Tighten Edges

Purpose and expected result

Tighten Edges darkens samples in proportion to a fixed local edge magnitude, then blends that filtered image with the source. On bright structures against a dark field this can contract or harden the bright-side transition. It is not a deconvolution, an unsharp mask, or a selectable-scale morphology operation.

Use it sparingly on a duplicate, star layer, or mask whose edges actually need contraction. The operation is mathematically nonlinear because it uses absolute gradient responses and clamps, even though the process accepts both linear and nonlinear images and does not change their recorded linearity state.

At a glance

Item	Current behavior
Menu path	Process > Morphology > Tighten Edges
Enabled when	An image or sequence is active, the same process is not already attached, and no attached process holds exclusive access
Inputs	One grayscale or RGB still/sequence; RGB can use stored or derived component channels; intended finite normalized samples; optional enabled active mask
Outputs	Replaces the target at the same geometry and pixel type; RGB/component reconstruction follows the selected Channel; a successful sequence uses a temporary replacement sequence; Take snapshot creates an auxiliary still
Recommended image state	Linear or nonlinear is accepted; visually easier to tune after a display stretch or on a dedicated mask/star layer; recorded processing state is preserved rather than explicitly changed
Preview support	Asynchronous full-image Preview with Linear/Autostretch display, original/result switch, selectable Preview area, and snapshot
Sequence support	Generic temporary replacement-sequence path with progress and cooperative cancellation; configured CFA auto-debayer can supply RGB frames
Mask support	Active target mask post-blends source and the Amount-blended process result; white applies, black protects, gray blends, and Invert Mask reverses weights
External dependencies	None; the AfterNight engine's own four-direction edge-tightening filter

Before you start

Reduce noise before tightening edges: the filter responds to every local gradient, including grain, ringing, hot pixels, mask stair steps, and compression artifacts. Inspect at 100% and include both a representative transition and smooth background.

Choose the process Channel before tuning. RGB filters stored R, G, and B independently and can produce colored rims. A luminance/lightness component can reduce that risk, but component conversion/reconstruction can change colour, and an integer image's reconstruction clips at its type's range (a floating image keeps values outside [0, 1]).

Procedure

1. Select a duplicate, mask, or star layer and open Process > Morphology > Tighten Edges.
2. Choose Channel. For RGB color data, compare RGB with CIE L* (Lightness) or CIE Y (Luminance) rather than assuming they are equivalent.
3. Set Value to 0.10..0.25 and keep Amount below 1.00 while finding a useful direction. This is conservative guidance, not a measured optimum.
4. Inspect bright and dark sides of edges, small stars, noise, all four frame borders, and any active-mask boundary. Use 100% zoom.
5. If using Preview area, ignore its one-pixel perimeter when judging the full result; cropping creates different edge neighbors there.
6. Select Apply and recheck the committed target, metadata/state, and Image History before continuing.

Complete parameter and action reference

UI label/action	Type and unit	Runtime default	Persisted-default exception	Valid values, step, or constraints	Effect of increasing, decreasing, or selecting	Dependencies and enable/visibility rules	Quality, artifact, runtime, memory, or disk trade-off	Scope: preview/apply, channel, mask, still/sequence	Persistence
Value	Double value plus slider, edge-subtraction scale	0.00	None	0.00..2.00; displayed spin step 0.50 with adaptive decimal stepping; slider has 2001 positions	Multiplies the modified Sobel magnitude before subtraction; 0 is identity for finite inputs within full scale	Apply disabled only at exactly 0; control disables during Apply	Larger values darken/clip more edge samples and amplify border, ringing, noise, and color artifacts; fixed kernel cost is unchanged	Preview/Apply; selected channel/component; before Amount and mask; still/sequence	Captured per execution; Reset/success returns 0.00
Amount	Double value plus slider, blend fraction	1.00	None	0.00..1.00; displayed spin step 0.50 with adaptive decimal stepping; slider has 1001 positions	Blends source to the already filtered/clamped result: 0 source, 1 full filter	Apply gate ignores Amount; with Value nonzero, Amount 0 permits an identity-looking Apply	Reduces visible effect without changing edge-map work; cannot undo clipping inside the filtered branch, only mix source back	Preview/Apply; after selected-channel reconstruction and before active mask; still/sequence	Captured per execution; Reset/success returns 1.00
Target Image	Image selector	Attached/active target	None	Open eligible still/sequence	Changes source, Channel list, Preview, active mask, and Apply destination	Active image required; duplicate/exclusive launch guards apply	Wrong target is destructively replaced after success	Preview/Apply source and active mask	Target lifetime; Reset does not change it
Channel	Combo box	RGB for RGB; disabled Grayscale for mono	None	RGB, Red, Green, Blue, CIE L* (Lightness), CIE a, CIE b, CIE c, CIE h, CIE X, CIE Y (Luminance), CIE Z, HSL Hue, HSL Saturation, HSL Lightness, HSV Hue, HSV Saturation, HSV Value, YCbCr Luma, YCbCr Blue Chroma, YCbCr Red Chroma	RGB filters three stored planes independently; a component filters one converted plane and reconstructs/copies the others	Component choices require RGB; mono selector disabled; disabled during Apply; the selection is captured when Apply starts	RGB can make color rims; component round trips add buffers/quantization/gamut effects; hue planes are treated as ordinary noncircular scalars	Preview/Apply; active mask post-blend; still/sequence	Dialog-local; Reset/success does not restore RGB
Preview	Checkbox	Checked	None	Checked/unchecked	Enables/disables asynchronous full-image or selected-area execution	Remains interactive during Apply	Full filter, float buffers, optional component conversion, and display autostretch add work	Preview only; mask-aware; still/current displayed sequence frame	Window lifetime; Reset does not change it
Preview pane	Image viewer	Filtered result	None	Pan with scroll bars or dragging; toolbar actions below	Shows mask-aware result without modifying the target	Exists only with Preview	Holds source/result/display buffers in addition to filter buffers	Preview only	Window lifetime
Fit To Window	Preview-toolbar action	Preview fitted when first displayed	None	Click or Ctrl+Shift+F	Fits the whole image uniformly in the viewport	Preview image required	Resampling can hide one-pixel rims and noise	Preview display only	Viewer lifetime
Zoom In	Preview-toolbar action	Current scale	None	Click; 1.2× relative zoom within viewer limits	Enlarges around viewport center	Preview image required	Rendering only	Preview display only	Viewer lifetime
Zoom Out	Preview-toolbar action	Current scale	None	Click; 0.8× relative zoom within viewer limits	Reduces around viewport center	Preview image required	Shows more context but hides fine artifacts	Preview display only	Viewer lifetime
Zoom 100%	Preview-toolbar action	Current scale	None	Click; absolute scale 1.0	Shows the viewer's one-pixel scale	Preview image required	Required for one-pixel edge/border artifacts	Preview display only	Viewer lifetime
View Channel	Preview-toolbar combo box	RGB for RGB; disabled Grayscale for mono	None	Stored and derived RGB/CIE/HSL/HSV/HSI/YCbCr display components	Selects only the displayed component	Separate from process Channel; RGB target required for components	Helps diagnose colored rims without changing pixels	Preview display only	Viewer lifetime; not in process history
Display Mode	Preview-toolbar combo box	Inherits target Linear or Autostretch when Preview is created	None	Linear, Autostretch	Changes display only; Autostretch uses prepared target/result statistics	Autostretch enables settings action	Strong stretch can exaggerate noise and rims	Preview display only	Preview-view state; not in process history
Autostretch Settings	Preview-toolbar dialog action	Disabled in Linear mode; inherits target-view settings	Shared-view values load when opened	Opens Preview autostretch controls; Apply Stretch is hidden	Tunes Preview display only	Autostretch mode only; disabled while its dialog is open	Changes visual judgment, never filter samples	Preview display only	Viewer-scoped; not a Tighten Edges parameter
Switch image	Preview-toolbar action	Filtered Preview shown	None	Click, Space, right-click, or Ctrl+Shift+O	Toggles result/original	Preview required	Comparison only; no recomputation	Preview display; still/current frame	Preview-window lifetime
Preview area	Preview-toolbar checkbox	Unchecked; first enablement centers a bounded 600×600 area or the full image when smaller	None	Checked/unchecked; move/resize overlay	Processes only the crop and composites it over the original Preview; Apply remains full-frame	Preview required	Faster tuning, but crop borders use zero neighbors and differ from full-image interior; inspect outside its one-pixel perimeter	Preview and Take snapshot; mask cropped consistently	Preview-window lifetime; Reset unchanged
Take snapshot	Preview-toolbar action	No snapshot	None	Click or Ctrl+Shift+K	Re-executes settings and opens <target> - Tighten Edges - SNAPSHOT	Preview required	Adds a complete execution and auxiliary image; Preview-area perimeter behavior is baked into the snapshot	New still; target unchanged; area/mask honored	Each action creates a separate image view
Apply	Button/action	Disabled while Value is 0; enabled otherwise, even when Amount is 0	None	One action	Captures Value/Amount, locks target, then filters the full still or every sequence frame	Apply visually stays enabled during work when Value is nonzero, although repeat clicks are ignored; Target, Channel, Value, Amount, Close, and Reset are disabled during Apply	Full float edge/filter buffers plus component conversion, blend, mask, and sequence work	Destructive still/sequence replacement after success	Records Value/Amount and, for a colour image, Channel; success resets Value/Amount only
Reset	Button/action	Enabled while idle	None	One action	Restores Value 0.00 and Amount 1.00	Does not change Target, Channel, Preview, mask, or Preview toolbar; disabled during Apply	Checked Preview schedules another full execution	Configuration only	Immediate; also invoked after successful Apply
Close	Button/action	Enabled while idle	None	One action	Closes without applying current settings	Disabled during Apply	A preview being computed is cancelled	Window/execution	Process settings do not persist after destruction
Sequence progress / Cancel	Shared per-frame progress and Cancel	Created for sequence Apply	None	Cooperative Cancel; frame/task text updates around temporary processing	Sequence target only; automatic CFA debayer can produce RGB frames	The filter observes Cancel between rows, so a frame stops promptly	Temporary encode/disk cost; Value, Amount, and Channel are captured once for every frame	Cancel removes temporary output; success replaces sequence, records one history step, and resets Value/Amount

How the algorithm works

Conceptual pipeline

1. The engine reads the selected stored image or component in its stored units, in single precision; double data is narrowed to float32.
2. For every plane, compute four signed 3×3 directional responses: horizontal, vertical, and two diagonals. Sum their absolute magnitudes and multiply by 0.05.
3. Subtract Value times that nonnegative edge map from the source, then clamp the filtered branch to the image's full scale (0..1 for float, the type's range for an integer image) and write it back in the target pixel type.
4. Reconstruct RGB for a selected component, blend source and filtered result by Amount, then post-blend the active mask.

Model and equations

For one plane S, written as fractions of full scale, and the four fixed derivative kernels K_j, the edge map is

\[ E(x,y)=0.05\sum_{j=1}^{4}\left|K_j*S\right|(x,y). \]

There is one fixed 3×3 neighborhood: no radius, iteration, pyramid, or scale selector exists. With Value v and Amount a,

\[ T(x,y)=\operatorname{clamp}(S(x,y)-vE(x,y),0,1), \]

\[ O(x,y)=(1-a)S(x,y)+aT(x,y). \]

Where T has not clipped, the net subtraction is approximately avE; after T reaches zero, lowering Amount mixes source back but does not change the clipped filtered branch. The active mask is a second blend after O.

The filter only subtracts a nonnegative edge magnitude. It does not add a high-pass residual or solve a point-spread function. Bright-side transitions can contract; dark-side pixels remain at the lower clamp rather than being sharpened symmetrically.

Boundary and failure behavior

Every neighbour outside the image is a constant zero, on all four sides. The zero border can create dark/colored frame rims. A Preview crop creates new boundaries, so its one-pixel perimeter is not Apply-equivalent.

The engine does not validate finite floating samples. Its clamp maps finite output below the range to its bottom and above it to its top; NaN is not sanitized. The filter observes Cancel between rows.

Parameter interactions and starting points

Combination	Conservative starting point	Too weak looks like	Too strong looks like	Adjustment order
Value × Amount	Value 0.10..0.25, Amount 0.25..0.50	Edge width/brightness barely changes	Dark rims, clipped bright-side detail, crunchy noise	Set modest Value, then blend with Amount
RGB × luminance component	Compare RGB with CIE Y (Luminance) or CIE L* (Lightness)	Luminance-only result may leave chromatic edges	RGB produces unequal channel rims or colored borders	Choose Channel first, then tune Value/Amount
Fixed scale × sampling	Inspect at 100%; resize/output scale matters	Coarse/soft structures are unaffected	Fine noise and single-pixel stars are eroded	Judge at final sampling; use another tool for broader scales
Preview area × borders	Ignore one-pixel crop perimeter	Interior change seems subtle	Crop seam/rim is mistaken for full-image behavior	Tune interior, then verify full Preview and frame borders
Amount × active mask	Start with a soft verified mask and modest Amount	Protected edge changes too little	Filter and mask boundaries combine visibly	Tune unmasked direction, then enable/inspect mask

Inputs, outputs, and metadata

• Output dimensions and target pixel type match the input. RGB processing returns RGB; grayscale remains grayscale. Component conversion reconstructs RGB through the shared Channel dispatcher.
• The result keeps the source image's header cards, metadata, and existing processing state for every Channel choice; Tighten Edges adds no explicit linear/nonlinear provenance marker.
• The target view is replaced in place. Non-float/component temporary paths do not explicitly guarantee the image-internal name/file-name, although the view identity remains.
• Image History records Tighten Edges, Value, and Amount, and for a colour image in channel: <channel> with the selected Channel, derived components included.
• Sequence Apply writes a temporary sequence and replaces the target only after success. Value, Amount, and Channel are captured once for every frame.

Performance

Cost is linear in pixel count with a fixed four-response 3×3 neighborhood, run on the threads the operation is granted. RGB processes three planes. The filter holds one float32 plane of accumulated edge magnitudes beside its working planes; double data is narrowed for computation. Amount adds a full-image blend and a mask adds another. Derived components add conversion and reconstruction buffers. Sequences add decode, encode, and temporary-disk work per frame.

Preview area reduces pixel work but introduces crop-border differences. No production timing or peak memory was measured.

Worked example

• Evidence mode: Source/equation-derived; not executed on project-owned astronomical data.
• Identity checkpoint: Value 0 gives T=S for normalized finite inputs; Apply is disabled. With Value nonzero and Amount 0, O=S, but Apply remains enabled and still records a process step.
• Bright-side checkpoint: for S=0.8, local edge magnitude E=0.2, Value 0.5, and Amount 1, T=O=0.7.
• Blend checkpoint: with the same S, E, and Value but Amount 0.25, O=0.775.
• Clip checkpoint: if vE>S, the filtered branch is 0; Amount 0.25 returns 0.75S, not a negative sample.
• Inspect in real use: both sides of transitions, noise, small stars, channel rims, all frame borders, Preview-area perimeter, mask boundary, and metadata.
• Limitations: These equations verify the implementation order, not halo reduction quality, a saved sequence, runtime, or memory use.

Warnings and limitations

Inspect every frame border

Neighbours outside the image count as zero on every side. Strong Value settings can create dark or colored border rims.

Preview-area perimeter is not Apply-equivalent

Cropping creates new zero neighbors. Judge the Preview-area interior, then switch to full Preview before Apply.

• Tighten Edges has one fixed 3×3 scale. Value is strength, not radius or number of scales; Amount is only a final source/result blend.
• The transform subtracts edge magnitude. It can contract bright structures but can also erode faint detail, deepen dark rims, and amplify visible noise.
• RGB planes are independent. Derived hue components are processed as ordinary linear scalars even though hue is circular; avoid HSL/HSV/CIE h* unless that noncircular behavior is explicitly intended.
• Apply can look enabled during execution, but repeat clicks are ignored; the other controls are disabled until it finishes.
• The kernel computes in float32, and a derived component's reconstruction of a floating image is not clamped to [0, 1].

Troubleshooting

Symptom or exact error	Verified cause/validation path	Recovery	Evidence source
Tighten Edges menu is disabled	No active image/sequence or generic process launch guard failed	Activate a target; close the same or exclusive process	action state/launcher
Apply is disabled	Value is exactly 0	Set a nonzero Value; Amount alone cannot enable Apply	settingsUpdatedImpl
Apply runs but result is unchanged	Amount is 0, or selected Channel has negligible local gradients	Raise Amount or choose the intended Channel	modulation blend/channel dispatcher
Dark/colored frame rim appears	Neighbours outside the image count as zero	Reduce Value/Amount; inspect/crop final border only after processing	Engine zero border
Preview-area seam differs after Apply	Crop created a different one-pixel neighborhood	Ignore crop perimeter and verify full Preview	Preview crop plus 3×3 edge map
RGB result shows color fringes	R, G, and B edge maps are independent	Try CIE Y/L* on a duplicate; lower Value/Amount	RGB filter branch

Related workflow steps

Use Unsharp Mask when you need a selectable Gaussian scale and symmetric detail addition, or Morphological Transformation for explicit erosion/dilation/opening/closing with a chosen structuring element. For star-layer-specific tightening inside recombination, see Profiled Starmask Recombination, which applies the same engine filter before its mask/addition stage. Verify Image History, Image Statistics, and frame borders after Apply.

References and provenance

• AfterNight's engine edge tightening (planEdgeTighten in src/AfterNightEngine/include/AfterNightEngine/Algorithms/Spatial/GeneralFilter.h) is authoritative for the four 3×3 responses, 0.05 magnitude scale, subtraction/clipping, float32 work precision, and constant-zero border. This process does not call an external Sobel or morphology implementation.
• Shared AfterNight process/channel/preview source is authoritative for Amount blending, component conversion, metadata/history behavior, masks, Preview area, and temporary sequence execution.