Quando si attiva un profilo per il proprio monitor mediante ColorSync, se il profilo contiene il tag vcgt, i valori gamma per il rosso, verde e blu vengono caricati nella scheda video e modificano il gamma attualmente presente. In questo modo ci si assicura che il profilo corrisponda precisamente (almeno per quanto riguarda il gamma) con le condizioni del monitor.
Il tag vcgt può essere a tabella (come il primo qui sotto) o a formula (come il secondo).
Questo profilo ha un tag vcgt a tabella con 256 entrate.
Questo profilo ha un tag vcgt a formula che indica un gamma di 2.2 (1/2.2 = .45)
Alcuni programmi per la creazioni di profili di monitor inseriscono un tag vcgt. Altri usano una tecnica diversa per raggiungere lo stesso scopo: per esempio Prove It! non inserisce il tag vcgt, ma lancia una applicazione alla partenza di Mac OS 9 (o Classic) che imposta le LUT della scheda grafica.
However, there is a little application that comes with the ColorSync SDK for programmers called "ProfileGammaTagger" that will allow you to save the 'vcgt' tag into your profile. I'm not sure if you can download the built binary, you may have to compile it with CodeWarrior. Someone from Apple may want to point out where to get the binary from.
Run it in classic after running the Prove It! startup app. It will prompt you to select a profile to add the Video Card Gamma Tag to. After selecting a profile, you can then select the gamma table to add it it. After saving, restart into Mac OS X, and select the profile you saved the tag into. You now have a Mac OS X compatible profile. I can't tell you whether using this method is as effective as using the startup app. It certainly appears so, and it works for me. (Tom Beckenham)
How do you "Load the video look up tables (VLUTs) with ramps"? I don't recall ever hearing this technique mentioned before. (David Remington)There are probably many ways of doing this, but here is one that does not require any extra software. I have a set of six monitor profiles at http://www.brucelindbloom.com/Vcgt.html which are useful for vcgt testing. They are named A through F. Profile "C.icc" has a ramp vcgt tag. ColorSync will load this into your VLUTs for you via:
OS 10.2.2: System Preferences > Displays > Color
OS 9.2.2: Monitors control panel > Color
Then select "C.icc" which causes your VLUTs to be immediately loaded with ramps. Then select a monitor profile that does not contain a vcgt tag. The VLUTs will remain unchanged (i.e. they'll still be ramps). I would not recommend leaving "C.icc" selected -- use it only as a temporary means to load the VLUTs and then select something else.
If you want to make these ramp VLUTs "stick" to your monitor profile, you may use Apple's "Profile Gamma Tagger" utility to pull the ramps out of the VLUT and place them into your monitor profile as a ramped vcgt tag (just like the one C.icc has). Then, whenever your monitor profile is selected, the VLUTs get loaded with a ramp (and you won't need to do that C.icc trick anymore). "Profile Gamma Tagger" may be found on Steve Upton's site at http://www.chromix.com. (Bruce Lindbloom)