Friday, June 19, 2015

Experimental "Volume" tool

I've been recently working on a new tool, inspired by this post in Pat David's blog: Clarity in GIMP.

The post describes a method to imitate the "Clarity" effect in Lightroom, by applying a UnSharp Mask (USM) filter at high radius restricted to the mid-tones of the image.

So I took this idea and tried to implement it in a flexible way in PhotoFlow. In order not to re-use the same terminology as Lightroom, the filter is called "Volume" (as it is intended to give some "volume" to your image...).

In the current version, a local contrast enhancement filter (USM for the moment) is strength-modulated with a smooth curve applied to the luminosity channel of the input pixel data. The screenshot below shows the graphical interface of the filter with the default parameter values. The five vertical sliders in the bottom allow to control the strength of the effect in various luminosity ranges. The example below corresponds to a possible selection of the mid-tones.


More specifically, the 5 sliders correspond to input values of 0% (blacks), 25% (shadows), 50% (mid-tones), 75% (highlights) and 100% (whites). For example, the default configuration show above is equivalent to this smooth tone curve:


The filter is still in an experimental form, but works already without issues and therefore it will most likely be included in the next release for user testing and feedback.

In future, a simple extension (which is already foreseen in the code and GUI) will be to add other local contrast enhancement filters, in addition to large-radius USM. Here are some possible choices:
  • Surface blur (or "bilateral smoothing")
  • G'MIC Local Normalization
  • G'MIC Local Variance Normalization

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