MorphologicalTransformation
MorphologicalTransformation changes structures by their shape. In astrophotography it is used mainly for star reduction: stars shrink from the outside and stay round.
Purpose
| Task | Setting |
|---|---|
| shrink stars, push back faint stars | Morphological Selection, Selection below 0.5 |
| enlarge stars (rarely) | Selection above 0.5 |
Whyreduce stars?
In dense star fields the stars cover the nebula, and stretching has turned even faint stars into prominent dots. Reduction makes stars smaller instead of only dimming them; faint stars recede, bright ones remain as accents.
Principle
For each pixel the operator looks at the neighbourhood defined by the structuring element and picks a value from it.
WhyMorphological Selection rather than erosion?
Morphological Selection picks a value from the sorted list of neighbourhood values. With Selection below 0.5 a darker neighbouring value is chosen – the star shrinks from the outside but stays round as long as the structuring element is circular. Pure erosion would be harsher and would wipe out faint stars entirely.
The effect mainly concerns structures about the size of the structuring element; large bright stars with halos hardly change.
Parameters
| Parameter | Effect | Typical choice |
|---|---|---|
| Operator | type of selection | Morphological Selection |
| Interlacing / Iterations | spacing of the pixels considered / repetitions | 1 / 1 |
| Amount | blend with the original; 0.25 barely visible | 0.5–0.8 |
| Selection | below 0.5 shrinks, 0.5 neutral (median), above 0.5 enlarges | 0.2–0.3 |
| Structuring Element | shape and size of the neighbourhood | size 5, circular; 3 if stars turn angular |
Structuring element: white cells are active. Circular means: corners black, the rest white (icon with a filled circle). If only the corners are white, the shape is inverted → choose the circle icon. The shape imprints itself on the stars; a square produces angular stars.
Operation
- Apply to a copy of the star image (after 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 →). Nothing there needs protecting. In the full image a star mask would be required.
- There is no real-time preview: apply, compare at 1:1 or 2:1 with Ctrl+Z / Ctrl+Y, adjust values.
- Judge the result finally only after the stars have been recombined.
Typical applications
Star reduction in the star image
Morphological Selection, Selection 0.2–0.3, Amount 0.5–0.8, structuring element 5×5 circular.
Example: Amount 0.60, Selection 0.25, 5×5 circular – faint stars clearly pushed back, medium stars smaller and round, bright stars with halos barely changed.
What a good result looks like
Typical errors
| Observation | Cause | Remedy |
|---|---|---|
| Effect too weak | Amount low | Amount 0.7–0.8 |
| Faint stars gone completely | Amount too high | Amount 0.5 |
| Stars angular or with a dark core | structuring element too large or square | lower Amount, 3×3 or circular |
| Stars larger instead of smaller | Selection above 0.5 | Selection 0.2–0.3 |
| Stars distorted | structuring element inverted | set circular again |
| Nebula changed | applied to the full image without a mask | apply to the star image |
When to use it – and when not
| Good for | Better another tool |
|---|---|
| shrinking stars, especially in dense fields | only dimming stars overall → factor s during 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 → |
| pushing back faint stars | greenish stars → 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 → |
BlurXTerminator (BXT) · HaLRGB workflow · Narrowband workflow (HOO/SHO) · OSC workflow with a quad band-pass filter · StarXTerminator (SXT)