LRGBCombination
LRGBCombination combines a separate luminance with a colour image. The luminance provides brightness, structure and sharpness, the colour image provides colour. For the result to look neither pale nor oversaturated, the brightness of both images must match.
Purpose
| Task | Setting |
|---|---|
| combine luminance with a colour image | only L enabled, apply to the colour image |
| balance remaining differences in brightness and saturation | Lightness, Saturation |
| smooth colour noise during combination | Chrominance Noise Reduction |
Principle
LRGBCombination works in the CIE L*a*b* colour space. L* is brightness, a* and b* are the colour components. The L* of the colour image is replaced by the luminance; a* and b* remain.
| Luminance relative to the colour image’s L* | Result |
|---|---|
| brighter | more brightness with the same colour information → pale, washed out |
| darker | oversaturated, dark |
| less contrast | flat, dull colours |
| more contrast | harsh, colours lost in the shadows |
So match luminance and colour image before combining: Measures at fixed spots whether a separate luminance and the colour image match in background, midtones and highlights, and adjusts the luminance with a curve – so the LRGB combination is neither pale nor oversaturated. Open fundamentals →.
Parameters
| Section | Effect | Typical choice |
|---|---|---|
| Channels / Source Images | which channels are replaced | only L enabled with the luminance; R, G, B disabled – the target image supplies the colour |
| Inherit astrometric solution | carry plate-solve data into the result | enabled |
| Lightness | midtone transfer of the luminance | start at 0.5 |
| Saturation | saturation of the result | start at 0.4 |
| Chrominance Noise Reduction | smooths only the colour components, not the structure | enabled; Smoothed wavelet layers 4, Protected wavelet layers 2 |
Operation
There is no real-time preview. Find values on duplicates:
- Drag the colour image’s tab onto an empty area → duplicate.
- Apply LRGBCombination to the duplicate.
- Compare with Ctrl+Z / Ctrl+Y.
- Several values: several duplicates side by side; dragging one window tab onto another window’s tab transfers zoom and position.
- Apply the values found to the actual colour image, close the duplicates.
After combining, the measuring previews can be dragged onto the result to check brightness and contrast.
Typical applications
Balancing remaining differences
| Result looks | Remedy |
|---|---|
| washed out, pale | raise Lightness slightly or lower Saturation slightly |
| oversaturated, dark | lower Lightness slightly or raise Saturation slightly |
Loss of detail after combining
Brightness comes entirely from the luminance. Structures that are brighter in the colour image than in the luminance – for example because a curve lowered them in the luminance and no measuring point was there – become darker and less distinct.
| Remedy | Procedure | Effect |
|---|---|---|
| 1 lower Lightness | try 0.35–0.45 | raises the whole luminance transfer; quickest |
| 2 additional measuring point | preview on the affected structure; reapply the luminance curve | fixes the cause rather than the symptom |
| 3 blend luminances | PixelMath, new image: Luminance * 0.7 + ColourImage_L * 0.3 (share 0.2–0.4); use as L | recovers structures that are well represented only in the colour image |
Side effects of a low Lightness: paler colours → Saturation 0.30–0.35; brighter background → 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 →.
What a good result looks like
Typical errors
| Observation | Cause | Remedy |
|---|---|---|
| Pale colours | luminance too bright relative to the colour | lower Saturation, raise Lightness; check the matching |
| Structures weaker than in the colour image | luminance too dark there | lower Lightness, add a measuring point, blend luminances |
| Flat result | luminance with less contrast than the colour image | repeat the matching before combining |
| Astrometric solution missing | Inherit astrometric solution off | enable it |
| Colour noise visible | colour image not denoised | Chrominance Noise Reduction or AI-based noise reduction with separate strength for fine- and large-scale noise and for brightness and colour. Several light passes keep more detail than one strong pass. Open fundamentals → |
When to use it – and when not
| Good for | Better another tool |
|---|---|
| combining a separate (real or synthetic) luminance with a colour image | assembling channels into a colour image → ChannelCombination or PixelMath |
| processing structure and colour separately and merging them | matching brightness → Measures at fixed spots whether a separate luminance and the colour image match in background, midtones and highlights, and adjusts the luminance with a curve – so the LRGB combination is neither pale nor oversaturated. Open fundamentals → (beforehand) |
HaLRGB workflow · Matching luminance and colour image · Narrowband workflow (HOO/SHO) · OSC workflow with a quad band-pass filter · PixelMath · Synthetic luminance