Because if the limited number of values that can be stored in an 8-bit integer and because humans don't notices small differences between dark colours as well as small differences in bright colours, sRGB encodes colours non-linearly so there are more bright colours and fewer dark colours.
However, looking at image editing tools for farbfeld I've found
that the tools do not take this into account. For example, making
a colour twice a bright, is not as simple as multiply the RGB
values with 2, rather the algorithm that shall be used is
x = F(2 ⋅ F⁻¹(x₀))
where F is sRGB's transfer function
⎧ -F(-t) if t < 0
F(t) = ⎨ 12.92 t if 0 ≤ t ≤ 0.0031306684425217108 [*]
⎩ 1.055 t↑(1/2.4) - 0.055 otherwise
[*] Approximate value. You will often find the value
0.0031308 here, but this value is less accurate and
only good enough when working with 8-bit integers.
pgps98aZmwvZH.pgp
Description: OpenPGP digital signature
