Squishes the contrast in an input HDR image, and saves the result. Both the
input and the output images are in EXR format. A great deal of tweaking of the
"alpha", "beta", "delta" and "theta" arguments is usually required before one
gets pleasing results.

The HDR contrast processing is performed using an implementation of:
	Raanan Fattal, Dani Lischinski and Michael Werman. "Gradient Domain
	High Dynamic Range Compression". Proceedings of the 29th annual
	conference on computer graphics and interactive techniques. Pages
	249-256. 2002.
The alpha, beta and saturation parameters are analagous to those described in
the paper, except that in this implementation, alpha is a proportion of the
average gradient magnitude, and beta is what would be 1-beta in the paper.
Gamma is the maximum gradient attenuation factor at a given level, which is
needed in order to cope with small gradients. The paper isn't clear on whether
the average gradient magnitudes used for the alpha scaling should be the
overall averages, or the average only over the current level in the pyramid. We
use a convex combination of these two averages, mediated by the theta
parameter, where 0 chooses the overall average, and 1 the average on the
current pyramid level.

Essentially, alpha controls the point at which contrast-magnification gives way
to contrast-attenuation, beta controls the overall extent to which
contrast-squishing is performed, and increasing theta causes large-scale
(spatially) contrast differences to be given greater importance, relative to
small-scale differences. Changing delta seldom has any great effect. Epsilon is
the termination threshold--increasing it will reduce the quality of the
solution, but improve runtime, while decreasing it will have the opposite
effect. One warning: if epsilon is too small, then the solution may never
converge, and you'll need to abort this program.

Usage: hdr_squish.exe [OPTION...]
  -h, --help                       show help
  -i, --input=INPUT                input file (EXR)
  -o, --output=OUTPUT              output file (EXR)
  -a, --alpha=FLOAT(=0.1)          alpha parameter
  -b, --beta=FLOAT(=0.1)           beta parameter
  -d, --delta=FLOAT(=1.1)          delta parameter
  -t, --theta=FLOAT(=0)            theta parameter
  -e, --epsilon=FLOAT(=0.0001)     epsilon parameter
  -s, --saturation=FLOAT(=1)       saturation parameter