[argyllcms] Re: Displaying sRGB graphics on wide gamut monitors - gamma problem?
- From: Sam Berry <samkberry@xxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 27 Oct 2010 16:18:03 +0100
On 27 October 2010 15:21, Anders Torger <torger@xxxxxxxxxxx> wrote:
> On Wednesday 27 October 2010, Graeme Gill wrote:
> > Please take some time to understand the difference between
> > calibration and profiling.
>
> This is what people keep saying to beginners like me :-). However, it
> seems to me quite easy. Calibration = through translation (matrix or
> lut) make the monitor display colours closer to some desired target.
>
This is profiling.
> Profiling = measuring the screen to see how it actually does display
> colours. One can profile a calibrated display or an uncalibrated
> display.
>
This is profile/calibration validation.
Calibration = Ensuring that display channels perform *individually* as
expected. Normally via display settings and 3 1-dimensional LUTs. This does
not alter colour in a major way, other than perhaps altering the white
point. A perfect monitor will show no change after calibration. This process
ensures the monitor conforms to a 2.2 gamma response with a D65 white point,
for example.
Profiling = Modelling the display using a matrix or a set of 1D LUTs and a
*3-dimensional* LUT. This is the step that allows the graphics application
to ensure the display shows the correct colours, regardless of varying
gamut. Prior calibration improves the effectiveness of profiling.
Validation = Ensuring that the final result of both of these steps is
adequate by measuring a set of patches and comparing to those predicted by
the profile.
>
> However, a source of confusion may be that I have misunderstood where
> the data is. I thought that for an .icc profile from dispcal vgct =
> calibration curves and rTRC/gTRC/bTRC = profile (*after* vcgt
> applied)... but I'm no longer that sure. I must try to look further into
> this, source code if I must. It will be impossible for me to find the
> source to my experienced (or imagined) problems if I don't have full
> understanding how calibration and profiling is applied.
>
The xTRC curves are there to allow the colour correction to be performed in
linear space, rather than in gamma space. Matrix gamma transformation
operations are meaningless unless performed on linear data. The curves allow
this transformation and its inverse to be performed.
>
> Anyway, I'm sorry for being clueless, I know it's not fun educating
> clueless beginners like me... I'm trying not to say clueless things, but
> it seems today like I need to spend some more time on this before
> leaving the clueless stage :-).
>
> /Anders
>
>
Sam Berry
www.satsumatree.co.uk
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