Am 18.10.2017 um 14:24 schrieb Kenneth:
> SO... getting back to POV-Ray's built-in gamma set-up chart...
> 
> As that chart was made many eons ago, I had a desire to see if it is still
> 'accurate' in the modern world ;-)  Leaving no stone unturned, I copied that
> image and took it into both my (older) Photoshop, and a recent version of GIMP--
>  to check not the thin horizontal bars, but the gray 'gamma' swatches that they
> are compared against.  (BTW, both of my apps are working in sRGB color/gamma
> space.) Using the 'eyedropper' tools there, I measured the gamma 2.2 swatch. Its
> brightness value in both apps reads as 186/255, or 0.72941    And 0.72941^2.2 =
> 0.49950, or almost exactly 0.5
> 
> I can't say that I know exactly what I'm doing with this experiment-- ha!-- but
> is  0.72941  the correct value for the gamma 2.2 swatch? (I assume it is, of
> course, but I wanted an expert's opinion.)

IF your display subsystem has a gamma of 2.2, THEN the corresponding
swatch, when displayed, should result in a physical brightness of 0.5.

In other words, the swatch should have a brightness value of N such that:

    (N/255)^2.2 = 0.5

The (N/255) part is the stored value normalized to the [0..1] range, and
the ^2.2 part is the effect of the display's non-linearity.


BTW, the swatch-based assessment has some more limitations than just the
zoom/interpolation issue:

- It relies on the image being displayed "as is", without any active
gamma correction, just the "native" effect of the display subsystem. If
for example your display is calibrated to the Adobe RGB colour space,
and the displaying software is aware of this fact and presumes the image
data to be stored in accordance to the sRGB standard, then this approach
will give nonsense results because the display software will introduce
another non-linear conversion that is not part of the native display.

- It naively presumes the display subsystem's native transfer function
to match the form f(x) = x^gamma. For display subsystems calibrated to
e.g. the sRGB, ITU-R BT.709 or ITU-R BT.2020 standards this is
specifically not the case (for the modern ITU-R BT.2100 standard, aka
HDR TV, it is probably not even close), and you'll get a gamma value
that is only valid for 50% physical brightness.

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