High Pass Filter

Purpose and expected result

High Pass Filter replaces the target with a neutral-centered detail layer. It subtracts the exact Gaussian blur of the source from the source, adds the fixed offset 0.5 (half of full scale), and clips the result. Smooth regions become middle gray; transitions become lighter or darker than gray according to the sign of the residual.

This process does not directly sharpen the image. There is no Amount control, blend mode, or luminance-only Channel selector. Apply destructively replaces the target with the detail layer. Use it to inspect or prepare frequency detail for a deliberate downstream layer/compositing workflow, not as a one-click final sharpen.

At a glance

Item	Current behavior
Menu path	Process > Details > High Pass; window title High Pass Filter
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; an optional active mask must match target geometry
Outputs	Destructively replaces a still, or after successful temporary processing a sequence, with a same-size, same-type neutral-gray detail layer; Take snapshot creates an auxiliary still; no file is written directly
Recommended image state	Finite, normalized data after calibration/registration/stacking and noise control; duplicate the target because the output is a layer, not the original plus detail
Channel behavior	No process Channel selector: every stored channel is blurred and subtracted independently; RGB neutral is (0.5,0.5,0.5)
Preview support	Full-image preview initially; optional movable 600×600 Preview area when enabled, original/result switching, histogram, snapshot, and display-channel selection; no Display Mode or Autostretch Settings
Mask support	Yes, but it crossfades the original and the neutral detail layer; it does not perform masked sharpening
Sequence support	Yes; one captured Radius and mask state is applied frame by frame to temporary storage, with frame progress and Cancel, before replacement
External dependency	None; the AfterNight engine's own detail filter; no Python, G'MIC, network, or separate installation

Before you start

• Duplicate the target. Apply replaces the image with a high-pass layer; it does not retain the original appearance.
• Reduce noise and hot-pixel residue first. High-frequency noise appears as real detail around neutral gray.
• Use finite, normalized samples. The residual is always clipped to [0,1]; out-of-range and non-finite data are not preserved safely.
• Debayer a CFA still before making a color-detail layer. Otherwise the filter extracts the sensor mosaic pattern as one grayscale plane.
• There is no luminance-only process Channel. Extract or duplicate a luminance image first if chromatic residuals are not wanted.
• Expect long runs at very large Radius. Above 32 the blur is composed of several exact passes, and the count grows with the square of Radius.

Procedure

1. Duplicate a finite, normalized still and open Process > Details > High Pass. Confirm Target Image and leave Preview checked.
2. Start with Radius 10.0. The histogram should cluster smooth areas near 0.5; enable clipping information to see residuals reaching black or white.
3. Toggle Switch image. Smaller useful radii isolate finer transitions; larger radii retain progressively broader structure in the residual.
4. Inspect at Zoom 100% for noise, bright/dark edge pairs, star rings, and independent RGB color residuals. Enable Preview area only to reduce tuning work; it starts unchecked and initializes a centered 600×600 area.
5. Check the full-frame preview and histogram. If the result will be composited elsewhere, prefer Take snapshot so the original target remains unchanged.
6. If Apply is intentional, select Apply. For a sequence, monitor frame progress; Cancel preserves the original. Radius returns to 10 after success.

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
Histogram plot	Graph, normalized intensity/count	Source or current mask-aware preview; RGB channels overlaid	None	Binning comes from Plot resolution	Shows the source/detail-layer distribution and approximate endpoint-bin clipping; a clean smooth residual clusters at 0.5	Show histogram controls visibility	Recalculation adds work; endpoint counts are resolution-dependent	Display only; stored channels	Recomputed after source, preview, or resolution changes
Left-drag in histogram	Graph gesture	No readout	None	Hold left button and move	Shows normalized coordinate readout	Histogram shown	Diagnostic only	Display only	Ends on release
Middle-drag in histogram	Graph gesture	Fitted/unpanned	None	Hold middle button and drag	Pans within graph scroll area	Useful beyond viewport	No output effect	Display only	Until reset/window close
Ctrl+mouse wheel in histogram	Graph gesture	Fitted size	None	Up 1.20×; down 0.8×; plain wheel scrolls	Zooms around cursor	Interactive graph enabled	Rendering area only	Display only	Window lifetime
Zoom in	Histogram-toolbar action	Fitted size	None	Each action 1.30×	Enlarges plot	Disabled with hidden histogram	Rendering only	Display only	Window lifetime
Zoom out	Histogram-toolbar action	Fitted size	None	Each action 0.70×	Shrinks plot	Disabled with hidden histogram	Rendering only	Display only	Window lifetime
Zoom reset	Histogram-toolbar action	—	None	One action	Fits plot and resets scrolling	Disabled with hidden histogram	Rendering only	Display only	Immediate
Show grid	Histogram toggle	Checked	None	Checked/unchecked	Shows or hides grid	Disabled with hidden histogram	Display-only aid	Display only	Window lifetime; hiding histogram resets grid internally
Show clipping info	Histogram toggle	Checked	None	Checked/unchecked	Shows or hides endpoint-bin counts	Disabled with hidden histogram	Binning-dependent estimate; useful for residual saturation but not an exact process-clipping count	Display only	Window lifetime
Absolute scales	Histogram toggle	Unchecked	None	Checked/unchecked	Checked normalizes each visible channel histogram to its own peak	Disabled with hidden histogram	Can conceal relative channel strength; never normalizes pixels	Display only	Window lifetime
Draw histogram areas	Histogram toggle	Unchecked	None	Checked/unchecked	Filled-area versus line drawing	Disabled with hidden histogram	Area overlap can obscure RGB residual differences	Display only	Window lifetime
Show histogram	Histogram toggle	Checked	None	Checked/unchecked	Hides/clears graph and disables toolbar, or reloads source/preview	Always available while idle	Hiding saves graph work, not filtering	Display only	Window lifetime; Reset unchanged
Plot resolution	Combo box, bins	8-bit (256)	None	6-bit (64), 7-bit (128), 8-bit (256), 9-bit (512), 10-bit (1024), 12-bit (4096), 14-bit (16K), 16-bit (64K), 100 levels, 1000 levels, 10000 levels	More bins refine the neutral peak and endpoint estimates	Disabled with hidden histogram	Higher bins cost graph memory/rendering; output precision unchanged	Display only	Window lifetime
Radius	Double value plus linear slider, requested Gaussian sigma in pixels	10.00	Label says Radius, but the engine treats it as the standard deviation of an exact Gaussian	0.10..1000.00; two decimals; spin step 0.50; slider is remapped to 1001 positions over the full range, about one unit per tick	Small values retain only the finest residual; larger values subtract a broader base, so broader structures remain in the layer; at 0.10 the Gaussian barely blurs and the layer is almost uniformly neutral	Enabled while idle	Cost grows with Radius; above 32 the blur is composed of several exact passes, about (Radius/32)² of them, so the largest values are very slow	Preview/Apply; all stored channels; before mask; still or sequence	Captured per execution; Reset and successful still Apply return 10.00
Target Image	Image selector	Attached/active target	None	Open image or sequence	Changes source, histogram, 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 unchanged
Preview	Checkbox	Checked	None	Checked/unchecked	Enables/disables asynchronous full-image or selected-area high-pass execution and changes histogram source/result	Process idle	Runs blur, residual, clamp, mask, and histogram passes	Preview only; all stored channels; still/current displayed sequence frame	Window lifetime; Reset unchanged
Preview pane	Image viewer	High-pass result in Linear mode; window initially uses shared 1000×700 default	Execution autostretch is not enabled for this process	Pan with scroll bars or dragging; toolbar actions below	Shows the mask-aware neutral detail layer without modifying target	Exists only with Preview	Holds source/result/display and blur buffers	Preview only	Window lifetime
Fit To Window	Preview-toolbar action	Preview fitted when first displayed	None	Click or Ctrl+Shift+F	Fits whole image uniformly	Preview image required	Good for broad residuals; resampling hides fine noise	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	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 context but hides fine residuals	Preview display only	Viewer lifetime
Zoom 100%	Preview-toolbar action	Current scale	None	Click; absolute scale 1.0	Shows one-pixel viewer scale	Preview image required	Preferred for noise, rings, and color residuals	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; it is not a process Channel	RGB preview required for components	Helps inspect colored residuals without changing pixels	Preview display only	Viewer lifetime; not in history
Switch image	Preview-toolbar action	High-pass 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	Unchecked	When first enabled without an explicit process size, initializes centered 600×600 or full image if smaller	Checked/unchecked; move/resize overlay	Limits preview/snapshot processing and composites the patch over the original	Preview required	Reduces tuning work; can hide broad-radius context; Apply remains 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 Radius and opens <target> - High Pass Filter - SNAPSHOT	Preview required	Allocates an auxiliary still; safest way to retain original target while creating a detail layer	New still; area/mask honored	Each action creates a separate view
Apply (still)	Button/action	Enabled at every valid Radius	None	One action	Captures Radius, locks the target, filters every stored channel, applies mask, and replaces the still	Still target and successful lock; controls disabled during commit	No progress dialog or Cancel is provided for a still; destructive layer replacement	Full still; all channels; mask crossfade	Records Apply high pass filter with radius <value>, then resets Radius
Reset	Button/action	Enabled while idle	None	One action	Restores Radius 10; if Preview is off, reloads source histogram	Does not change Target, Preview, histogram, mask, or preview toolbar	Checked Preview schedules another execution	Configuration only	Immediate; also invoked after successful still Apply
Close	Button/action	Enabled while idle	None	One action	Closes without applying current Radius	Disabled during still Apply	Requests preview cancellation, which reaches the blur between rows	Window/execution	Settings discarded at destruction
Apply (sequence)	Same visible Apply action	Shared frame progress dialog with Cancel	None	One action	Applies the captured Radius and mask to every frame in temporary storage	Sequence target and successful lock	Decode/encode, temporary disk, and the blur per frame add cost; Cancel stops the current frame between rows and removes temporary output	Sequence target; every frame	Success replaces the sequence and records one history step, then resets Radius

How the algorithm works

Neutral-centered residual

For a stored channel sample I(p), the engine computes the exact Gaussian base B_r(p) with standard deviation equal to the displayed Radius r, then returns

\[ H_r(p)=\operatorname{clip}_{[0,1]}\left(I(p)-B_r(p)+0.5\right). \]

The fixed 0.5 represents zero residual. If a region is constant, its blur is the same constant and its output is 0.5, regardless of the original level. Where the source is locally brighter than its base, the layer rises above gray; where it is darker, the layer falls below gray. A hard transition therefore creates paired light/dark structure. Values below 0 or above 1 are clipped, so residual magnitude beyond 0.5 is lost.

Radius controls the low-pass base, not the intensity of the result. Increasing it removes a broader base and leaves a wider band of structures in H. There is no Amount control. A conventional high-pass sharpening composite would require a separate blend of this layer with the original; this process does not perform that step.

Exact Gaussian base

The base, in the preview and at Apply, is a separable Gaussian: taps sampled at each pixel centre out to four standard deviations, renormalized to sum to one, applied along the rows and then down the columns. Beyond each edge the image is mirrored without repeating the edge sample. The engine's widest kernel has a radius of 128 pixels, which four standard deviations reach at Radius 32; a wider Radius is composed of several exact passes of equal width whose variances add up to r², so the base is still the Gaussian of the requested Radius.

At the minimum Radius 0.10 the Gaussian barely blurs, so B≈I and the layer is almost uniformly the neutral 0.5. The minimum does not preserve the source and does not reveal detail.

Channels, types, mask, and clipping

Grayscale uses one stored plane. RGB runs the same scalar operation separately on R, G, and B; there is no shared luminance or process Channel conversion. Neutral RGB is gray, but unequal channel residuals can create colored edges.

Every type is filtered in its stored units: the blur accumulates in double precision, and the offset and clip are stated in the type's own codes (half of its span above its lowest value, and its full range), so the layer means the same thing for an integer image as for a float one. Up to Radius 32 the result is written back once, rounded to the nearest code for an integer image; a wider Radius is computed on a single-precision working copy of fractions of full scale and converted back once. Double data is filtered at single precision. The midpoint may round to the nearest available integer code.

If an active mask has effective normalized weight m(p), the shared process path computes

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

Mask inversion substitutes 1-m. This crossfades between an ordinary image and a neutral detail layer. It is not the result of adding masked detail to the original, and a mask edge can create an abrupt visual transition between those two representations.

Parameter interactions and starting points

Combination	Conservative starting point	Too weak looks like	Too strong looks like	Adjustment order
Radius × feature size	Radius 10, then lower while inspecting at 100%	Small-radius layer is nearly uniform gray except very fine noise/edges	Large radius retains broad gradients/halos rather than only fine detail	Choose the widest feature that should remain, then reduce Radius
Radius × clipping	Watch endpoint bins and bright/dark edge pairs	Residual stays close to 0.5	Peaks reach 0/1, losing residual magnitude	Reduce Radius or preprocess extreme transitions
RGB × luminance-only goal	Inspect individual View Channels	RGB residual may be acceptable	Colored rims appear because channels differ	Extract luminance first; High Pass has no process Channel
Radius × mask	Prefer a snapshot/unmasked layer	Masked area receives little detail-layer representation	Boundary visibly switches between original and gray-centered layer	Create the layer first; composite deliberately downstream

These are source-derived workflow checkpoints, not executed astronomical quality measurements. A useful radius depends on sampling, seeing, noise, feature scale, and the intended downstream blend.

Inputs, outputs, and metadata

• Output dimensions, stored channels, pixel format, and source pixel type match the source. Integer output is rounded; float/double residuals are clipped.
• The result keeps the source image's header cards and metadata. The process sets no processing-state transition.
• The inherited linear/nonlinear state can therefore describe the source rather than the new neutral detail layer. Treat it as unreliable provenance.
• Image History records only Apply high pass filter with radius <value>; it does not record the fixed offset 0.5, mask/inversion, lack of Channel choice, or how many blur passes ran.
• Take snapshot creates a separate still from current preview scope. Display channel and histogram settings are not baked into its samples.
• Successful Apply resets Radius to 10; Target, Preview, histogram, mask, and preview toolbar remain.
• A sequence is processed frame by frame into temporary storage with one captured Radius and replaces the original only after success; Cancel/error removes the temporary output.

Performance

• The exact Gaussian costs two separable passes of 2 × ceil(4 × Radius) + 1 taps per sample, so work grows with Radius as well as pixel and channel count. Above Radius 32 each additional pass costs as much as one blur at 32: Radius 100 takes ten passes, and Radius 1000 close to a thousand.
• RGB channels are blurred separately on the threads the operation is granted. Memory includes working planes for the blur, destination, preview presentation, mask, and histogram buffers.
• Preview area can reduce preview work after it is enabled; Apply always uses the full still. Histogram recomputation adds binning/rendering cost.
• The blur checks for cancellation between rows, so closing or cancelling stops it promptly.
• No approved astronomical workload has been timed and peak memory has not been measured.

Worked example

• Evidence mode: Source-and-equation-derived; not executed on approved astronomical data.
• Constant checkpoint: If I=0.20 throughout a region, B=0.20, so H=0.20-0.20+0.50=0.50. The same is true for any constant normalized level.
• Edge checkpoint: If a local source sample is 0.80 and its blurred base is 0.60, the layer sample is 0.70; if source is 0.40 over the same 0.60 base, it is 0.30.
• Mask checkpoint: With original 0.20, high-pass 0.70, and mask 0.25, the committed sample is 0.20×0.75 + 0.70×0.25 = 0.325; it is not 0.20 plus one-quarter of the residual.
• Inspect: Smooth histogram mass near 0.5, endpoint clipping, star rings, background noise, separate RGB residuals, and the Radius-only history step.
• Limitations: No sequence, visual improvement, compositing result, production runtime, or memory result is claimed.

Warnings and limitations

• Apply replaces the original with a neutral-gray detail layer. It does not sharpen or blend that layer back into the source.
• There is no Amount or process Channel control. RGB channels are independent; luminance-only work requires a separate image/component workflow.
• Radius 0.10 produces an almost neutral layer because a Gaussian that narrow barely blurs. The control minimum is not an identity image output.
• Very large Radius values are slow: above 32 the blur needs about (Radius/32)² exact passes.
• Residuals clip at black/white. Strong edges and out-of-range float data can lose magnitude; NaN and infinity are unsupported.
• Double input is filtered at single precision, and integer output is rounded to whole codes.
• Mirror borders pull reflected structure into the border region at large radii.
• Masks crossfade original and layer representations; they do not implement masked high-pass sharpening.
• The preview is linear-only and starts full-frame. No Display Mode or Autostretch Settings action is exposed.
• Still Apply and snapshots have no progress dialog; a sequence Apply shows frame progress and Cancel.

Troubleshooting

Symptom or exact error	Verified cause/validation path	Recovery	Evidence source
Result is gray instead of sharpened	The output is source-blur+0.5, not source plus detail	Undo or use a duplicate/snapshot; composite the layer deliberately elsewhere	High-pass equation
Radius 0.1 gives almost uniform gray	A Gaussian that narrow barely blurs, so the residual is near zero	Raise Radius	Exact Gaussian base
Bright/dark rings surround stars	A high-pass residual naturally has opposite-sign edge lobes; larger Radius broadens them	Reduce Radius; inspect before downstream blending	Residual equation
Colored edges appear	R, G, and B are processed independently with no luminance option	Extract a luminance component before High Pass	Process initialization and channel loop
Histogram reaches black/white	Residual exceeded the fixed ±0.5 representable range	Reduce Radius or handle high-contrast transitions before filtering	Fixed offset and clamp
Masked result looks partly normal and partly gray	Shared mask crossfades original and detail layer	Create an unmasked layer and use an intentional later composite	Mask equation
Preview is different from an autostretched target	High Pass preview exposes Linear mode only	Compare under matched Linear display conditions	Process initialization
Preview or Apply is slow at a large Radius	Above 32 the blur is composed of about (Radius/32)² exact passes	Reduce Radius, or enable a smaller Preview area while tuning	Composed exact passes
An error is reported	Invalid data or insufficient memory	Verify finite normalized data and memory; retry on a smaller duplicate	Engine/process error path

Related workflow steps

Complete calibration, registration, integration, gradient correction, color calibration, and appropriate noise reduction before extracting a final-scale detail layer. Use Unsharp Mask for direct single-radius sharpening or Sharpen Alpha for a multiscale alpha reconstruction. Use Image History to revert an accidental destructive Apply and Saving and Exporting to preserve a snapshot layer for an external/compositing workflow.

References and provenance

• Peter Kovesi, “Fast Almost-Gaussian Filtering” — the repeated-box approximation of a Gaussian; this process does not use it, and its base is the exact Gaussian.
• AfterNight engine and process sources — authoritative for the exact Gaussian and its composed passes, mirror border, fixed 0.5 offset, clipping, type behavior, live controls, histogram, preview, mask, metadata/history, and sequence path.