[argyllcms] Re: Calibration + profiling or Profiling only

  • From: János, Tóth F. <janos666@xxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 11 Sep 2010 23:59:19 +0200

>>> I don't know if you want to watch the movies on a Windows box. If that's
the case I can recommend Media Player Classic - Home Cinema:

The ICC based color management in MPC-HC is an interesting thing but there
are some serious problems with it.
The biggest is that it works with EVR but EVR is a bad joke when you compare
it to madVR.
On the other hand, madVR supports full 3DLUTs. It is theoretically better
than a little ICC file. (Of course, it also depends on the creator software.
But I follow the development of the yCMS software and I made some tests. I
like it.)
By the way, madVR works with much higher precision than EVR-CP (custom
preset, the default is much worse).
* May be it can be better when they fix the bug with 10 bit output for EVR.
But madVR will also support 10 bit in the future, so...

>>> I'm not sure I understand what you mean by 'plain tonal response'. Can
you elaborate further.

A strange curve which doesn't really follow an exponential curve but
averages around an exponential curve with a low exponent (~1.73).

>>> If you can get a reasonable gray balance with hardware gains only and
the gamma isn't too far off, you could get away with skipping the VGA LUT

The gray balance is reasonable but the 10 bit/color HDMI connection can
improve it. ArgyllCMS/dispcal will try it, at least.
And the gamma is much lower than my target. I want pure-power gamma 2.35
with black offset.

>>> All three aim towards the same target, so I think it's only a difference
how much 'work' each of the three does.

Ok, let me redraw this...

The white POINT can be corrected with the hardware RGB gains. It will
permanently reduce the contrast ratio and the number of the available shades
(the amount of this is unknown now but I guess it's good because it is done
by a 12 bit controller).
Now, the VGA LUT will correct the white balance across the full gray ramp.
Because the curves are not fully coherent, there is a chance that it will
decrease the R and G at the middle of the grayscale because I decreased the
B to get a better WP at the 100% IRE.
Of course, it shouldn't be a big thing, but I am talking about the
accumulation of these little things when there is numerous correction steps.
Do you get the idea...?

Now, let's continue with the gamut emulation.
It cannot be done by the VGA LUT. But the VGA LUT will correct the gamma
curve as well. So,
I think the gamut emulation is tricky and it mess with the WB and gamma too.
Let's see an example. (But tell me if I am wrong. I am not sure.)
You have an oversaturated green, so you have to add some red and blue. If
you do so, you will increase the luminance. So, you have to decrease all of
the three values to get the same chroma with correct luma.
(Of course, in the real world, you apply a pre-calculated correction formula
to end up with the correct results. But this is how I imagine it...)

As I imagine it now, the initial gamma curve can be different from the
target because you have to play with the luminance anyway. (Of course, it
should be somewhere close.)
Now, here is the trick:
- You can correct the gamma with the VGA LUT and mess with the numbers
during the gamut emulation to get back to the target luminance.
- Or you can start from the native limunance, do the gamut correction and
target the desired luminance.
Do you get it?

I can't really see it because I never examined any gamut conversion math
This is why I ask you...

>>> Is Rec709 the same as « sRGB »?

The primaries are the same. But sRGB has a standard output tonal response
curve while there is no standard curve for Rec709 decoding. (It specifies
the encoding only and we know that the decoding is not an inverse-encoding
but some pure-power curve, may be gamma 2.35. It is a complicated

>>> I think I completely missed the fact that you want to watch « movies ».
Forget everything I just wrote L

Why? Color management is always the same in theory. The targets and the
process may differ but the idea is the same: achieve your target (mostly one
of the standardized) characteristic(s)...

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