[Shotwell] what does the Shadows selector do exactly ?

[Lucas Beeler]

Hi Lucas,

The simple answer to your question "what does the Shadows value
selector do" is that it remaps tone values upwards in the darker areas
of an image, so as to bring out hidden detail in underexposed areas.

The complete answer is that the Shadows transformation increases the
exposure of pixels by a variable amount based on their luminant
intensity in an adaptive and non-linear way. The Shadows algorithm
builds a cubic, Hermite polynomial approximation of the Gamma
distribution PMF where both the k and omega parameters are 2, and then
uses this curve to boost exposure accordingly. If you look at
Wikipedia's illustration of the Gamma distribution PMF here
(http://en.wikipedia.org/wiki/File:Gamma_distribution_pdf.svg) the
x-axis corresponds to luminant intensity and the y-axis corresponds to
the magnitude of the exposure boost. You can create a tone-mapping
curve in UFRaw that does pretty much the same thing, but that task is
beyond the scope of this mailing list.

Take care,
Lucas