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Re: [OM] More on monitors and printing

Subject: Re: [OM] More on monitors and printing
From: Andrew Dacey <frugal@xxxxxxxxxxxxxxx>
Date: Sun, 22 Feb 2009 11:36:51 -0400
I probably should have explained this better. In untagged RGB (no  
colour space) the RGB values are completely arbitrary, there's no  
indication of what a particular RGB value is actually supposed to  
represent. A good analogy would be in the CMYK world you often  
specify colours as percentages of each ink, that's worthless if you  
have no idea what a particular printer's inks look like (one  
printer's "Cyan" might be a little bit bluer while another's might be  
a bit greener).

Tagging RGB values to a specific colour space tells you what the  
values are actually supposed to represent (in terms of LAB values).  
Then you actually know what "colour" a particular RGB value is  
supposed to represent.

Calibration involves adjusting the individual Red, Green and Blue  
outputs for the monitor so that they output the exact LAB colour that  
the colour space demands. Ideally, you'd be calibrating the monitor  
so that its gamut completely covers the colour space, or at least  
gets as close as you can get. CRT monitors frequently allow control  
over the individual electron guns so it is possible to calibrate  
them. An LCD usually only gives you brightness control (and possibly  
contrast) so it's impossible to adjust the individual Red, Green and  
Blue channels independently, which makes it impossible to actually  
calibrate the monitor to a specific colour space.

Where profiling differs is you're simply taking a "fingerprint" of  
the monitor's characteristics. The software sends various untagged  
RGB values to the monitor and measures what LAB colour the monitor  
displays. Based on that information, it can build a table which  
determines what RGB values need to be sent to the monitor in order to  
produce the proper LAB value. What this means is that the image file  
might specify that a pixel is R 125, G 90 and B 50 in sRGB space, but  
what's sent to the monitor is R 100, G 95 and B 42. Whereas with a  
calibrated display (that's been calibrated to sRGB) you'd be sending  
R 125, G 90 and B 50.

The difference in the 2 really comes into play with issues with  
gamut. A calibrated display has been adjusted to best cover the  
colour space's gamut, whereas the profiled display hasn't had its  
gamut adjusted at all. If you're dealing with 2 displays that have  
widely differing gamuts then you will notice differences between the  
2 when viewing an image that has colours that are out of gamut for  
one display but not the other.

Now of course the big question is how does this impact printing? Well  
assuming that you've profiled your printer as well, you can use "soft- 
proofing" to get a preview of what the printed image will look like.  
Since your monitor is a known quantity (profiled) and your printer is  
as well, and the colour management knows how to translate between the  
2, it can show you a representation of what the print will look like  
when printed. In all likelihood, it's not going to look the same as  
what you had on screen, but you can now make adjustments to the image  
to get it looking as close  to the screen display as possible, or at  
least to create as pleasing an image as possible (since some colours  
may just be outside the capabilities of the printer).


On Feb 21, 2009, at 10:52 PM, Wayne Harridge wrote:

> Calibrating, profiling, sounds like the same thing to me.
>
> You're adjusting the "system" (hardware & software) to produce a  
> consistent
> and known output from a specified input.
>
> I would have thought that if the gamut of each monitor was  
> different then
> the profiling hardware/software would let you know that the screen 
> (s) could
> not be fully "profiled" (calibrated).
>
> ...Wayne
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