Open Photography Forums  
HOME FORUMS NEWS FAQ SEARCH

Go Back   Open Photography Forums > Color Management Discussion > CM Theory and Practice

CM Theory and Practice Profiles, color spaces, perception, science.

Reply
 
Thread Tools Display Modes
  #1  
Old November 12th, 2010, 08:38 AM
Doug Kerr Doug Kerr is offline
Senior Member
 
Join Date: May 2006
Location: Alamogordo, New Mexico, USA
Posts: 8,196
Default Display calibration and profiling

When we seek to "prefect" the way in which our display system present the colors of an image file, we typically use software packages that, operating with a colorimeter "head", perform in sequence two quite different processes, calibration and profiling. These two are often confused.

[By the way, the details that follow pertain to Windows, and only through XP that I can be sure of.]

• In performing calibration, the software determines the response of the display system (driver plus monitor) and, by formulating a new suite of entries for the "lookup tables" (LUT) in the display adapter, seeks to give the display system a response matching a standard color space (often sRGB). In general, it is not possible to perfectly attain this (more on this shortly).

• In performing profiling, the software determines the response of the display system (driver plus monitor) with the freshly-determined "calibration" in place. It then describes that response in an icc profile file.

Profile-aware image editing or handling applications then can refer to this icc profile and properly transform the color representations in an image file into representations that, fed to this particular display system, will cause it to generate the colors described in the file.

One objective of the calibration stage of the process is that the calibrated display will do the best attainable job of correctly presenting the colors in an image file fed to the display by an application that does not make use of the icc profile (a "non-profile aware" application).

The LUT tables in the display adapter, one for each "channel", R, G, and B, describe transfer functions between the channel value sent to the display system and the channel signal sent to the monitor. We cannot, merely by changing the entries in these tables, overcome all the imperfections in display behavior. Most notably, we cannot that way compensate for the primaries of the display not having the chromaticities assumed by the standard color space.

Thus the importance of the icc profile in "perfecting" the display process.

Different software packages may use different strategies for deciding, if it can't "perfect" the display by calibration, which "imperfect" situation should it put into effect.

Note that the icc profile for a display only works properly if the LUT settings in the display driver are those chosen and implanted by the calibration-profiling software just before it developed the profile.

Are the lookup table settings held in non-volatile memory in the display adapter? Not usually. In fact, when the system is (re) started, those tables are filled with standard "factory" values from RAM in the adapter.

How then does the calibration established by our software package stay in effect? A little program (an "LUT loader"), usually installed by the profiling-calibration software, each time the system is (re)started, automatically runs and loads into the LUT that suite of entries. Where does it get those?

Normally, these LUT settings are stored in a "caboose" of the icc profile file, called the vcgt tag, from “video card gamma table” (“gamma table” referring to the LUTs). This is not part of the profile proper, but the icc standard for profile files recognizes that this caboose may exist.

Which icc profile file does the "LUT loader" get the entries from? One that it was told to use during the calibration-profiling process. (That is held in the Registry.)

Not the one we told some profile-aware application to use? Nope. Not the one we told Windows was the preferred profile for our display? Nope.

What in fact does it do when we tell Windows that a certain icc profile is the one we want use for our display? Does is make the operating system, itself, apply the profile when colors are sent to the display? Nope. That only happens in a profile-aware image application, before the colors are sent to the OS.

What is does is, when a profile-aware image application starts up, it asks Windows, "has the owner 'nominated' a profile for use with the display system here?" Windows reports, "yes, this one". The application then starts by putting that profile into effect, which remains in force unless and until the user tells the application to use another profile.

What if we have used more than one calibration-profiling software package? Then there might me more than one LUT loader in operation, and the LUT will end up with the settings from the caboose of the icc profile file used by which ever one runs last during the startup.

What happens if the icc profile we tell a profile-aware application was made by one calibrating-profiling package, but the one whose caboose is used by the LUT loader to populate the LUT at startup was made by another package? Might that mean that we don't get the intended color performance? Could be.

Best regards,

Doug
Reply With Quote
  #2  
Old November 12th, 2010, 02:49 PM
Andrew Rodney Andrew Rodney is offline
Member
 
Join Date: May 2006
Posts: 467
Default

I’d say a critical reason to calibrate (and what you would calibrate for, the targets) is to produce a screen to print match. I’ve kind of spelled it out here:http://www.luminous-landscape.com/tu...too_dark.shtml
__________________
Andrew Rodney
Author "Color Management for Photographers"
http://www.digitaldog.net/
Reply With Quote
  #3  
Old November 12th, 2010, 02:56 PM
Doug Kerr Doug Kerr is offline
Senior Member
 
Join Date: May 2006
Location: Alamogordo, New Mexico, USA
Posts: 8,196
Default

Hi, Andrew,

Quote:
Originally Posted by Andrew Rodney View Post
I’d say a critical reason to calibrate (and what you would calibrate for, the targets) is to produce a screen to print match.
Well, my thought is that to attain this we need to profile, or preferably calibrate and then profile, for the display chain, and profile for the printer.

Maybe by "calibrate" you mean "profile" or (where applicable) "calibrate and profile".

Best regards,

Doug
Reply With Quote
  #4  
Old November 12th, 2010, 04:02 PM
Andrew Rodney Andrew Rodney is offline
Member
 
Join Date: May 2006
Posts: 467
Default

In the chain of events, calibration takes place first, then the profile reflects those conditions. Calibration is placing a device into a desired (optimal?) state. Profiling describes that state.

Not all devices need to be calibrated. A display is unstable, and its behavior should be calibrated to reflect a condition that produces a match to a print if your goal is WYSIWYG and described in the link above. Some devices are very stable, they don’t necessarily need calibration. An Epson printer is an example. Its behavior isn’t ideal IMHO (its not all that linear in ink delivery). But it can profiled and since its quite stable, the canned profiles Epson provides do a pretty good job.
__________________
Andrew Rodney
Author "Color Management for Photographers"
http://www.digitaldog.net/
Reply With Quote
  #5  
Old November 12th, 2010, 04:11 PM
Doug Kerr Doug Kerr is offline
Senior Member
 
Join Date: May 2006
Location: Alamogordo, New Mexico, USA
Posts: 8,196
Default

Hi, Andrew,

Quote:
Originally Posted by Andrew Rodney View Post
In the chain of events, calibration takes place first, then the profile reflects those conditions. Calibration is placing a device into a desired (optimal?) state. Profiling describes that state.
Yes.

Quote:
Not all devices need to be calibrated. A display is unstable, and its behavior should be calibrated to reflect a condition that produces a match to a print if your goal is WYSIWYG and described in the link above.
But calibration alone will not (necessarily) attain that. Profiling (and of course the use of that profile) is (usually) needed to complete the desired situation.

Quote:
Some devices are very stable, they don’t necessarily need calibration. An Epson printer is an example. Its behavior isn’t ideal IMHO (its not all that linear in ink delivery). But it can profiled and since its quite stable, the canned profiles Epson provides do a pretty good job.
Yes.

Best regards,

Doug
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are Off
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Monitor Profiling: Why does my color on my monitor look right to me but not others? Cody White CM Theory and Practice 39 June 21st, 2010 10:56 AM
The Spyder 3 Pro display calibration and profiling system Doug Kerr CM Theory and Practice 3 June 28th, 2009 11:19 AM
Monitor calibration data Doug Kerr CM Theory and Practice 2 May 26th, 2009 03:19 PM
Question: SpectraView Users, Settings Screenshot Edward Bussa Image Processing and Workflow 25 February 4th, 2009 11:45 PM
Calibration and profiling of monitors Doug Kerr CM Theory and Practice 3 April 19th, 2007 02:33 PM


All times are GMT -7. The time now is 01:29 AM.


Posting images or text grants license to OPF, yet © of such remain with its creator. Still, all assembled discussion © 2006-2017 Asher Kelman (all rights reserved) Posts with new theme or unusual image might be moved/copied to a new thread!