it yet ... ;-)

Unfortunately Color management is one of those things that is not trivial to 
setup or to understand.  There is a significant learning curve but in return 
for getting past that learning curve you end up with a very powerful tool 
that allows you to get consistently better results with less effort.  Because 
of the steep learning curve many photographers give up before they get far 
enough along to leverage the tools it provides.  There are more resources 
than ever available that can help you in your quest to learn about and use 
color management and the tools to support it - software, meters and targets - 
are becoming better and lower in cost. 

The most important thing to keep in mind is that to get the best results you 
need profiles for YOUR devices and YOUR work flow.  Even devices of the same 
make and model will have sample to sample variation and a profile that I 
created for my camera might not give the same results for your camera of the 
same model because of this.  On the other hand it might be "close enough" for 
your camera if your work flow is the same as mine.  But things like different 
setting during RAW conversion or using a different RAW conversion program 
would make the profile invalid even for the same device.   So a consistant 
imaging work flow is critical.  Software like ArgyllCMS and LProf along with 
the supported meters and targets give you the tools you need to create these 
profiles without having to spend a furtune.


PS: Digikam apparently uses some of the LProf code base.  For what exactly I 
am not sure but it appears to use some of the profile checker functionality.  
See http://sourceforge.net/project/screenshots.php?group_id=146038&ssid=21739 
for a screen shot.

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