Curves Transformation

Purpose and expected result

Curves Transformation is a point-controlled remapping tool for deliberate tone and color shaping. It is intended for users who have completed calibration and, for most workflows, the main linear-to-nonlinear stretch. You define a spline from normalized input intensity on the horizontal axis to normalized output on the vertical axis, then apply that spline to RGB samples or to one selected color-space component.

The expected result is one modified still image or replacement sequence with the same dimensions and pixel type. Channel count and pixel format are normally preserved, except that automatic CFA debayering can turn a one-channel mosaic sequence into RGB before the curve runs. A curve above the diagonal brightens the affected range; a curve below it darkens that range. Component curves can change lightness, saturation, chroma, hue, or color balance without requiring channel extraction.

At a glance

Item	Current behavior
Menu path	Process > Intensity Transformations > Curves Transformation
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 intended normalized grayscale or three-channel RGB still image or sequence; a one-channel CFA still is technically treated as grayscale; an optional active mask must match the target geometry
Outputs	Destructively replaces the target still or sequence after success; dimensions and pixel type are preserved, while automatic sequence CFA debayering can change one-channel CFA to RGB; no auxiliary image or file
Recommended image state	Usually nonlinear for finishing contrast, saturation, or color balance; a gentle first stretch is possible but offers no statistics-driven protection
Preview support	Asynchronous full-image RT preview; the graph uses the current preview result for its histogram
Sequence support	Supported; the same locked curve, interpolation, and selected component are applied to every decoded frame through a temporary replacement sequence. The sequence reader honors the view's automatic CFA debayer setting.
Mask support	Supported by post-transform blending; white applies the result, black keeps the source, gray blends, and Invert Mask reverses the weights
External dependencies	None; the curve is the bundled engine's native interpolation, run on its threads; no external tool, model, catalog, or network access

Before you start

• Finish background correction and color calibration while the image is still linear. Curves can make faint gradients and color casts harder to correct.
• For a primary display stretch, compare Histogram Transformation, Generalized Hyperbolic Transformation, or ArcSinh Stretch. Curves has no automatic black-point, median, or highlight-protection calculation.
• Duplicate the target or be prepared to use Image History. Apply replaces the current target after processing succeeds.
• Prefer floating-point data while making strong component curves. Color-space conversion works in a 32-bit float plane (64-bit for a Float64 image) and an integer image's reconstruction is rounded once to its codes, so integer images incur output quantization.
• Keep normalized floating samples within [0,1]. The transfer spline is defined only on that domain.
• Do not use a one-channel CFA still as a grayscale target: Curves does not debayer still images and would remap the mosaic samples directly. For a CFA sequence, decide whether the view's automatic debayer option should convert every decoded frame to RGB; a successful replacement then remains RGB.
• If a mask is active, remember that the curve editor is an unmasked diagnostic, while its histogram and clipping readout count the preview's mask-blended result. Judge the mask-aware result in the preview pane.

Procedure

1. Select the target and open Process > Intensity Transformations > Curves Transformation. Confirm Target Image and leave Preview selected. The right pane should show the current full-image result.
2. Choose Channel. Start with RGB to apply one curve independently to all three stored channels, or choose a lightness/value component when you want to limit direct color-balance changes.
3. Leave Interpolation: Cubic for a smooth general-purpose curve. Choose Steffen or Monotone Cubic when avoiding overshoot between monotonic control points matters.
4. Click the existing diagonal to add an interior control point, then drag it. Drag upward for more output at that input range and downward for less. The curve and preview update together.
5. Add neighboring points to anchor ranges you want to protect. Use Show guide to compare with the identity diagonal and Show histogram to see where samples are concentrated.
6. Inspect shadows, transitions, highlights, and color at 100% zoom. Check the graph's Clipping Low/High readout as a coarse warning, then use Image Statistics for exact checks.
7. Select Apply. A successful still-image run replaces the target, writes an Image History step containing Channel and Interpolation, then resets the curve to identity, selects RGB when available, and resets graph zoom. A sequence run reports progress, replaces the sequence only after the temporary output succeeds, offers Cancel, and performs the same UI reset. The new identity curve keeps the interpolation the combo shows.

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
Curve editor: add/select/drag point	Graph gesture, normalized x/y	Identity endpoints (0,0) and (1,1)	None	Left-click within about 5.5 screen pixels of the drawn curve to add/select; y clamps to [0,1]; endpoint x remains 0/1; interior x stays ordered and inside (0,1)	Raising a point brightens its neighborhood; lowering it darkens; horizontal movement relocates the affected input range	Editor enabled while idle; Apply stays disabled for the exact two-endpoint identity	Closely spaced or extreme points can create abrupt contrast, noise emphasis, or spline overshoot	Defines the selected Channel transform for preview/apply, mask post-blend, still/sequence	Until Reset or successful Apply rebuilds the identity curve
Curve editor: delete point	Graph action	—	None	Right-click an interior point; endpoints cannot be deleted	Removes that constraint and rebuilds the interpolant	Must hit an existing interior point	May broaden the influence of neighboring points	Same transform scope as editor	Immediate; no undo inside the dialog
Curve editor: coordinate readout	Graph gesture	Hidden	None	With histogram shown, left-drag away from the curve	Shows normalized x and displayed histogram height, not an editable node value	Suppressed when selecting/dragging a curve point or when histogram is hidden	Diagnostic only; histogram height is display-normalized	Unmasked graph diagnostic	Gesture lifetime
Curve editor: pan	Mouse gesture	No offset	None	Middle-drag after zooming	Scrolls the enlarged editor	Useful only when content exceeds the scroll area	Display only	Graph only	Until Zoom reset, Reset, or successful Apply recenters the graph
Curve editor: Ctrl+wheel zoom	Mouse/wheel gesture	Fit	None	Ctrl+wheel up/down scales around the pointer by 1.2/0.8	Enlarges or reduces the graph while keeping the pointer location anchored	Interactive zoom is always enabled	Display only	Graph only	Until Zoom reset, Reset, or successful Apply restores fit
Zoom in	Toolbar action	—	None	Repeated action, 1.30x per click	Enlarges the editor	Always available while idle	Display only	Graph only	Until Reset or successful Apply restores fit
Zoom out	Toolbar action	—	None	Repeated action, 0.70x per click	Reduces the editor	Always available while idle	Display only	Graph only	Until Reset or successful Apply restores fit
Zoom reset	Toolbar action	—	None	Single action	Fits the editor to its parent area	Always available while idle	Display only	Graph only	Not persisted
Show grid	Checkable toolbar action	Selected	None	On/off	Shows or hides the eight-by-eight graph grid	Always available while idle	Display only	Graph only	Window lifetime; Reset does not change it
Show guide	Checkable toolbar action	Selected	None	On/off	Shows or hides the identity diagonal y=x	Always available while idle	Display only; useful for seeing brightening/darkening	Graph only	Window lifetime; Reset does not change it
Draw histogram areas	Checkable toolbar action	Cleared	None	On/off	Fills displayed histogram channels instead of outlines	Disabled whenever Show histogram is cleared	Rendering preference only	Unmasked graph diagnostic	Window lifetime; Reset does not change it
Show histogram	Checkable toolbar action	Selected	None	On/off	Shows the current preview-result histogram and enables Draw histogram areas	Histogram is available only while Preview exists; clearing it disables area mode control but retains its checked state	Histogram recomputation adds work after preview updates	8-bit graph diagnostic of the mask-blended preview; display-channel selection follows the preview viewer, not process Channel	Window lifetime; Reset does not change it
Clipping Low / Clipping High	Read-only overlay, percent	Current preview histogram endpoints	None	Three decimals; sums endpoint counts across histogram channels	Reports the share of samples at or below 0 and at or above 1 in the image the graph's histogram counts	Visible with the curve editor; updated with its current histogram image	Counts the preview's mask-blended result, or the target while the preview shows its original; values just inside 0 or 1 are not counted	Diagnostic only; preview/still source while configuring a sequence	Recomputed, not persisted
Intensity gradient	Read-only graph strip	Black-to-white, or black-to selected preview-display channel	None	Normalized 0–1	Shows graph direction and current preview display-channel color	Always drawn; follows the preview viewer's display composition	Display only	Graph only	Window lifetime
Interpolation	Combo box	Cubic	Reset and successful Apply keep the displayed selection and rebuild the identity curve in it; reopening shows Cubic	Linear, Cubic, Akima, Steffen, Monotone Cubic; two points always evaluate linearly; selected Akima or Steffen falls back to Cubic below five points	Changes how the curve passes between the same nodes	Rebuilds the spline; controls lock during Apply	Cubic/Akima can overshoot between points before final output clamp; Steffen and Monotone Cubic preserve monotonicity between monotonic nodes; Linear can show corners	Preview/apply, all selected channel modes, still/sequence	Current selection applies, and Reset or a successful Apply keeps it for the new identity curve
Target Image	View selector	Active view at launch	None	Any open grayscale/RGB still or sequence view	Retargets channel choices, preview, histogram, and Apply	Target must be lockable; unsupported exclusive/duplicate process state blocks launch or execution	Recalculation cost follows target size and type	Source for preview/apply and active mask	Window 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, HSL Hue, HSL Saturation, HSL Lightness, HSV Hue, HSV Saturation, HSV Value, YCbCr Luma, YCbCr Blue Chroma, YCbCr Red Chroma; mono Grayscale	Selects stored channels or one converted color-space component	Component choices require a three-channel target; mono selector is disabled	Individual/color-component curves can change balance, gamut, hue, or saturation; conversions add full-image work and quantization	Preview/apply, still/sequence; active mask blends after RGB reconstruction	Resets to RGB after success/Reset; remains Grayscale for mono
Preview	Check box	Selected	None	On/off	Enables/disables asynchronous full-image RT preview updates	Requires a valid target; stale preview generations are superseded	Full-image component conversions and histogram recalculation can be expensive	Preview only; target remains unchanged; active mask is shown	Window lifetime; Reset does not change it
Preview image pane	Image display	Current transformed target	None	Shared viewer zoom/pan/display-channel behavior	Shows the mask-aware full-image result	Updated only while Preview is selected and work completes	Holds an additional result image; component modes also hold conversion buffers	Still target preview; current sequence frame while configuring a sequence	Window lifetime
Apply	Button/action	Disabled for exact identity curve	None	Single action after any non-identity curve	Starts destructive still/sequence execution	Target must lock; all mutable controls disable during execution	Curve evaluation is parallel; component conversions and sequence temporary output add CPU/memory/disk cost	Replaces target; mask post-blend; still/sequence	Success resets the curve to identity, selects RGB when available, and resets graph zoom; the new curve keeps the shown interpolation; graph toggles remain
Reset	Button/action	Enabled while idle	None	Single action	Restores an identity curve in the shown interpolation, RGB channel when available, and graph zoom	Does not reset target, Preview, the visible Interpolation selection, Grid, Guide, Histogram, or Area states	No image change	Configuration only	The visible Interpolation selection remains and the rebuilt identity curve uses it
Close	Button/action	Enabled while idle	None	Single action	Closes immediately when idle; an active worker follows shared deferred-close/cancellation handling	Disabled with the other controls during Apply	No output by itself	Window and active execution	No settings persist after the window is destroyed
Sequence progress	Progress dialog	Appears during sequence Apply	None	Frame progress	Reports temporary replacement-sequence processing	Sequence target only	Temporary output uses disk space proportional to the encoded sequence	Sequence Apply only	Run lifetime
Cancel	Sequence progress action	Available while sequence work runs	None	Cooperative request	Stops before replacement, closes/removes temporary output, and preserves the source sequence	Sequence progress dialog only	Current frame may finish before cancellation is observed	Sequence Apply only	Run lifetime

How the algorithm works

Conceptual pipeline

1. The editor maintains ordered control points (x_i,y_i) on the normalized [0,1] square. Endpoints remain at x=0 and x=1; all y values clamp to [0,1].
2. After every edit, AfterNight builds a Linear, natural Cubic, Akima, Steffen, or Monotone Cubic interpolant through the points. With two points, it uses Linear; selected Akima and Steffen use Cubic until there are at least five points.
3. RT preview and Apply run one plan: an exact table over every code of an unsigned eight- or sixteen-bit image (each entry held in single precision, so a result is the nearest code to the curve within that rounding), and an 8,193-entry table read linearly over a floating image (or a signed or 32-bit integer read as fractions into a Float32 copy), whose largest error against the curve the plan measures and reports (under one millionth for a smooth three-point Cubic; an overshoot held to [0,1] counts in it). The preview is therefore the result Apply writes; a Preview area shows exactly what Apply writes there.
4. RGB/Grayscale maps stored samples directly. A component mode converts RGB to that color space in a 32-bit float working plane (64-bit for a Float64 image), maps one component, and reconstructs RGB. An image described as sensor codes or a physical quantity, as files are read, is converted as fractions of full scale. The result keeps the source's cards, acquisition record, and solution, and is marked nonlinear.
5. The shared process framework then blends an active mask. A successful still swaps the result into the target; a sequence is written to a temporary replacement and committed only after success. Sequence readers first honor automatic CFA debayer, so a decoded CFA frame can already be RGB here.
6. After either successful commit, the process resets its identity spline, channel, and graph zoom; the identity spline keeps the interpolation the combo shows.

Model and equations

Let the ordered control points be

\[ (x_0,y_0),(x_1,y_1),\ldots,(x_n,y_n), \qquad 0=x_0 < \cdots < x_n=1. \]

The selected interpolation constructs a continuous function S(x) that passes through every point, so S(x_i)=y_i. The stored result for a normalized sample x is

\[ y=\operatorname{clamp}_{[0,1]}(S(x)). \]

The clamp means interpolation overshoot does not survive outside the display range: it becomes black or white. It does not prevent a Cubic or Akima curve from changing direction between points. The natural Cubic has zero second derivative at the two endpoints; the engine reproduces the GNU Scientific Library's rules for these interpolants. Steffen and Monotone Cubic are the exposed methods that guarantee no new extrema between control points.

For RGB, the same scalar function is evaluated independently:

\[ (R',G',B')=(S(R),S(G),S(B)), \]

with the clamp applied to each result. A nonlinear S does not generally preserve R:G:B ratios, so RGB mode can change saturation even though it uses one shared curve.

For a component mode, write C(R,G,B) for the conversion and C^{-1} for RGB reconstruction. If component k is selected, AfterNight computes

\[ (c_0,c_1,c_2)=C(R,G,B),\qquad c'_k=S(c_k),\qquad (R',G',B')=C^{-1}(c_0,\ldots,c'_k,\ldots,c_2). \]

The encoded components are normalized: CIE L* maps 0..100 to 0..1, CIE a* and b* use their signed ranges around neutral 0.5, CIE c* maps 0..200, and CIE h* maps -180..180 degrees to 0..1. HSL/HSV hue is also normalized and treated as an ordinary line, not as a circular angle. Strong hue curves near the wrap boundary can therefore produce discontinuous-looking color changes. A floating image keeps a CIE or YCbCr reconstruction outside [0,1] where the inverse conversion puts it; an integer image saturates at its range. CIE components are reconstructed through the exact inverse of the XYZ matrix, and YCbCr through unrounded luma-chroma coefficients.

Boundary and failure behavior

• Apply is disabled only for the exact two-point identity. Adding a point on the diagonal makes the spline non-identity by structure even when its output is visually identical.
• Control-point y values and the curve's results clamp to [0,1]. A floating input outside the curve's [0,1] domain takes the curve's value at the nearer end.
• CIE, HSL, HSV, and YCbCr conversions preserve the target pixel type. The component is a float plane; an integer result is rounded to the nearest code.
• There is no NaN-specific guard in the curve evaluator and no per-pixel error report. Use finite normalized input.
• The output is recorded as nonlinear with provenance curves-transformation, states no transfer function, and its lineage records the curve step (an::applyChannelScope) with the Channel and Interpolation text Image History shows, not the nodes.
• Image History records only Channel and Interpolation, not control-point coordinates, graph toggles, or mask settings. History cannot reconstruct the exact curve.
• Sequence cancellation is cooperative. A cancelled or failed run does not replace the source sequence and its temporary output is removed.
• A one-channel CFA still follows the Grayscale path and is remapped without demosaicing. A CFA sequence with automatic debayer enabled is decoded to RGB before execution and the successful replacement sequence is RGB.
• Reset and successful Apply rebuild the identity spline in the interpolation the combo shows.

Parameter interactions and starting points

Combination	Conservative starting point	Too weak looks like	Too strong looks like	Adjustment order
Tone curve + anchor points	Add one gentle point, then one anchor on each side	Little visible separation in the intended range	Flat/steep transitions, crushed endpoints, or amplified noise	Place anchors, move the main point, inspect histogram and 100% view
Interpolation + node spacing	Cubic with a few well-spaced points; Steffen for monotonic safety	Curve cannot shape a narrow range	Cubic/Akima bows or oscillates between close points	Simplify nodes first, then compare Steffen/Linear
RGB + saturation	Use a shallow S-curve after color calibration	Contrast change is negligible	Channel ratios diverge; bright colors saturate or clip	Set tone contrast, inspect RGB clipping and star color, reduce curvature
Lightness/value + color preservation	Try CIE L* or HSV Value with a gentle curve	Subject remains flat	RGB reconstruction clips gamut or changes perceived saturation	Choose component, shape curve, inspect high-saturation regions
Saturation/chroma + hue	Use HSL Saturation or CIE c* with broad, small changes	Color remains muted	Color noise, neon saturation, gamut clipping	Adjust saturation/chroma first; use hue curves only for deliberate remapping
Mask + curve strength	Tune the unmasked curve gently, then judge the blended preview	Protected region changes too little	Mask edge or transition becomes obvious	Inspect mask, reduce curve strength, then soften/correct mask upstream

Inputs, outputs, and metadata

• One still target produces one modified still; one sequence produces one replacement sequence. Dimensions and pixel type do not change. Channel count and pixel format normally remain unchanged, but automatic sequence CFA debayer converts a one-channel mosaic replacement to RGB.
• Output keeps the source pixel type. An unsigned eight- or sixteen-bit channel curve maps every code through an exact table; a signed or 32-bit integer is read as fractions into a Float32 copy, curved through the 8,193-entry table, and rounded back to the nearest code; a component mode works on a float plane and rounds the reconstruction once, to the nearest code.
• The source target is replaced only after successful processing. Sequence work uses a temporary file and swaps it in after success.
• The source's FITS/XISF cards, acquisition record, and astrometric solution are kept; the output states no transfer function and its lineage records the curve step (an::applyChannelScope) with the Channel and Interpolation text, not the nodes. No auxiliary map, curve preset, or curve file is written.
• Image History receives Curves Transformation with the selected Channel and Interpolation. It omits spline nodes and toolbar states.
• The processing state is nonlinear with provenance curves-transformation.

Performance

Curve evaluation is linear in the number of image samples and runs on the engine's threads, the preview through the same plan as Apply, recalculating an 8-bit histogram when it is shown.

RGB/Grayscale and individual stored-channel modes need the least auxiliary work. CIE, HSL, HSV, and YCbCr modes allocate a full converted image and perform forward and reverse conversion. Sequences add temporary output proportional to the encoded sequence size. Disable Preview while placing many points on a large target, or prototype on a duplicate crop; graph zoom itself does not change processing cost or precision.

Worked example

• Evidence mode: Source-and-test-derived; not executed on project-owned astronomical data.
• Input condition/provenance and settings: A color-calibrated nonlinear RGB image. Select Channel: CIE L* (Lightness) and Interpolation: Steffen. Keep endpoints (0,0)/(1,1), add a broad midtone point slightly above the diagonal, and add low/high anchors on the diagonal.
• Expected checkpoint: Source logic maps only encoded L* through the spline and reconstructs RGB with the original a/b components; a floating image keeps an out-of-gamut value outside [0,1], and an integer image saturates at its range. The curve should remain monotonic when the nodes are monotonic.
• Inspect at 100%/statistics/history: Check dark noise, star cores, saturated colors, mask transitions, clipping statistics, and the Channel/Interpolation history description.
• Limitations: No real-image contrast, color-preservation, clipping, or timing outcome was measured. The exact nodes are intentionally qualitative and are not stored in history.

Warnings and limitations

Apply is destructive and the curve is not saved

A successful Apply replaces the target and immediately resets the spline to identity. Image History does not record node coordinates, so it cannot reproduce the executed curve after Apply, Reset, or reopening the process.

• Do not assume a shared RGB curve preserves color ratios. Nonlinear per-channel mapping can change saturation and push channels into the output clamp.
• CIE a/b and YCbCr chroma are signed concepts encoded around normalized neutral 0.5; broad ordinary tone curves are usually inappropriate for them.
• Hue is circular, but the spline is not. Curves across the 0/1 hue boundary can create a seam-like color change.
• The histogram and coordinate readouts are 8-bit and display-normalized, and the histogram and clipping readout count the mask-blended preview. The histogram places a whole code on a bin boundary in its own bin, by exact interval arithmetic. They are diagnostic, not a full-precision output measurement.
• Preview and Apply run one plan: an exact table over every code of an unsigned eight- or sixteen-bit image and an 8,193-entry linear table over a floating one, so the preview is the result Apply writes.
• The output's processing state is recorded as nonlinear (curves-transformation); treat any non-identity curve result as nonlinear in workflow decisions.
• There is no node list, numeric point editor, preset, save/load, or undo within the process window.
• Successful Apply resets the curve, channel, and graph zoom. Reset does the same without changing the image. In both cases the rebuilt identity curve keeps the interpolation the combo shows.
• A one-channel CFA still is processed as grayscale, not debayered. A CFA sequence becomes RGB when its automatic debayer option is enabled.

Troubleshooting

Symptom or exact error	Verified cause/validation path	Recovery	Evidence source
Apply is disabled	The curve is the exact two-endpoint identity	Add or move a point; use Reset when no change is intended	CurvesTransformationProcess::settingsUpdatedImpl; CurvesWidget::splineIsIdentity
Selected Akima/Steffen looks like another spline	The current implementation substitutes Cubic below five points; any two-point curve is Linear	Add enough well-spaced points or choose the effective method explicitly	ToneCurveModel::effectiveInterpolation
Curve overshoots or flattens against black/white	Cubic/Akima interpolation crossed [0,1], then output clamped	Remove close points, reduce curvature, or use Steffen, Monotone Cubic, or Linear	evaluateCurve; buildCurveProgram
Saturated colors shift after an RGB curve	The same nonlinear function changes each channel value independently, not by a common ratio	Use a gentler curve or compare a lightness/value component	CurvesTransformationProcess::executeTypedProcessImpl
Hue changes abruptly around one color	Hue is encoded on a linear 0..1 axis even though physical hue wraps	Avoid crossing the wrap boundary; prefer a localized color tool	ChannelScope.h; applyProcessChannelScope
Preview and Apply differ	Preview and Apply run one curve plan, so a difference comes from a changed mask, target, or display setting, or a preview still on its way	Compare identical mask/display state, wait for the current preview, and inspect statistics after Apply	curvesProgramFor; shared RT preview/mask path
History cannot reproduce the result	The step description stores Channel and Interpolation only	Record node values externally or preserve a duplicate result	CurvesTransformationProcess::executeTypedProcessImpl
Sequence remains unchanged after Cancel	Cancellation intentionally removes temporary output before replacement	Retry with final settings; keep enough temporary disk space	ImageRTPreviewControl::runOverSequence
The next curve is identity/RGB after Apply	Successful still and sequence commits call the process reset path	This is expected; rebuild the curve and reselect the intended channel	Shared resetSettingsAsync() and CurvesTransformationProcess::resetSettings
A CFA sequence becomes RGB after Apply	Automatic sequence CFA debayer was enabled, so the reader decoded each mosaic frame to RGB before Curves ran	Disable automatic sequence debayer when preserving a one-channel CFA representation is required; do not stretch CFA mosaics as grayscale	Shared sequence executor, demosaic, and concrete sequence readers

Related workflow steps

Complete Background Neutralization and Photometric Color Calibration or Spectrophotometric Color Calibration before nonlinear work. Use Histogram Transformation, Generalized Hyperbolic Transformation, or ArcSinh Stretch for the main stretch, then use Curves for intentional tone/color finishing. Continue with Color Saturation, targeted noise reduction, and sharpening while monitoring Image History and Image Statistics.

References and provenance

• GNU Scientific Library 2.8: Interpolation — the definitions of the Linear, natural Cubic, Akima, and monotonic Steffen interpolants and their point requirements. AfterNight implements the same rules itself and does not use the library.
• AfterNight project source — authoritative for editor gestures, interpolation fallbacks, the shared preview/Apply curve plan, color-component paths, masks, sequences, history, and the processing-state record.