RangeSelection (Range Mask)
RangeSelection creates a mask (a “range mask”) that selects a brightness range. This limits a global process – such as a curve – to the nebula while protecting the background, or the other way round.
Purpose
| Task | Setting |
|---|---|
| process only areas brighter than the background | Lower limit just above the background |
| process only the background | the same mask, Invert |
Whya mask?
Curves and many other processes change all pixels of the same brightness in the same way. Faint nebula and background often sit at almost the same value; lifting the midtones brightens the background as well. The mask limits the effect to the intended area.
Principle
| Mask value | Effect of the following process |
|---|---|
| white | full processing |
| grey | partial |
| black | protected (shown in red when the mask is displayed) |
The process is called RangeSelection (Process → MaskGeneration); the result is the range mask.
Parameters
| Parameter | Effect | Typical choice |
|---|---|---|
| Lower limit | values below become black | just above the background (e.g. 0.15 in a stretched image) |
| Upper limit | values above become black | 1.0 |
| Fuzziness | soft transition at the limits instead of a hard edge | 0.1–0.2 |
| Smoothness | blurs the mask, prevents visible edges | 3–10, up to about 12 |
| Screening | mask graded by brightness – faint structures would hardly be processed | off |
| Lightness | mask from L* instead of the RGB channels; colour images only | off |
| Invert | inverts the mask | off (on: background only) |
Operation
- Adjust RangeSelection with the Real-Time Preview, apply → new image, e.g.
range_mask. - Attach the mask: drag the tab of
range_maskonto the left tab bar of the target image (or Mask → Select Mask). - Hide the red overlay for judging: Mask → Show Mask (the mask stays active).
- Apply the actual process.
- Remove the mask: Mask → Remove Mask.
Typical applications
Lifting nebula, protecting background
Lower limit just above the background, then Remaps tonal values along a freely shaped curve – for contrast, saturation, hue and for shifting individual tonal ranges. Acts globally; locally only with a mask. Open fundamentals → on RGB/K with a slightly raised midtone point (example: 0.30 → 0.33–0.35) and an upper fixed point (example: 0.80 → 0.80).
Processing only the background
The same mask with Invert. Example: Removes an excess of one colour, usually green, by limiting that channel to the level of the other two – a correction of colour errors or a deliberate creative tool. Open fundamentals → against a green cast in the background while nebula with real green or teal components stays protected.
What a good result looks like
Typical errors
| Observation | Cause | Remedy |
|---|---|---|
| Hard transitions in the result | mask too sharp | raise Fuzziness or Smoothness |
| Faint structures not processed | Lower limit too high | lower the Lower limit |
| Background brighter anyway | Lower limit too low | raise the Lower limit |
| Faint structures barely processed despite a suitable limit | Screening active | Screening off |
| Following processes act only partly | mask still active | Mask → Remove Mask |
When to use it – and when not
| Good for | Better another tool |
|---|---|
| limiting an effect by brightness | emphasising local contrast → Raises local contrast – structures that differ only slightly from their immediate surroundings stand out without shifting the global tonal values. Open fundamentals → |
| processing background separately from nebula | protecting or processing stars separately → Splits an image into a starless image and a star image, so that nebula and stars can be processed separately and recombined at the end with a screen blend. Open fundamentals → |
CurvesTransformation · HaLRGB workflow · Narrowband workflow (HOO/SHO) · OSC workflow with a quad band-pass filter · SCNR