Matching luminance and colour image
Before a separate luminance is combined with a colour image, the brightness of both must match. The procedure measures at fixed spots, evaluates the pattern of deviations and adjusts the luminance with a curve.
Purpose
| Task | Procedure |
|---|---|
| check whether luminance and colour image match | measuring at previews |
| correct deviations specifically | curve from the measured values |
| balance remaining differences | Lightness and Saturation in Replaces the brightness of a colour image with a separate luminance – structure and sharpness from the luminance, colour from the colour image. Open fundamentals → |
Principle
Whycompare?
LRGBCombination replaces the colour image’s L* with the luminance; a* and b* remain. If the luminance is brighter than the colour image’s L* at a spot, that area gets more brightness with the same colour information – it looks paler. If it is darker, it looks oversaturated and dark. Only when both brightnesses match is the image balanced.
The goal is for background, faint structures and bright areas to relate similarly in both images. The luminance is adjusted, not the colour image.
Whyadjust the luminance and not the colour image?
The colour image is colour-balanced (white balance, background). Any curve there also changes saturation and colour balance. The luminance is monochrome; a curve there changes only brightness.
Operation
Extracting the colour image’s luminance
Whynot measure directly on the colour image?
A readout on the colour image returns channel values or their median. In a red nebula R is high, G and B low; the median then lies near G or B and underestimates the brightness. Only L* is comparable, because exactly that is replaced in the combination.
- ChannelExtraction on the colour image.
- Color space CIE L*a*b*, enable only L.
- Apply → greyscale image, e.g.
ColourImage_L.
Defining measuring spots
Measurements by mouse click are hard to reproduce. Previews have exact coordinates, can be transferred to the second image, are saved in the project and can be measured again after every change.
- Draw three small previews (e.g. 20×20 px) in the luminance – with Alt+N or via Preview → New Preview… with coordinates.
- Rename them (double-click the preview tab):
| Preview | Position |
|---|---|
Dark | star-free background |
Mid | faint structure, e.g. outer nebula |
Bright | bright part of the object |
| optional further ones | area of a second structure colour, e.g. OIII |
- Drag the preview tab onto the other image window’s tab bar → a preview with identical position and size is created there. Both images must have the same dimensions.
Do not place them on stars or star remnants – these distort the value.
Measuring
- Recommended: Statistics – open the process, select the preview at the top, read the median. The median over an area is more stable than a single point.
- Alternative: Readout – Edit → Readout Options: Probe size 9×9 or 15×15, Mean or Median, Apply Global; measure at the same coordinates in both images.
| Measuring spot | Luminance | ColourImage_L | Difference |
|---|---|---|---|
| Dark | |||
| Mid | |||
| Bright |
Typical applications
Evaluating the pattern
| Pattern | Meaning | Correction of the luminance |
|---|---|---|
| all values similar (a few hundredths) | matches | none |
| only Mid differs | different midtones | curve: fix Dark and Bright, move Mid |
| Dark and Mid higher, Bright lower | luminance with less contrast | gentle S-curve |
| Dark and Mid lower, Bright higher | luminance with more contrast | inverse S-curve |
| only Dark clearly different | different background | black point or curve point |
| all uniformly higher or lower | different global brightness | curve or Lightness in LRGBCombination |
| only one structure colour differs | synthetic luminance contains this structure too weakly or too strongly | check the weighting of the synthetic luminance |
Matching with a curve
Each measured value becomes a curve point in 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 →: Input = luminance value, Output = value from ColourImage_L. Remove any mask first, reset the curve, channel RGB/K.
| Goal | Dark | Mid | Bright |
|---|---|---|---|
| midtones only | output = input | target | output = input |
| match contrast | target | target | target |
Compromise with additional signal
If the luminance contains additional signal (e.g. Ha in a synthetic or HaL luminance), faint structures are deliberately brighter there. Then do not set the Mid point fully to the target but between luminance and target value: part of the additional detail remains without strongly washing out the colours of these areas.
Example
Example (HaLRGB): comparison after stretching.
| Measuring spot | Luminance | ColourImage_L |
|---|---|---|
| Dark | 0.127 | 0.13 |
| Mid | 0.25 | 0.18 |
| Bright | 0.658 | 0.67 |
Pattern “only Mid differs” (Ha signal in the luminance) → midtones only, with compromise: Dark 0.13 → 0.13, Mid 0.25 → 0.21, Bright 0.66 → 0.66.
The same data set before combining, after processing the starless images: Dark 0.14 / 0.10, Mid 0.26 / 0.20, Bright 0.61 / 0.64. Pattern “less contrast” → gentle S-curve: 0.14 → 0.105, 0.26 → 0.21, 0.61 → 0.63.
What a good result looks like
Typical errors
| Observation | Cause | Remedy |
|---|---|---|
| Measured values fluctuate | measuring by click, star in the area | previews and Statistics (median) |
| Curve entirely below or above the diagonal although contrast should be matched | input error, e.g. mistyped output | check values |
| Dark clouds black after the curve | Dark point lowered too much | lower it less |
| Ha detail lost after combining | Mid point set fully to the target | compromise between luminance and target |
| Structure weaker after combining | no measuring spot on this structure | additional spot, Replaces the brightness of a colour image with a separate luminance – structure and sharpness from the luminance, colour from the colour image. Open fundamentals → |
| Preview cannot be transferred | images of different size | ensure identical dimensions |
When to use it – and when not
| Good for | Better another tool |
|---|---|
| before every combination of a separate luminance and a colour image (LRGB, HaLRGB, synthetic luminance) | colour image without a separate luminance → no matching needed |
| again, whenever one of the two images has been changed afterwards (curves, saturation, LHE) | remaining differences after matching → Replaces the brightness of a colour image with a separate luminance – structure and sharpness from the luminance, colour from the colour image. Open fundamentals → |
CurvesTransformation · HaLRGB workflow · LRGBCombination · Narrowband workflow (HOO/SHO) · OSC workflow with a quad band-pass filter · Synthetic luminance