[argyllcms] Re: Getting back to using argyllcms. Some questions.

  • From: "Hal V. Engel" <hvengel@xxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 28 Nov 2008 20:37:18 -0800

On Friday 28 November 2008 19:36:37 Leonard Evens wrote:
> On Fri, 2008-11-28 at 19:18 -0800, Hal V. Engel wrote:
> > On Friday 28 November 2008 18:44:06 Leonard Evens wrote:
> > > On Sat, 2008-11-29 at 12:16 +1100, Graeme Gill wrote:
> > > > Leonard Evens wrote:
> > > > > I understand correctly, that term refers to a table stored in my
> > > > > video card.  Does the monitor have its own LUT which is used to
> > > > > `calibrate' the monitor?
> > > >
> > > > Generally LCD's have their own lookup tables they process the
> > > > video data through before sending to the actual panel. It is
> > > > this mechanism they use to simulate the range of controls
> > > > that people expect to be on monitors. For color calibration
> > > > and profiling purposes, these simulations usually degrade
> > > > the resulting images.
> > > >
> > > > Graeme Gill.
> > >
> > > I reviewed for myself how LCD monitors work.   Is there anything, such
> > > as the brightness of the light soure, which one adjusts at the hardware
> > > level.  Or it it just all done by  the internal LUT.
> >
> > For consumer grade LCDs the only adjustment that does not use the
> > internal LUT is the back light level (brightness).  Everything else
> > (contrast, color controls, gamma...) uses the internal LUT.
> >
> > Hal
> Is the back light level denoted Brightness or is it denoted Contrast?

I have a Samsung 245BW and I suspect it's controls are probably the same as 
your monitor or at least very similar.  For my monitor brightness is the back 
light level and contrast changes the internal LUT.  But at contrast settings 
below about 75 ( the default value) the changes to the LUT cause the white 
level to change likely because it starts changing the upper values of the LUT.  
So it sort of mimics the way the contrast control works on a CRT but it also 
causes a reduction in the dynamic range at settings below the default setting.  
For my monitor I found that settings slightly higher than default resulted in 
the best "contrast" setting - IE. the highest dynamic range with an even 
distribution of shades when viewing a gray step wedge. 

On my monitor the RGB controls change the per channel gamma and do not affect 
the white point at all unless fairly extreme settings are used.  Higher 
settings decrease the gamma and lower settings increase the gamma but also 
result in a lower dynamic range (IE. reduced contrast ratio) below the default 
setting.  So the RGB controls are effectively gamma and black point controls.  
I figured this out through lots of testing and measuring with different 
control settings.

For my Samsung monitor I have found that using native white point and native 
white level is the best approach.  The back light is around 6600K which is 
close enough to 6500K that I am not concerned with the difference.  I then use 
the RGB controls to adjust the black point and the gamma.  I keep the gamma 
control set to it's default setting because it seems to cause some response 
anomalies when other settings are used.  I have found that to get the display 
gamma near 2.2 that my RGB controls have to be set somewhat higher than the 
default 50 setting which has a gamma of about 2.3 with my display.  You can 
check the gamma in dispcal by using the "5) Check All" command and looking at 
how close you are to 0% for the 50% target; you should be able to get the 
error down below +-1%.   Be sure to use the -g parameter if you want a gamma 
something other than the 2.4 default for dispcal.  

I have a two monitor setup and the other monitor is a CRT with both RGB gain 
and RGB offset controls (IE. I can adjust both the white point and the black 
point).  It is not capable of getting the same white levels as the LCD so I 
set the white level of the LCD to match the CRT.  For the CRT I adjust it's 
white point and black point to closely match the LCD since the LCD has a fixed 
white point.  When I do this the LCD and CRT are fairly closely matched 
although the CRT has somewhat deeper blacks.


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