[Shotwell] suggestions

[Lucas Beeler]

Hi Florin,

> I would guess the Shadows function is inspired by Picasa & Co. A big
> weakness of those programs is that the filter is easy to use but
> primitive. It can lift the shadows, sure, but often the pictures end up
> looking washed out.

I wrote the shadow detail boosting algorithm in Shotwell with my own
needs as a serious amateur photographer in mind. Though there are
exceptions, I think you'll find Shotwell's shadow control to be
superior to Picasa's in at least two key respects:

(i)
Effectiveness: Shotwell's shadow detail control appears to blend in a
higher absolute pixel value multiplier than the one used in Picasa.
The result is that if you move the shadows slider into the top third
of its parameter range, you get a more aggressive boosting of shadow
detail than is possible in Picasa.

(ii)
Image quality: Shotwell's shadow detail control uses a smooth Hermite
polynomial curve to blend in and blend out the intensity of the
exposure boosting transformation. The Hermite polynomial is a single
curve segment (i.e. not a spline) of degree 4 whose shape is directly
controlled through its coefficients. The advantage of using a single
polynomial segment is that the blending function is infinitely
differentiable, so there are no points of discontinuity, even at
higher derivatives. These higher derivatives control things like the
rate at which tonal intensity change accelerates or decelerates. While
this might not seem important, the human visual system is acutely
sensitive to movement, change, and the rate of change, so
discontinuities in the higher derivatives of blending functions can
lead to perceptible visual artifacts like banding and posterization.

In terms of how well the transformation is constrained to the lower
end of the intensity range (and doesn't bleach out the photo), we at
Yorba are satisfied with it, at least for now. Once again, I think
this is something that we do better than Picasa, but others might not
think so. How the blending curves are parameterized is as much an art
as a science and individual aesthetic tastes definitely play a role. I
would just tell you to use Shotwell's shadow control for a while and
to see what you think of it.

Thank you so much for your interest in Shotwell.

Regards,
Lucas