Vibrancy and Saturation

Thom Hogan posted a recent article on the difference between color saturation and vibrancy, and it's well worth the read for those interested in how to manage colors in post.  Most cameras have a "vivid" setting that allows users to dial up color saturation in camera, but often these settings blow out colors we'd rather leave alone.  Ken Rockwell is fond of VIVID++!!!BANGBANG!ONE!, but notes that this usually destroys skin tones, for example.

Color correction is a big subject about which I know comparatively little, but he mentions a significant issue when increasing color saturation:  linearity.  A heedless linear increase in color saturation across all spectra will lead to poor results.

While Photoshop has (I'm led to believe) a vibrancy control, the Gimp does not.  (There is a vibrancy plugin, but as far as I know it's not standard.)

A suitable method for increasing saturation in a non-linear fashion is to decompose the image into an appropriate color space and modify the saturation curve.  For the following example, I'll use an image from my shots at Hopewell Furnace.

The source image is the one of a sheep chewing:

The subject matter is fine, but the background could possibly use a bit more color to help offset the sheep.  (Or at least, that's what I'll say for sake of the post.)

To achieve this, I decomposed the image (Colors > Components > Decompose) into the HSV (hue-saturation-value) color space.  Other spaces will work, too, but this is a convenient one for working directly on saturation.

The tool will decompose each element of the color space into its own layer, with saturation in the middle.  After turning off the visibility of the hue layer (the top one), I used the curves tool:

The after shot looks a bit like this:

The important thing to note here is that the high-valued part of the saturation curve is kept mostly constant to avoid oversaturating the colors that are already well saturated.  The bottom half of the curve is practically black and white, so I attack it gradually to bring it up.

n.b., this aggressive approach is bound to make things a bit uglier than they should be because there's not much data to work with.  So while the results here are instructive, they are not wonderful.

After applying the curve, I recomposed the layers (Colors > Components > Compose).

The end result looks like so:

And a side-by-side 100% crop: