[Digikam-users] Display calibration and profiling

[Geert Janssens]

On Friday 4 September 2009, Ricardo Amézquita wrote:
> I’m quite new at photography and I’m trying to setup a color managed work
> flow, but there is something I don’t understand. Maybe this have been asked
> before, but I haven't found a clear answer in the list, so here it goes.
>
> I just got an Eye One Display and I used it to calibrate and profile my
> screen using Linux. I can load the profile using dispwin, and I can see the
> difference in the appearance of my photos. I can also load this profile in
> Digikam, and I can see the difference between Color Managed View on and
> off. What is not clear for me is if I have to load the profile using
> dispwin and turn  Color Managed View on at the same time. I assume that any
> program not using color management output the image in an standard RGB
> color space (I assumed it to be sRGB, but there is no reason why this must
> be the case), and that the X server with a loaded color profile would
> correct this image to show it right to on the display. I also assume that
> if the program is color aware, I can activate the color management for the
> program, and it would correct the image and show the image fine on an X
> server without color profile loaded. But what I don’t know is what would
> happen if I load a color profile in the X server and I select color
> management for the display in Digikam. Will the correction be applied
> twice?.
>
Color management is a confusing matter. I have subscribed to the argyllcms 
list and read a book about it to better understand it. And I'm still a novice 
in the whole matter...

To better answer your question, I should first explain there are two things 
regarding displays: it can be calibrated and profiled.

The argyll tools do both things unless you explicitly disable one of them.

In computers, colors are most often described as RGB values. These values are 
sent to the monitor, which will display a certain color for that value. 
However, due to the physics of monitors, each monitor will show a different 
color in response to the same RGB value. Even on one single monitor the 
displayed color will change over time (aging) or when you fiddle with the 
display controls.

Here calibration comes into play. By calibrating a monitor, you alter it's 
output curves in such a way that you it displays a very specific color for a 
very specific RGB value.

However, there's no such thing as a perfect color device. The same goes for 
monitors.

And here profiling will be important. Profiling is used to measure the exact 
behaviour of your monitor. It doesn't alter the monitor's output like 
calibration does. It just sends a set of color values to the monitor and 
measures the exact color response. Based on these measurements it creates a 
color profile for the monitor, which basically describes how this monitor 
translates certain color values into colors.

There are also a number of theoretical profiles, like sRGB or AdobeRGB. These 
profiles define how an imaginary monitor would display certain colors based on 
a certain set of color values (in this case RGB values). These theoretical 
profiles are very interesting because they are hardware independent. That's 
why they are often used as embedded profiles in images. The image's rgb values 
together with the profile define exactly which true colors the image is 
composed of.

The behaviour of these imaginary monitors is different from your real world 
monitor, but with the profiles of both you can calculate how to compensate for 
this difference. So once you have profiled your monitor, a color management 
aware application can display any image with an embedded profile as it was 
intended by compensating for the difference in behaviour between the embedded 
profile and your monitor, at least within the physical limitations of your 
monitor.

It's worth noting that calibration and profiling are mostly used together, but 
it is perfectly possible to only profile your monitor, without calibration. 
This would mean you have to reprofile the monitor on regular intervals to 
compensate for aging. When you combine calibration and profiling, you only 
have to recalibrate at regular intervals. And as an additional bonus, your 
images generally look better in non-color management aware applications if you 
calibrate the monitor. Some argue though that only profiling (without 
calibration) is a better solution for cheap LCD monitors. I'm to ignorant 
about all this to comment on that.

So far the theory. Now in practise.

The calibration information and the profiling information are both stored in 
the profile generated by the argyll tools.

Dispwin loads this profile into the X windowing system. X however only uses 
the calibration information and alters the output tables of your monitor (or 
display driver) to get the output in a calibrated state. As a result, it 
should give you better color output on the complete display.

As explained before however, this is only part of displaying images as they 
are intended. Particularly those images that have an embedded profile other 
than sRGB.
[ N.B. I assume calibration brings the monitor as close as possible to the 
sRGB state, but I may be completely wrong here. In any case the principle 
remains valid even if this assumption is not correct, only sRGB images will be 
affected then as well. ]
For those images your calibrated monitor would still display the wrong colors. 
There are test images available on the web to demonstrate this very clearly. 
Such a test image has got the R, G and B values swapped, and an embedded 
profile to match this. Even on a calibrated monitor the color are completely 
wrong.
Color management aware applications though, can now use the profiling 
information in the image, together with the profiling information in your 
monitor's profile to display the image as really intended (or as close as 
possible within the physical limits of the monitor).
That's what digikam's color management does.

So to conclude, your image will only display as it really was intended if you 
load the profile with dispwin, define the same profile in Digikam and look at 
the image in a color managed view.

I hope this helps explaining it.

Regards,

Geert

> I have done some tests doing
>
> - profile loaded with dispwin, Color management Off
>
> - profile cleared with dispwin, Color management On
>
> - profile loaded with dispwin, Color management Off
>
> and in all cases my images look different, meaning that my assumptions are
> wrong.
>
> Can any one please explain me what is the correct way to set up my system?
>
>
> Regards
>
>
> Ricardo