SCNR¶

Purpose and expected result¶

SCNR (Subtractive Chromatic Noise Reduction) suppresses green in a three-channel RGB image, or in every frame of an RGB sequence, with one of four per-pixel protection methods. Two methods hold green at a ceiling taken from red and blue; two multiply green by a protection mask taken from red and blue, mixed by Amount. Preserve Lightness keeps each pixel's CIE L* lightness while taking the suppressed pixel's colour.

On Apply the process replaces the target's samples, weighed by the view's image mask when one is enabled, and records one Image History step. The image's size, sample representation and metadata are kept, and the result is recorded as nonlinear; the change is recorded in the image's lineage. SCNR always targets green; it cannot suppress red, blue, cyan, magenta or yellow.

At a glance¶

Item	Current behavior
Menu path	Process > Color Calibration > SCNR Filter; window title SCNR followed by the view name
Enabled when	The active view shows a colour image. The Target Image list offers only views showing a three-channel RGB image. The process is nonexclusive.
Inputs	One three-channel RGB still or sequence of any of the eight sample representations, a protection method, Amount (mask methods only), Preserve Lightness, and the view's enabled image mask, if any
Outputs	Replaces the target as one Image History step; a sequence is written whole into a new sequence of the same container, which the view then shows. No preview, mask, report or new view is created.
Recommended image state	After gradient removal, background neutralization and colour calibration. SCNR records its result as nonlinear, so steps that need linear data refuse or warn about it afterwards.
Preview support	None. Apply runs the full image or sequence; the window stays open for another run.
Sequence support	Yes: every frame, whatever frames the player has selected, with the settings read at Apply; progress and Cancel are shown
Mask support	Yes: the result is weighed by the view's enabled mask, read when Apply is pressed; inversion is honoured
External dependencies	None

Before you start¶

Finish gradient correction, background neutralization and PCC, SPCC or manual colour calibration first. SCNR removes green; it does not estimate a neutral background or a calibrated colour response.
Decide whether the residual green is unwanted. Green can be real signal (OIII contribution, reflection nebulosity, stars), and the mask methods reduce green even where it is not the strongest channel.
Know how the image is described. With Preserve Lightness, an image whose description states linear RGB is read as linear light; any other colour image is read as sRGB-encoded. The lightness kept is computed in that reading.
Image History keeps the step and it can be undone. There is no preview or Reset, and a second Apply suppresses again on top of the first.
If you use an image mask, check that it is enabled, has the image's width and height, and is or is not inverted as you intend. It is read when Apply is pressed.
For a sequence, every frame must be three-channel RGB as SCNR reads it. If the sequence viewer demosaics the frames it shows, the frames are demosaiced before SCNR, by the method the sequence states from the one RAW Demosaic Algorithm setting.

Procedure¶

Activate the RGB target and open Process > Color Calibration > SCNR Filter. Confirm Target Image.
Start with Maximum Neutral for the gentler hard ceiling: it reduces green only where green exceeds both red and blue. Average Neutral, the default, holds green to the mean of red and blue and removes more.
For a graded correction choose Maximum Mask or Additive Mask. Amount becomes enabled; start below 1.00.
Leave Preserve Lightness selected to keep each pixel's lightness; clear it to change green alone, with red and blue left as they were.
Check the view's image mask: disable it for a global run, or leave it enabled for a weighted one.
Choose Apply. The dialog and the mask are read at that moment; changing a control afterwards does not reach the run. A still runs on a worker without a progress window; a sequence shows an SCNR progress window with Cancel.
When the result is shown, an SCNR Image History step is added whose description is Protection Method <method> preserve lightness true or ... false, and the window stays open with Apply and Close enabled again.
Compare at 100% under the same display conditions: green star cores, nebular colour, dark chroma noise and any clipping. Undo before trying other settings rather than stacking runs.

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
Protection Type	Choice	Average Neutral	None	Average Neutral, Maximum Neutral, Maximum Mask, Additive Mask	Selects the green ceiling or protection mask: Average Neutral holds green at `(R+B)/2`; Maximum Neutral at `max(R,B)`; Maximum Mask uses `max(R,B)` and Additive Mask `min(1,R+B)` as the mask	Selecting a mask method enables Amount; a neutral method disables it	Average Neutral removes the most green; Maximum Neutral keeps green up to the stronger of red and blue; Additive Mask protects more than Maximum Mask at the same pixel and Amount	Apply; green, and all three channels with Preserve Lightness; still/sequence	Window lifetime; no Reset or preset
Amount	Slider and decimal spin box	`1.00`, disabled under the default method	None	`0.00`–`1.00`; spin step `0.05`; slider `0`–`1000`	Higher suppresses more green in the mask methods; `0` leaves green as it was and `1` applies the mask outright	Enabled only for Maximum Mask and Additive Mask; ignored by both neutral methods; keeps its value when the method changes	High values can desaturate legitimate green	Apply; green; still/sequence	Window lifetime
Target Image	Image/sequence selector	The launching view	None	Open views showing a three-channel RGB image	Chooses the image or sequence Apply changes	Reattaches the window to the chosen view	A wrong target will be changed	Apply; still/sequence	Window lifetime
Preserve Lightness	Checkbox	Selected	None	Selected/cleared	Selected: the result keeps the original pixel's CIE L* and takes a* and b* from the suppressed pixel, so red and blue move too. Cleared: only green changes	Always enabled	Three colour conversions per pixel; the reconstructed colour can leave the unit interval and is then held to it	Apply; all three channels when selected; still/sequence	Window lifetime
Apply	Action	Enabled	None	One run at a time	Reads the dialog and the view's mask, then starts SCNR on a worker	Apply and Close disable while it runs and re-enable when the run lands	Destructive; no preview; Image History keeps the image it replaces	Apply; still/sequence	The window stays open; settings are not saved
Close	Action	Enabled	None	Button or title-bar close	Closes without starting a run	The button disables during a run; a title-bar close while a run is in flight hides the window, which closes once the run has landed; it does not stop the run	Unapplied settings are discarded	Dialog only	Not persisted
Sequence progress window	Read-only progress	Shown only for a sequence	None	Title SCNR; text `Executing SCNR to sequence <view name>`	Shows the run over the sequence's frames	Belongs to the run, not to the SCNR window	None	Apply; sequence only	Per run
Cancel in the progress window	Action	Available while a sequence runs	None	One request	Stops the run between frames	Sequence only; a still run has no Cancel	A stopped run leaves the sequence as it was, writes nothing into Image History and leaves no temporary sequence behind; nothing is reported	Apply; sequence only	Per run

How the algorithm works¶

Conceptual pipeline¶

On Apply, read the protection method, Amount, Preserve Lightness and the view's enabled mask (and whether it is inverted) on the GUI thread. The worker reads only those values.
Refuse an image that is not three-channel RGB with SCNR needs an RGB image.
Make a working copy of the samples as fractions of full scale: an unsigned integer as a fraction of its largest value, a signed integer counted from its smallest, in Float32 - Float64 for a Float64 image and the two 32-bit integer representations.
Run the engine's chromatic-noise reduction (reduceImageChromaticNoise) with a white level of 1: compute green from the selected method; with Preserve Lightness, rebuild the pixel in CIE L*a*b*; hold all three channels to [0,1]. A pixel the image's validity rejects is left as it was in all three channels.
Bring the working copy back to the image's representation, to the nearest value and saturated, keeping the image's description, validity and metadata, with a lineage entry an::reduceImageChromaticNoise, detail SCNR: <method>, amount <amount>, lightness preserved (or not preserved), the method spelled without spaces (AverageNeutral) and Amount to six decimals.
If the view had an enabled mask, weigh the result by it against the source.
For a still image the result is an edit of the view: Image History records the step before the view shows it. For a sequence, see Inputs, outputs, and metadata.

Model and equations¶

Let (r,g,b) be one pixel as fractions of full scale and A be Amount.

Average Neutral computes

\[ g'=\min\left(g,\frac{r+b}{2}\right). \]

Maximum Neutral computes

\[ g'=\min\big(g,\max(r,b)\big). \]

Both are hard ceilings and do not read Amount. Average Neutral is never gentler than Maximum Neutral because (r+b)/2 <= max(r,b).

Maximum Mask uses m = max(r,b); Additive Mask uses m = min(1, r+b). Both evaluate

\[ g'=g\,\big[(1-A)(1-m)+m\big]=g\,\big[1-A(1-m)\big]. \]

At A=0, g'=g; at A=1, g'=mg. Additive Mask's m is never smaller than Maximum Mask's, so it keeps at least as much green at the same pixel and Amount. The mask methods do not first ask whether green exceeds red or blue.

Without Preserve Lightness the result is (r, g', b), held to [0,1].

With Preserve Lightness the engine converts the original pixel to CIE L*a*b* (D65 white, sRGB primaries) and keeps its L*_0, converts the suppressed pixel (r, g', b) and keeps its a*_1, b*_1, converts (L*_0, a*_1, b*_1) back to RGB, and holds each channel to [0,1]. The RGB samples are read through the transfer the image states: linear when the image's description says its colour is linear RGB, sRGB-encoded for any other colour image. Where the rebuilt colour leaves the unit interval, the hold means the lightness is not kept exactly.

For a view mask value alpha, the result is

\[ O=(1-\alpha)\,I+\alpha\,P, \]

where I is the source and P the processed image; an inverted mask swaps the weights. A one-channel mask weighs all three channels; a three-channel mask weighs each channel by its own.

Boundary and failure behavior¶

A view that does not show a three-channel RGB image is not offered as a target. A sequence frame that is not three-channel RGB stops the run with SCNR needs an RGB image.
Only green is suppressed. There is no channel selector, cast detector, threshold or spatial model.
Every result sample is held to [0,1] of full scale and then written in the image's representation. No count or map of held samples is produced.
A failure - the engine refusing memory, a mask of another size, a frame that is not RGB - is reported in a message titled Error running SCNR whose text is An error occurred when executing the SCNR process: followed by the reason, and in the log; the image and its history are left as they were and the window stays open.
If another run already holds a sequence view, SCNR shows the view's busy warning titled SCNR and starts nothing.
If Image History cannot keep the image the run replaces, the history says so once, in a message titled Image History, SCNR says nothing more, and the image and its history are left as they were.
A still run has no Cancel. Closing the window during a run hides it until the run lands; it does not stop the run.

Parameter interactions and starting points¶

Combination	Conservative starting point	Too weak looks like	Too strong looks like	Adjustment order
Maximum Neutral + Average Neutral	Try Maximum Neutral once after colour calibration	Green remains above the red/blue mean but not above both channels	Average Neutral removes legitimate green or changes star or nebular colour	Compare Maximum Neutral first; undo before testing Average Neutral
Additive Mask + Amount	Additive Mask below `1.0`	Residual unwanted green or chroma noise	Broad green desaturation, especially in dark pixels	Choose the method, raise Amount gradually, inspect real green structures at 100%
Maximum Mask + Amount	Use when Additive Mask protects too much	Weak red and blue leave green under Additive Mask	Dark or weakly coloured areas lose green strongly	Switch method first, then retune Amount from a lower value
Preserve Lightness + clipping	Selected; inspect all channels	Cleared, the result is darker where green was removed	Selected, the rebuild is held at `0` or `1` or shifts red and blue	Test one method and Amount, compare Preserve on and off through undo, inspect histograms
View mask + SCNR	Settle the global settings unmasked, then enable a soft mask	A dark or disabled mask changes little	A hard or colour mask creates local, channel-dependent transitions	Check the mask and its inversion, then Apply once

Inputs, outputs, and metadata¶

Inputs are three-channel RGB stills or RGB sequence frames of any of the eight sample representations. Width, height and representation are kept.
The result keeps the image's description, validity rule and mask, acquisition record, plate solution and header cards. Its processing-state cards say it is nonlinear display RGB made by the operation scnr, and its meaning no longer claims a linear encoding. The lineage gains one entry, an::reduceImageChromaticNoise, naming the method, Amount and whether the lightness was preserved.
An enabled view mask weighs the result against the source, honouring inversion; it must have the image's width and height and one channel or three.
On a still image the result is one SCNR Image History step, Protection Method <method> preserve lightness true|false; it does not record Amount or the mask.
On a sequence, SCNR takes the view for the run, reads every frame of the sequence - every one, whatever the player has selected, and demosaiced when the view demosaics them - writes each result frame into a new temporary sequence of the same container, publishes it whole, and shows it in the view as one Image History step. A run that fails or is cancelled leaves the sequence as it was and nothing in the temporary directory, and releases the view.
Apply creates no preview, new view, mask, clipping map, report or saved file.

Performance¶

Work is proportional to the number of pixels. A run holds two working copies of the image in Float32 (or Float64), the result in the image's representation and, for Image History, the image it replaces. Preserve Lightness adds three colour conversions per pixel. An enabled mask adds a copy of the result and one blend. A sequence additionally writes a temporary output sequence. No GPU, network or preview cache is involved, and no timing is claimed here.

Worked example¶

Evidence mode: source-and-test-derived; not run on approved astronomical data.
Input condition and settings: one pixel of a Float32 image, (R,G,B) = (0.20, 0.60, 0.40), Preserve Lightness cleared, no view mask.
Expected checkpoint: Average Neutral gives G' = 0.30; Maximum Neutral gives G' = 0.40; at Amount 1.0, Maximum Mask gives G' = 0.24 and Additive Mask G' = 0.36; at Amount 0.5, Maximum Mask gives G' = 0.42. Red and blue stay 0.20 and 0.40. With Maximum Neutral the history step reads Protection Method Maximum Neutral preserve lightness false.
Inspect at 100%/statistics/history: compare green structures and star cores, clipping with Preserve Lightness, mask boundaries and the history step. Note Amount separately, because the history does not record it.
Limitations: the arithmetic shows the equations only; it is not evidence of perceptual quality, removal of a real cast, colorimetric accuracy or runtime.

Warnings and limitations¶

SCNR always suppresses green. It has no colour selector and no test that the green is unwanted.
SCNR is a nonlinear colour change, and the result is recorded as nonlinear: an image declared linear is declared nonlinear afterwards.
Preserve Lightness reads a linear-RGB image as linear light and any other colour image as sRGB-encoded, under a D65 white. It is not a profile-aware conversion.
Average and Maximum Neutral are full-strength ceilings; Amount is disabled and not read. The mask methods act everywhere, not only where green dominates.
Apply is destructive, has no preview or Reset, and the window stays open for repeated runs. Undo between trials.
The view mask weighs the result only; a three-channel mask weighs each channel differently.
All result samples are held to [0,1] of full scale with no clipping count.
History records the method and Preserve Lightness only.
A sequence run can be cancelled only between frames; a still run cannot be cancelled.

Troubleshooting¶

Symptom or exact error	Verified cause/validation path	Recovery	Evidence source
The target is not offered, or the menu action is disabled	The view does not show a three-channel RGB image	Select an open RGB still or sequence	`showsRgbImage`; `ColorToolsUnitsTest.AnImageATooDoesNotReadIsRefused`
Amount is disabled	Average Neutral or Maximum Neutral is selected; neither reads Amount	Choose Maximum Mask or Additive Mask	`ColorToolsCase.ScnrOffersTheAmountToTheMaskMethodsAndStaysOpen`
Amount `0` still changes something	The mask methods leave green as it was at `0`, but Preserve Lightness's round trip and the hold to `[0,1]` can move samples	Clear Preserve Lightness for an identity check	Engine equations
Legitimate green disappears	A ceiling or mask method suppressed real green; the mask methods do not require green to dominate	Undo; try Additive Mask with a lower Amount, or a view mask	SCNR equations
Red or blue changed	Preserve Lightness rebuilt the pixel from its original L* and the suppressed colour	Undo; compare with Preserve Lightness cleared	Engine source
Error running SCNR	The engine refused memory, the mask did not fit, or a frame was not RGB	Read the message and the log, free memory or fix the mask, and run again	`ColorToolsWindowTest.ARunTheEngineRefusesSaysWhyAndChangesNothing`
The mask had no or the wrong effect	The mask was disabled, inverted, or changed after Apply was pressed	Check the mask before Apply	`ColorToolsWindowTest.TheViewsMaskWeighsTheResult`
A setting changed during the run had no effect	The dialog is read when Apply is pressed	Run again with the new setting	`ColorToolsWindowTest.ARunReadsTheDialogAsItWasWhenApplyWasPressed`
History cannot reproduce the result	The step omits Amount and the mask	Note Amount and the mask before Apply	`ColorToolsCase.TheHistoryStepsDescribeWhatRan`
Busy warning titled SCNR	Another run holds the sequence view	Wait for it to finish, then run again	`runInPlace` busy path
Sequence does not change after Cancel	A stopped run publishes nothing	Run again	`ColorToolsWindowTest.AStoppedRunPublishesNothing`

Run Background Extraction, Background Neutralization, and one of Color Calibration, Photometric Color Calibration, or Spectrophotometric Color Calibration before SCNR. Those steps estimate gradients, offsets or calibrated channel balance; SCNR is a later selective green subtraction.

Use Histogram Transformation to establish the intended nonlinear display state, and Color Saturation for broader chroma adjustments. A view mask can localize SCNR, but SCNR itself does not generate a cast mask.

References and provenance¶

The AfterNight SCNR window, the shared colour-tool support, the engine's chromatic-noise reduction and the focused tests are authoritative for the four methods, the fixed green channel, the hold to the unit interval, the metadata, the mask and the execution behavior.
International Color Consortium, How to interpret the sRGB color space, checked 2026-07-13. Context for the sRGB transfer and D65 white used by Preserve Lightness; AfterNight does not perform an ICC profile conversion here.
CIE, CIE 1976 Lab* Colour Space, checked 2026-07-13. Context for L*, a*, b*; the SCNR protection methods are AfterNight implementation formulas, not CIE standard operations.