Re: [Openexr-devel] f-stops and steps. How are they related?

[Florian Kainz]

> As a side note, concerning my monitor gamma adjustment, I adjusted the 
> gamma (which is set to 1.65 on the video card driver) so that the set of 
> gamma patterns that are available on the Net (and I tested all of which 
> I could find) does look OK. So in the end, I don't really know the exact 
> total gamma I have [...] 


If you care, you can estimate the total gamma of your setup with a series
of test images.  Create an 8-bit image with the following pixel pattern:

    BBBBBBBBBBGGGGGGGGGG
    WWWWWWWWWWGGGGGGGGGG
    BBBBBBBBBBGGGGGGGGGG
    WWWWWWWWWWGGGGGGGGGG
    BBBBBBBBBBGGGGGGGGGG
    WWWWWWWWWWGGGGGGGGGG
    BBBBBBBBBBGGGGGGGGGG
    WWWWWWWWWWGGGGGGGGGG
    BBBBBBBBBBGGGGGGGGGG
    WWWWWWWWWWGGGGGGGGGG
    BBBBBBBBBBGGGGGGGGGG
    WWWWWWWWWWGGGGGGGGGG

Pixels marked B are black: r = g = b = 0.
Pixels marked W are white: r = g = b = 255.
Pixels marked G are gray: r = g = b = v.

Start by making a test image with a medium gray value, for example
with v = 128.  Display the image on your screen, and view it from a
distance so that the black and white lines visually merge into a gray
patch.  Compare the left and right halves of your test image; if one
side appears to be brighter or darker than the other side, make a new
test image with a different v value for the right half.  Repeat until
the left and right half look the same when viewed from a distance.

Now you can compute your setup's gamma from the v value you used for
your final test image:

The intensity/luminance of the left half of the test image is

    Imax * 0.5,

where Imax is your monitor's maximum intensity.  The intensity of the
right half of the test image is

    Imax * pow (v/255, gamma)

Since the left and right half of the test pattern look the same,

    Imax * 0.5 = Imax * pow (v/255, gamma)

or

    gamma = log (0.5) / log (v/255)