Sharpen Alpha

Purpose and expected result

Multiscale Sharpen Alpha decomposes the selected image or component into opacity-weighted detail layers at successively broader scales, increases the opacity of those layers, and alpha-composites them back over the residual base. It is not the same model as Unsharp Mask: there is no single radius and no additive high-pass equation exposed to the user.

The expected result is a same-size still or replacement sequence with stronger detail over several scales. Channel count normally matches the input, but configured CFA sequence frames can be auto-debayered to RGB before filtering and temporary replacement. Amplitude controls the layer boost, Number of Scales controls how many octave-like supports are available, Anisotropy changes how the bases are smoothed, and Amount blends the complete sharpened result back with the source. Strong settings can amplify noise, form star rims or broad halos, exaggerate color edges, and flatten highlights or shadows through the final [0,1] clamp.

At a glance

Item	Current behavior
Menu path	Process > Details > Multiscale Alpha Sharpen; window title Multiscale Sharpen Alpha
Enabled when	An image or sequence is active, the same process is not already attached to it, and no attached process holds exclusive access
Inputs	One grayscale or RGB still or sequence; a still CFA mosaic is treated as one grayscale sample plane unless debayered first; sequence auto-debayer can convert configured CFA frames to RGB before filtering; an optional active mask must match target geometry
Outputs	Destructively replaces the still or, after successful temporary processing, the sequence; Take snapshot creates an auxiliary still; no file is written directly
Recommended image state	Usually finite, normalized data after calibration, registration, stacking, gradient correction, color calibration, and noise reduction; test final-scale sharpening on a duplicate
Preview support	Asynchronous full-image or movable-area preview, original/result switching, snapshot, and display-channel selection; this process currently exposes no Display Mode or Autostretch Settings controls
Sequence support	Yes; numerical parameters and Channel are captured once when Apply starts and used for every frame; temporary output replaces the sequence only after success
Mask support	Yes; the active mask blends the Amount-modulated result with the original after filtering
External dependency	Bundled G'MIC 3.5.4/CImg runtime, run through AfterNight's optional G'MIC adapter; no separate installation or network access. In a build without G'MIC, Preview and Apply are disabled and their tooltip says why
Non-finite samples	An image holding a NaN or infinite sample is refused before G'MIC sees it, and left unchanged

Before you start

• Reduce objectionable noise first. Alpha-detail layers do not distinguish real small structure from shot noise, ringing, hot-pixel residue, or compression edges.
• Debayer CFA data before color sharpening. A CFA still is exposed as one grayscale plane and can sharpen the sensor mosaic itself.
• Use finite, normalized samples. The G'MIC result is clamped to [0,1] even for float or double input. An image holding a NaN or infinite sample is refused with "the image holds a sample that is not a finite number, which G'MIC cannot process" and left unchanged.
• This process needs G'MIC, which AfterNight normally includes. In a build without it, Preview and Apply are disabled and their tooltip reads "This build of AfterNight carries no G'MIC library, so this process cannot run."
• Duplicate the target or verify Image History. Apply replaces the target only after successful processing.
• Decide between RGB and a luminance/lightness component. The RGB path sends all stored channels through the same alpha-layer command; a derived component is filtered as one plane and reconstructed afterward.
• Inspect a smooth background, fine stars, a bright high-contrast edge, and broad structure at Zoom 100%. The embedded preview is linear-only for this process, so compare it with a linear target view or account for the difference.

Procedure

1. Select the target and open Process > Details > Multiscale Alpha Sharpen. Confirm Target Image, choose RGB or one deliberate component, and leave Preview checked.
2. Start with Amplitude 1.0, Number of Scales 4, Anisotropy 0.5, and Amount 1.0, the values the window opens with and Reset restores.
3. Use Preview area around representative fine detail and background. Change Number of Scales first: more scales add progressively broader supports.
4. Adjust Amplitude for the alpha-layer boost, then reduce Amount to mix the complete result with the source. Amount is the safer final strength control.
5. Compare Anisotropy values while watching real edges and background texture. Its bundled branch changes the scale bases; it is not a halo-removal switch.
6. Toggle Switch image, inspect Zoom 100%, and check the full frame for broad halos before Apply. Confirm target, Channel, mask, and inversion.
7. Select Apply. For a sequence, monitor progress; Cancel is cooperative inside G'MIC, and the original sequence remains until temporary work succeeds.

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
Amplitude	Double value plus slider, raw G'MIC alpha boost	1.00	G'MIC's GUI expresses the same raw value as 100%; AfterNight displays and passes the raw 0..4 value	0.00..4.00; spin step 0.10 with adaptive stepping; slider has 4001 positions	0 leaves layer opacity unboosted; increasing toward 4 raises opacity multipliers up to 1 + Amplitude across the scale stack	Enabled while idle; disabled during Apply	Higher values emphasize detail/noise and increase clipping/halo risk; decomposition cost is largely unchanged	Preview/Apply; selected channel/component; still/sequence; before Amount and mask	Captured per execution; Reset and successful Apply return 1.00
Number of Scales	Integer value plus slider, decomposition levels	4 when a new process opens	None	1..10; step 1; G'MIC limits the effective value to floor(log2(min(width,height)))	1 yields no detail split and is neutral; higher values add bases at sigma 0.25 + 2^k for k=0..N-2 (subject to the image-size limit)	Enabled while idle; disabled during Apply	More scales add broader halos/support and more blur, alpha-layer, and composite work; 10 reaches very broad nominal support	Preview/Apply; selected channel/component; still/sequence	Captured per execution; Reset and successful Apply return 4
Anisotropy	Double value plus slider	0.50	None	0.00..1.00; spin step 0.05 with adaptive stepping; slider has 1001 positions	0 builds each base with Gaussian blur; values above 0 use a bilateral blur with spatial sigma 0.25+2^k, range sigma (1-Anisotropy)×100, then Gaussian blur with sigma 0.5+Anisotropy	Enabled while idle; disabled during Apply	Changes decomposition shape and cost; high values can preserve/redirect strong edges but can also form directional or edge-local artifacts; normalized AfterNight input makes the fixed range constants an important limitation	Preview/Apply; selected channel/component; still/sequence	Captured per execution; Reset and successful Apply return 0.50
Amount	Double value plus slider, final blend fraction	1.00	None	0.00..1.00; spin step 0.05 with adaptive stepping; slider has 1001 positions	Shared post-filter blend: source × (1-Amount) + sharpened × Amount; 0 is neutral and 1 selects the complete G'MIC result	Enabled while idle; disabled during Apply	Reduces all scale effects together but does not skip G'MIC work at 0; active mask blends after Amount	Preview/Apply; selected channel/component; still/sequence; before mask	Captured per execution; Reset and successful Apply return 1.00
Target Image	Image selector	Attached/active target	None	Open eligible image or sequence	Changes source, Channel list, Preview, active mask, and Apply destination	Active image required; duplicate/exclusive launch gates apply	A 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 processes the multichannel G'MIC image; a component filters one converted plane and reconstructs/copies the others	Component choices require RGB; mono selector is disabled; disabled during Apply; the selection is captured when Apply starts	RGB alpha calculation can couple channel extrema and produce color changes; component round trips add buffers, quantization, and possible gamut clipping	Preview/Apply; active mask post-blend; still/sequence	Dialog-local; Reset and successful Apply retain the selected Channel
Preview	Checkbox	Checked	None	Checked/unchecked	Enables or disables asynchronous full-image or selected-area execution	Process idle	Runs the complete G'MIC decomposition; frequent changes allocate float/layer buffers	Preview only; mask-aware; still/current displayed sequence frame	Window lifetime; Reset unchanged
Preview pane	Image viewer	Sharpened result in Linear mode	No execution-autostretch support is enabled for this process	Pan with scroll bars or dragging; toolbar actions below	Shows the mask-aware result without modifying the target	Exists only with Preview	Holds source/result/display buffers plus G'MIC layer workspaces	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	Preview image required	Useful for broad halos; resampling can hide fine noise/ringing	Preview display only	Viewer lifetime
Zoom In	Preview-toolbar action	Current scale	None	Click; 1.25× relative zoom	Enlarges around viewport center	Preview image required	Improves local inspection; increases panning	Preview display only	Viewer lifetime
Zoom Out	Preview-toolbar action	Current scale	None	Click; 0.8× relative zoom	Reduces around viewport center	Preview image required	Shows broader 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	Preferred for noise, star rims, and color edges	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 color shifts without changing pixels	Preview display only	Viewer lifetime; not in process history
Switch image	Preview-toolbar action	Sharpened result shown	None	Click; Space; right-click; 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	Checked with an initial 600×600 area	None	Checked/unchecked; move/resize overlay	Processes only the area and composites it over the original preview; uncheck for full-image preview	Preview required	Reduces tuning cost but can hide broad-scale halos; Apply is always full-frame	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> - Multiscale Sharpen Alpha - SNAPSHOT	Preview required	Adds a complete execution and auxiliary image	New still; target unchanged; area/mask honored	Each action creates a separate image view
Apply	Button/action	Enabled, including Amplitude/Amount 0 and Scales 1	None	One action	Captures four numerical settings, locks the target, and processes the full still or every sequence frame	Target must lock; target, Channel, and process controls are disabled during Apply	G'MIC receives a cooperative abort flag; channel conversion, mask, sequence encode, and temporary storage add cost	Destructive still/sequence replacement after success	Records four settings and, for a colour image, a Channel label; successful Apply resets numbers, including Scales 4, but retains Channel
Reset	Button/action	Enabled while idle	None	One action	Restores Amplitude 1, Scales 4, Anisotropy 0.5, and Amount 1	Does not change Target, Channel, Preview, mask, or preview toolbar; disabled during Apply	Checked Preview schedules another complete 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	Requests active preview cancellation; G'MIC cancellation is cooperative	Window/execution	Process settings do not persist after destruction
Sequence progress / Cancel	Shared per-process/frame progress and Cancel	Created for sequence Apply	None	Cooperative Cancel; task and frame text update around temporary work	Sequence target only	G'MIC polls the abort flag; cancellation after return discards the current result	Captured numerical settings and Channel are used for every frame	Cancel/error removes temporary output; success replaces sequence, records one history step, and resets numbers	Execution snapshot only

How the algorithm works

Alpha-detail decomposition

AfterNight checks the selected image or component for non-finite samples and passes it through its G'MIC adapter, one G'MIC session per run, as planar float32 fractions of full scale to bundled G'MIC 3.5.4 with this command:

sharpen_alpha Amplitude,NumberOfScales,Anisotropy

The unexposed G'MIC minimize_alpha argument therefore keeps its default 1. G'MIC separates any existing opacity, then calls split_alpha without spatial subsampling. For N > 1, it repeatedly creates a smoother base and represents the difference between the previous layer and that base as an RGBA alpha-detail layer. The exact alpha difference is chosen so alpha-compositing the layer over its base reconstructs the input.

For scale index k=0..N-2, the base support begins at

\[ \sigma_k=0.25+2^k\ \text{pixels}. \]

At Anisotropy 0, this is a recursive Gaussian blur using G'MIC's default Neumann boundary. At values above 0, the bundled script instead applies a bilateral blur with spatial sigma sigma_k, range sigma (1-Anisotropy)×100, and then a Gaussian blur with sigma 0.5+Anisotropy. The operation is recursive: each broader base is derived from the preceding base, not directly from the original.

G'MIC then increases opacity across the detail stack from an unboosted factor through a maximum of 1 + Amplitude, clamps layer values/opacity to its 0..255 alpha-compositing domain, and uses merge_alpha to reconstruct the image. Amplitude 1 is the raw value corresponding to 100% in G'MIC's own GUI; AfterNight's maximum 4 corresponds to 400%. Amplitude 0 leaves the opacity multipliers at 1, so reconstruction is neutral apart from floating-point rounding and the [0,1] clamp. Number of Scales 1 performs no split and is likewise neutral.

Amount, channels, mask, and range

Let I be the source and S the clamped G'MIC result. The shared process layer applies Amount a as

\[ M=(1-a)I+aS. \]

Thus Amount 0 returns the original samples after G'MIC has still run, 0.5 halves the complete source-to-sharpened difference, and 1 selects S. If the effective normalized mask weight is m, the final result is

\[ O=(1-m)I+mM. \]

Mask inversion replaces m by 1-m. Mask blending therefore occurs after the global Amount blend.

Integer types are converted to float32 [0,1], a tightly packed float32 image is handed over in place, and double is narrowed to float32. Every successful G'MIC result is clamped to [0,1] before conversion back to the original pixel type, rounded to nearest and held to the type's range. The bundled alpha script contains fixed 0..255 and range-sigma constants even though AfterNight normally supplies normalized data. This scale mismatch is part of the current implementation: do not assume Anisotropy matches the visual behavior of the standalone G'MIC plug-in on its usual sample scale.

RGB stays a multichannel planar G'MIC image. Alpha differences use vector channel comparisons and a shared opacity for each detail layer, so a strong edge in one channel can influence the layer opacity used to reconstruct all channels. A selected derived Channel is converted to one plane, processed by the same command, and reconstructed through the shared component path. A derived component's reconstruction saturates an integer image at its type's range and keeps a floating image's values outside [0, 1].

Boundaries, errors, and cancellation

• Gaussian scale bases use Neumann boundary conditions (edge samples extend with zero normal derivative). The bilateral branch is followed by a Neumann Gaussian. Edge support is therefore not the reflective model used by Unsharp Mask.
• G'MIC command failures are reported with G'MIC's message; a still is committed only after success.
• An image holding NaN or infinity is refused before the command runs and is left unchanged.
• G'MIC receives a live abort flag, so Cancel is cooperative within the command. AfterNight also checks cancellation after the call and discards the result.
• Sequence processing writes temporary output. Cancel/error removes it; success replaces the sequence and records one history step.

Parameter interactions and starting points

Combination	Conservative starting point	Too weak looks like	Too strong looks like	Adjustment order
Scales × Amplitude	Scales 4, Amplitude 1; compare Scales 5 deliberately	Only the finest visible structures change	Broad star halos, crunchy multiscale contrast, clipped peaks	Choose largest useful scale first, then raise/lower Amplitude
Amplitude × Amount	Keep Amplitude near 1, reduce Amount below 1	Detail difference vanishes as Amount approaches 0	All layer boosts arrive at full strength and clipping becomes obvious	Establish decomposition with Amplitude; use Amount as final global mix
Anisotropy × Scales	Compare 0 and 0.5 at modest Scales	Gaussian branch may soften edge localization	Bilateral/extra-blur branch can produce edge-local transitions or unexpected scale behavior	Fix Scales, compare Anisotropy endpoints, then retune Amplitude
RGB × luminance component	Compare RGB with CIE Y (Luminance) or CIE L* (Lightness)	Luminance-only can leave chromatic noise	Shared alpha opacity can exaggerate a dominant channel edge	Inspect View Channel, choose process Channel, then tune strength
Amount × mask	Tune at modest Amount before localizing	Protected regions change little	Hard mask transitions reveal inconsistent sharpening	Tune globally, verify mask/inversion, inspect the full frame

These are source-derived starting points, not executed image-quality measurements. Sampling, seeing, noise, prior deconvolution, and final resize all change the useful scale range.

Inputs, outputs, and metadata

• Output dimensions match the input. Channel count normally matches, except that sequence auto-debayer can convert configured CFA frames to RGB before the filtered temporary replacement is written. Source pixel type is kept; integer results are rounded and double data is processed at float32 precision.
• The G'MIC leg is clamped to [0,1]. At Amount 1, out-of-range float samples are lost; partial Amount can retain part of the original out-of-range contribution; Amount 0 restores the source after unnecessary filter work.
• The result keeps the source image's header cards and metadata. The process does not update linear/nonlinear processing-state provenance.
• Image History records Amplitude, Number of Scales, Anisotropy, and Amount, and, for a colour image, in channel: <channel> with the selected Channel, derived components included.
• The Channel selector is not part of SharpenAlphaParams, but the run captures it when Apply starts, together with the four numerical settings, and uses it for every sequence frame.
• A still CFA mosaic follows the grayscale path. Sequence preprocessing may auto-debayer configured CFA frames before temporary replacement.
• Successful Apply resets the four numbers to the values the window opens with, while retaining Target, Channel, Preview, mask, and preview-toolbar state.

Performance

• Work scales with pixel count, channel count, and the effective number of scales. Each added level creates a broader blur/bilateral base, an alpha-detail layer, and an additional alpha composite.
• Gaussian support doubles by scale (1.25, 2.25, 4.25, 8.25, ... pixels), so high scale counts add much broader work than a linear-radius progression. G'MIC/CImg may use platform parallelism internally.
• The bilateral branch adds guide/range computation plus a second Gaussian per scale. Amount does not reduce decomposition work, even at 0.
• Memory includes source/destination float images, multiple RGBA detail layers, smoother bases, component/mask buffers, and preview copies. Sequences also need temporary disk roughly comparable to an output sequence.
• Preview area reduces tuning work; Apply remains full-frame. No approved astronomical performance workload was run, and peak memory was not measured.

Worked example

• Evidence mode: Source-and-equation-derived; not executed on approved astronomical data.
• Input condition and settings: Finite normalized data; Scales 4, Amplitude 1, Anisotropy 0, Amount 0.5.
• Scale checkpoint: G'MIC forms three recursive Gaussian bases with nominal sigmas 1.25, 2.25, and 4.25 pixels. Scales 5 adds an 8.25-pixel base.
• Blend checkpoint: If a source sample is 0.20 and the clamped reconstructed sample is 0.32, Amount 0.5 gives 0.20×0.5 + 0.32×0.5 = 0.26 before mask.
• Inspect at 100%/history: Compare star rims, background noise, and separate display channels; after Apply, verify four numerical settings and the available Channel label in history.
• Limitations: No visual improvement, production runtime, sequence output, or memory claim is made without approved-data execution.

Warnings and limitations

• This is alpha-layer boosting, not deconvolution. It cannot recover information absent from the input.
• Noise, hot-pixel residue, ringing, and compression edges are detail and can be boosted with real structure.
• More scales add broad supports and can create wide halos around stars or large transitions. Amplitude controls layer opacity, not a physical PSF or radius.
• Anisotropy switches to a different bilateral-plus-Gaussian construction for every nonzero value. It is not continuous with the exact 0 code path in all numerical details and is not a Reduce Halos control.
• The bundled command's fixed 0..255 alpha/range constants operate on AfterNight's normally normalized float input. Standalone G'MIC presets are not numerically interchangeable without validation.
• RGB detail layers share opacity across channels; strong single-channel edges can affect color. Derived component round trips can shift gamut and, in an integer image, clip at its type's range (a floating image keeps values outside [0, 1]).
• Every G'MIC result is clamped to [0,1]; highlights, shadows, and HDR float data are not safely preserved. Double input is narrowed. An image holding NaN or infinity is refused by name; without G'MIC in the build, the process cannot run and Preview and Apply stay disabled.
• The process preview is linear-only. It does not expose Display Mode or Autostretch Settings, so comparison with an autostretched target can mislead.

Troubleshooting

Symptom or exact error	Verified cause/validation path	Recovery	Evidence source
Amount 0 is unchanged but slow	The full G'MIC decomposition/clamp runs before the shared Amount blend restores the source	Disable Preview or close the process when no filter is wanted	Typed modulation order
Scales 1 is unchanged	split_alpha only decomposes when the effective scale count is greater than one	Raise Scales to at least 2	Bundled split_alpha branch
Broad halos surround stars	High Scales and Amplitude boost broad alpha-detail layers	Reduce Scales first, then Amplitude or Amount; use a mask	Scale supports and opacity boost
Noise becomes crunchy	Fine alpha layers boost noise with signal	Denoise first; reduce Amplitude/Amount; protect background	Alpha-detail model
Colors shift or rims appear	RGB layers use shared opacity derived from multichannel comparisons	Try CIE Y/L*; inspect View Channel; lower strength	Bundled sub_alpha; component dispatcher
Anisotropy behaves unexpectedly	Every nonzero value selects the bilateral-plus-extra-Gaussian branch and fixed range constants act on normalized data	Compare exact 0 with modest nonzero values; do not copy standalone plug-in presets blindly	Bundled split_alpha; normalized hand-off
Preview looks darker than target	This process preview has no execution autostretch controls	Compare in Linear mode or use a snapshot/duplicate under matched display settings	Sharpen process initialization
Cancel waits briefly	G'MIC abort handling is cooperative	Wait for its next polling point; use a smaller Preview area next time	G'MIC adapter abort flag
Error: the image holds a sample that is not a finite number	The image has a NaN or infinite sample, which G'MIC cannot process	Repair or crop the non-finite samples, then retry; the image was left unchanged	GmicProcessSupport.cpp
Preview and Apply are disabled; tooltip says the build carries no G'MIC library	This build was made without G'MIC	Use a build that includes G'MIC, or another sharpening process	GmicProcessSupport.cpp
A G'MIC error is reported	G'MIC command failure, invalid data, or memory exhaustion	Verify finite normalized data and memory; retry on a duplicate	Process error path

Related workflow steps

Complete calibration, registration, integration, gradient correction, color calibration, and appropriate noise reduction before final sharpening. Compare Unsharp Mask for a single Gaussian residual model or High Pass Filter when a neutral detail layer is required. Use Image History to compare/revert Apply and Saving and Exporting only after the final output scale has been checked.

References and provenance

• G'MIC sharpen_alpha command reference — official command purpose, arguments, and upstream defaults.
• G'MIC split_alpha command reference — official alpha-detail decomposition and reconstruction contract.
• G'MIC sub_alpha command reference — official minimal-opacity difference definition used by the decomposition.
• G'MIC blur command reference — official Gaussian/Deriche and boundary-condition defaults; the bundled script selects the default Gaussian and Neumann behavior.
• AfterNight and bundled G'MIC 3.5.4 sources — authoritative for the shipped script, live controls, normalized float32 hand-off, clamp, channel/mask order, preview, history, sequences, and cancellation.