Adjustments > Levels: best-in-class Auto Levels feature

I'm not exaggerating when I call PD's Auto-Levels "best in class",
because it consistently outperforms the same feature in GIMP, Paint.NET,
and Photoshop.

I have a lot of experience with auto-leveling, as PD's White Balance
tool is really just an auto-leveler.  (Same with the Shadow/Highlight
tool, though it takes a slightly different approach to the problem.)
Because of this, I'm pretty familiar with the shortcomings of
traditional Auto-Level algorithms.

The biggest problem with most auto-levelers is that in trying to fix
lighting issues, they introduce nasty color casts.  By independently
redistributing each RGB channel, auto-leveled images almost always take
on tonal shifts that screw up the original composition.  Skin tones in
particular tend to experience severe cooling, which makes subjects look
like corpses standing under fluorescent lights.

To get around these issues, PD's algorithm is custom-designed, and it
uses a mix of per-channel and uniform corrections that produce really
excellent lighting fixes without screwing up an image's existing
tonality.  Here are two easy testing photos to compare results:

http://nicobass.deviantart.com/art/Lara-Croft-89-163521265
http://nicobass.deviantart.com/art/lara-Croft-90-167090912

CG images are excellent for testing light-adjustment algorithms, as
they're ideally lit under very specific conditions, and they don't have
any auto-correction applied in advance by a camera.  These two images
are particularly tricky for auto-leveling, as one is deliberately
over-exposed while the other is deliberately under-exposed, and they
have completely opposite tonal temperatures.

Comparing the auto-leveling results of GIMP, Paint.NET, Photoshop, and
PD shows that PD's results are far and away the most faithful to the
original tonalities, while still greatly improving contrast and
saturation.  Clicking through the individual channel histograms shows
how PD selectively mixes per-channel and luminance corrections to arrive
at a really nice result, with no clipping issues and no severe
temperature shifts in either image.  Compare this to say Paint.NET's
corrections, which completely destroy both images, or GIMP's
corrections, which achieves reasonably good contrast fixes, but at the
cost of the original warm vs cool tonality of each image.

In case it's not obvious, I'm quite pleased with PD's implementation.
:)  A standalone one-click menu entry for this is forthcoming, pending
some additional testing.