Local Adjustments

Local adjustments - Lab

(12 september 2020)

Introduction

Local editing in RawTherapee is based on RT-spots, which are similar in principle to the U-Point concept originally used in Nikon Capture NX2 © and subsequently in the Nik Collection ©, DxO PhotoLab © and Capture NXD ©. RT-spots use algorithms developed specifically for RawTherapee by Jacques Desmis.

This approach is completely different to the more familiar local editing methods used in applications such as the GIMP, Photoshop © etc., which primarily use selection tools such as lassos, magic wands etc., associated with brushes, layers and blend masks. These methods can be time consuming and difficult to use accurately when complex shapes are involved.

An RT-spot consists of either an ellipse or a rectangle with a variable-diameter spot at the center. The shapes have four control points, which can be adjusted independently or symmetrically, and future developments will provide enhanced shape manipulation.

• Two types RT-spot shape
• 

  Ellipse.

• 

  Rectangle.

The RT-spot algorithm uses shape detection based on ΔE (the change in the visual perception of two given colours) to select the parts of the image to be modified inside the ellipse or rectangle . The extent to which these modifications are applied can be finely controlled allowing for very precise selections. Further refinement is possible with additional parametric masks but the shape-detection algorithms should be sufficient for the vast majority of local editing requirements. RT-spots can also be used to exclude the algorithm from influencing certain parts of the image . The modifications that can be carried out are extensive and incorporate most of the functions available in RawTherapee’s global adjustment tools along with some additional tools available only in the Local Adjustment tab.

The tools are grouped in the following modules (Title - position in pipeline):

Color & Light - 11

Adjust color, lightness, contrast and correct small defects such as red-eye, sensor dust etc. Other functions include a graduated filter, L*a*b* curves and blend modes.

Shadows-Highlights & Tone Equalizer - 5

Adjust shadows & highlights either with shadows & highlights sliders or with a tone equalizer. Can be used instead of, or in conjunction with the Exposure module. Can also be used as a graduated filter.

Vibrance & Warm-Cool - 3

Adjust vibrance (essentially the same as the global adjustment). Carry out the equivalent of a white-balance adjustment using a CIECAM algorithm.

Log Encoding - 0

Adjust under-exposed or high-dynamic-range images using a log-encoded algorithm.

Contrast Attenuator-Dynamic Range Compression & Exposure - 10

Modify exposure in L*a*b* space using Laplacian PDE algorithms to take into account dE and minimize artifacts.

Common Color Mask - 13

A tool in its own right. Can be used to adjust the image appearance (chrominance, luminance, contrast) and texture as a function of Scope.

Soft Light & Original Retinex - 6

Apply a Soft-light blend (identical to the global adjustment). Carry out dodge and burn using the original Retinex algorithm.

Blur-Grain & Denoise - 1

Can be used to blur backgrounds, soften skin, add film grain and denoise.

Tone Mapping - 4

Same as the tone mapping tool in the main menu. The main menu tool must be de-activated if this tool is used.

Dehaze & Retinex - 9

Dehaze and Retinex (Advanced mode only). Useful for dehaze, local contrast with high values and simulation of ‘clarity’.

Sharpening - 8

Uses RL deconvolution sharpening. View at 1:1

Local Contrast & Wavelets - 7

Local contrast (essentially the same as the global adjustment).

CBDL - 2

Contrast by detail levels. Can be used to remove sensor or lens marks.

Each tool module can be toggled between Basic, Standard & Advanced modes. The default mode can be set in RawTherapee’s Preferences window.

Getting start

Activating local adjustments

• In the tab bar, select the "hand" icon (Local Adjustments tab)
• Turn on the "Local Adjustments" power button (if it is not already activated) and expand the "Settings” module.
• Select "Add"

Original

Preparation

Position the RT-spot at the desired location. In this case, we want to increase the saturation of the red flower and reduce the luminance (lightness) without affecting the rest of the image:

• Move the center of the RT-spot so that it is located on an area representative of what you want to change.
• Position the 4 delimiters well beyond the flower.
• Select the "Lockable Color Picker" and locate 3 colors: a) one on the red flower, b) one on the blue sky, c) one on a green leaf.
• In the example the 3 colors are:
  • red flower L=48.6 a=74.4 b=47.0
  • blue sky : L=68.6 a=-3.1 b=-16.6
  • green leaf : L=48.3 a=-28.3 b=51.4

Preparation

Raw file: [1]

Adding the Color & Light tool

In the settings menu, choose "Add tool to current spot".

• You will see a list of choices : "Color & Light - 11",..., "Log Encoding – 0" etc.. For each RT-spot you can associate 1 or more tools from the list. The processing order in the pipeline corresponds to the number at the end of the tool description : "Log Encoding - 0" is first (if it is activated), "Color & Light -11" is the last one. This is also the case for the associated masks.
• Select "Color&Light (Defects) - 11"

Adding the Color & Light tool

Adjusting luminance (lightness) and chrominance

• Set "Lightness" to -70
• Set "Chrominance" to 130

• Review the results
• The red flower now has a new color L= 41.0, a = 65.6, b = 52.8
• the sky is unchanged
• the green leaf is unchanged

Adjusting luminance (lightness) and chrominance

Color Tool Scope and Transition Value

In the "Settings" module

• Observe the effect of moving the "Scope (color tools)" slider
  • if you reduce the value (default 30) only a part of the reds will be affected.
  • if you increase the value, the sky, then the green leaf, then the whole image will be taken into account (Scope=100)

Leave the Scope value at 100 and in the "Settings" module select "Show additional settings"

• Observe the effect of moving the "Transition value" slider:
  • reduce the value to 5
  • increase the value to 100 and see the result

Previewing the adjustment area using deltaE - ΔE

You can preview the areas of the image that will be affected by any changes. The preview does not show the changes themselves or the transitions, but allows you to set the scope of any adjustments. There are two possibilities:

• Use the "Preview ΔE" button located in "Settings". This will only work if you have activated one (and only one) of the tools in "Add tool to current spot" menu.
• Use the "Preview ΔE" option in the "Mask and modifications" menu associated with a particular tool (standard and advanced modes only). In this case the GUI takes into account any adjustments made with the tool and works regardless of the number of activated tools.

You can vary the intensity and color of this preview with "ΔE preview color " in the "Shape detection" section of the "Settings" module . The preview will also let you see the effect of varying the other sliders in the Shape detection section.

Previewing the modifiable area