Split CFA¶

Purpose and expected result¶

Split CFA opens new grayscale images or sequences derived directly from a Bayer mosaic. It can expose all four photosite positions, extract the red photosite as an Ha-oriented plane, average the two green photosites, or derive Ha and OIII planes for a dual-band OSC workflow. The source is never modified.

The Ha/OIII methods are sensor/filter assumptions, not a spectroscopic decomposition. They are meaningful only when red photosites primarily record Ha and green/blue photosites primarily record OIII.

At a glance¶

Item	Current behavior
Menu path	Process > Color Spaces > Split CFA; the window's title is Split CFA followed by the target's name
Enabled when	An image or sequence view is active. The target selector lists only views shown as a one-channel mosaic; a target that does not state a two-by-two mosaic is refused when OK is pressed.
Inputs	One undebayered RGGB, BGGR, GRBG, or GBRG mono image/sequence, with the arrangement and phase its description states. An X-Trans mosaic and an image that states no filter array are refused by name.
Outputs	One, two, or four new grayscale views/sequences in the mosaic's sample type. Dimensions depend on Method/Resampling. Source remains open and unchanged.
Recommended image state	Calibrated linear CFA data before Debayer. Ha/OIII requires appropriate dual-band acquisition.
Preview support	None. OK executes immediately.
Sequence support	Supported; every frame is split and written into new sequences of the source's container, with progress and Cancel.
Mask support	Unsupported and ignored.
External dependencies	None.

Before you start¶

Confirm the file is still mono CFA data and its stated Bayer pattern is correct. The stated arrangement and phase decide plane names and the Ha/OIII positions.
Calibrate before splitting when possible. Splitting first changes geometry and removes CFA metadata from the outputs.
For Ha/OIII, confirm the filter's red leak/cross-talk and sensor response are acceptable. This process performs no continuum subtraction or spectral fit.
Plan to plate-solve an output again if astrometry matters: no output states the source's solution, because its pixel grid is no longer the mosaic's. A positional plane keeps the mosaic's other header cards, but drops its world-coordinate cards and the solver's PLTSOLVR card with the solution.

Procedure¶

Select a CFA mono view and open Process > Color Spaces > Split CFA.
Verify Target Image and choose a Method.
For Extract Ha/OIII, choose whether to keep native unequal dimensions, upsample Ha, or downsample OIII.
Click OK. The window closes after success; sequence work shows progress.
Verify output names, dimensions, and sample values.
Use Merge CFA only with four unmodified, matching positional planes in CFA0–CFA3 order. Ha/Green/OIII products are not lossless merge inputs.

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
Method	Selector	Split CFA Channels	None	Split CFA Channels, Extract Ha, Extract Ha/OIII, Extract Green	Chooses four positional planes, the red photosite plane, derived Ha+OIII, or the mean of both green photosites.	Enables Ha/OIII Resampling only for Extract Ha/OIII.	Four-plane and Ha/OIII sequence modes write more sequences; derived modes are not reversible.	Execute only; still/sequence; no mask	Window lifetime; reopening restores default.
Ha/OIII Resampling	Selector	No Resampling	None	No Resampling, Upsample Ha, Downsample OIII	Keeps Ha at half width/height and OIII full size; or resamples one by a factor of two with the engine's Lanczos filter to match the other.	Enabled only for Extract Ha/OIII.	Upsampling invents samples and costs memory; downsampling discards OIII resolution; Lanczos can ring near sharp transitions.	Ha/OIII outputs only; still/sequence	Window lifetime.
Target Image	CFA mono image/sequence selector	Launching view	None	Open views shown as a one-channel mosaic	Chooses the source without modifying it.	Only compatible views listed; a target that no longer states a two-by-two mosaic is refused when OK is pressed.	Sequence length controls runtime/disk.	Execute only	Current window only.
OK	Button	—	None	Valid target	Checks the target's mosaic, reads the dialog, runs the extraction on a worker, opens the output view(s), and closes after success.	Disabled while running.	Output count is ½/4; sequences write that many new sequences.	Still/sequence; no preview/mask	No persistence.
Close	Button	—	None	Click action	Closes without output.	Disabled during execution. A window closed during a run stays hidden until it lands.	No source change.	Process window	No persistence.

How the algorithm works¶

Conceptual pipeline¶

Each 2×2 Bayer cell is indexed by position, independent of pattern label: CFA0 (even x, even y), CFA1 (odd x, even y), CFA2 (even x, odd y), CFA3 (odd x, odd y).
The mosaic's stated arrangement and phase map those four positions to Red, Green1, Green2, and Blue; the greens are numbered in plane order.
The chosen method copies positional samples or derives Ha/Green/OIII through the engine.
Each output is a new image in the mosaic's sample type. Sequence frames run the same operation and are written into new sequences.

Model and equations¶

The colour at each position under the four arrangements at zero phase is:

Pattern	CFA0	CFA1	CFA2	CFA3
RGGB	Red	Green1	Green2	Blue
BGGR	Blue	Green1	Green2	Red
GRBG	Green1	Red	Blue	Green2
GBRG	Green1	Blue	Red	Green2

A mosaic cropped at an odd offset states a shifted phase, and the names follow it: an RGGB mosaic cropped one column in is read as GRBG and its planes are named Green1, Red, Blue, Green2.

Split CFA Channels copies each position into a half-width, half-height plane. For odd source dimensions, the last column and/or row is discarded. Because the operation is pure copying, those four planes can reconstruct the retained even rectangle exactly when their order/type/dimensions are preserved.

Extract Ha copies the red position, as the stated phase places it, from each 2×2 cell, also at half width and height, in the mosaic's sample type. Extract Green computes

\[ G=\frac{G_1+G_2}{2} \]

for each cell in normalized sample space; an integer result is rounded to the nearest code, ties to even.

Extract Ha/OIII works in these units: an unsigned integer as a fraction of its largest value, a signed one counted from its smallest value, a floating sample as it is, in 32-bit float, or 64-bit float for a 64-bit float mosaic and for 32-bit integers. Odd dimensions are cropped to an even top-left rectangle. Ha copies the red position into a half-resolution plane. For OIII, the global means of the finite Green1, Green2 and Blue samples are μ₁, μ₂, μ_B; their shared target mean is

\[ \bar\mu=(\mu_1+\mu_2+\mu_B)/3. \]

Each non-red photosite is multiplied by μ̄/μᵢ; a position whose mean is within double-precision epsilon of zero is left unscaled. Each red position is filled from the equalized OIII samples among its eight neighbours, the four axial ones weighted by √2 and the four diagonal ones by 1. A non-finite sample counts toward neither a mean nor a fill and is written as zero. Optional matching resamples one product by a factor of two with a separable, eight-tap Lanczos filter (sinc(d) sinc(d/4), |d| < 4) on half-pixel centres, reading the nearest edge sample beyond the picture. Both products are then written back in the mosaic's sample type in the same units they were read in - an unsigned integer as a fraction of its largest value, a signed one counted from its smallest value - an integer to the nearest code.

Boundary and failure behavior¶

Every method reads only complete 2×2 blocks; an odd last row/column is not used.
The target is checked when OK is pressed. An image that states no filter array is refused with the image states no colour filter array, an image of more than one channel with a mosaic has one channel, and an X-Trans mosaic with the XTrans filter array is not a two-by-two arrangement, which is all this process takes apart, each under Error running Split CFA. There is no run-anyway question.
Split CFA Channels refuses a mosaic whose valid samples are stated in a mask with an image whose valid samples are stated in a mask cannot be split. Extract Ha, Extract Green and Extract Ha/OIII refuse, before any work, a mosaic that states any sample invalid - a validity rule or a mask - with an image that states some of its samples invalid cannot have a narrowband channel extracted. Ha/OIII refuses a mosaic in which a green or blue position holds no finite sample.
The Lanczos result is not clamped: beside a sharp edge a floating product can fall slightly outside 0–1, and an integer product saturates at its range.
A failure is reported in a message titled Error running Split CFA that begins An error occurred when executing the Split CFA process: followed by the reason, and in the log. No output view is opened.
A sequence another run holds, or a still image whose edit is still being written, shows the busy warning. Cancel on a sequence's progress window stops the run quietly; nothing is opened and the files it was writing are removed.

Parameter interactions and starting points¶

Combination	Conservative starting point	Too weak looks like	Too strong looks like	Adjustment order
Pattern metadata + Split	Verify pattern from acquisition metadata; use four planes first	Labels/colors do not match sensor positions	Wrong pattern silently relabels derived channels	Verify metadata -> inspect CFA0–3 -> derive channels
Ha/OIII + Resampling	No Resampling for diagnosis	Dimensions prevent later comparison/composition	Upsampling looks sharper without adding real resolution; downsampling loses detail	Inspect native -> choose one target geometry -> resample once
Ha/OIII + filter assumptions	Use only with reviewed dual-band response	Cross-talk/continuum remains	Derived planes are mistaken for spectrally pure masters	Verify filter/sensor -> inspect native planes -> compare externally if needed

Inputs, outputs, and metadata¶

Method	Outputs	Native dimensions for source `W×H`	Names
Split CFA Channels	4 grayscale images/sequences	`floor(W/2) × floor(H/2)` each	`<target> - CFA0 - <color>` through CFA3
Extract Ha	1 grayscale image/sequence	`floor(W/2) × floor(H/2)`	`<target> - Ha`
Extract Green	1 grayscale image/sequence	`floor(W/2) × floor(H/2)`	`<target> - Green`
Extract Ha/OIII, No Resampling	2 grayscale images/sequences	Ha `floor(W/2)×floor(H/2)`; OIII even-cropped `W×H`	`<target> - Ha`; `<target> - Oiii`
Extract Ha/OIII, Upsample Ha	2 matching full/even size	Ha resampled to OIII	Same
Extract Ha/OIII, Downsample OIII	2 matching half size	OIII resampled to Ha	Same

Each output's history begins Split CFA: <what> extracted from RAW image '<target>', where <what> is CFA0 to CFA3, Ha, Green or OIII; a sequence's says RAW sequence. Each new view opens beside the source.

Every output keeps the mosaic's sample type, any of the eight AfterNight stores. A positional plane keeps the mosaic's acquisition record, meaning and header cards, states no filter array, and states no plate solution; its lineage names the engine's block split and the plane. The solution goes with the cards that state it: the mosaic's world-coordinate cards and the solver's PLTSOLVR card describe the mosaic's grid and not the half-size plane's, so a plane carries none of them; plate-solve the plane again if you need a solution. An Ha, Green or OIII product keeps the acquisition record, drops the plate solution and the header cards, withdraws any decision about how it is written, and is marked as a resolved measurement (demosaiced). The source remains unchanged.

A sequence's frames are read as recorded, every frame of the sequence, and each output sequence is written in the source's container in AfterNight's temporary directory, published only when every frame is written. Save or export it when a durable file is wanted.

Performance¶

Simple split/Ha/Green work is linear in pixels and output count. Ha/OIII adds a floating working copy of the mosaic, two global passes, the red-site fill and, when asked, a 64-bit float working plane for the Lanczos resampling; a sequence computes the pair once per frame. Test one still before a long sequence and budget disk for up to four new sequences.

Worked example¶

Evidence mode: Source-and-focused-test-derived.
Input condition/provenance and settings: a small 16-bit RGGB mosaic in a view named target, split with each method; the same mosaic cropped one column in; a 32-bit float grey image with no filter array.
Expected checkpoint: four planes named target - CFA0 - Red, target - CFA1 - Green1, target - CFA2 - Green2, target - CFA3 - Blue; after the crop, Green1, Red, Blue, Green2. The grey image is refused under Error running Split CFA naming the missing filter array, and nothing opens.
Inspect at 100%/statistics/history: verify positional values, dimensions and history; a plane keeps the mosaic's OBJECT card, states no solution and carries no world-coordinate or PLTSOLVR card, an Ha product is marked demosaiced.
Limitations: tests do not validate dual-band spectral quality.

Warnings and limitations¶

There is no preview, mask, channel selector, or Reset.
Wrong Bayer metadata produces wrong labels and derived channels.
Ha/OIII is a dual-band heuristic, not continuum subtraction or spectroscopy.
No output states a plate solution or carries the cards of one; solve the derived image again if needed.
Odd dimensions lose the final row/column. Resampling uses Lanczos and can ring.
Split/derived outputs are new views; only the four untouched positional planes are losslessly mergeable back to the retained even rectangle.
X-Trans mosaics cannot be split. Demosaic one with Debayer under From metadata, or open the RAW file with automatic debayer, which demosaics it by the X-Trans counterpart of the RAW setting's method, instead.

Troubleshooting¶

Symptom or exact error	Verified cause/validation path	Recovery	Evidence source
Split CFA target absent	View is not shown as a one-channel mosaic.	Open undebayered data and correct CFA metadata.	`isViewControlSupported()`
`the image states no colour filter array`	The image states no Bayer pattern.	Reopen with correct RAW/CFA metadata; do not guess if the pattern is unknown.	`checkBayerMosaic`
`the XTrans filter array is not a two-by-two arrangement...`	The mosaic is X-Trans.	Split CFA takes apart only a two-by-two mosaic. Use Debayer with From metadata, which demosaics an X-Trans mosaic by the chosen method's X-Trans counterpart.	`checkBayerMosaic`; `GeometryUnits.OnlyATwoByTwoMosaicIsTakenApart`; `GeometryUnits.DebayerReadsAnXTransArrayFromTheImageAndRefusesANamedBayerPatternOverIt`
`an image whose valid samples are stated in a mask cannot be split`	The mosaic carries a validity mask.	Use a mosaic without one.	`splitMosaicPlanes`; `GeometryProcessCase.AMosaicWhoseValidityIsAMaskIsRefusedByName`
`... cannot have a narrowband channel extracted`	Extract Ha, Extract Green or Extract Ha/OIII on a mosaic that states some sample invalid, by a validity rule or a mask.	Use a mosaic that states every sample valid.	`refuseUnlessAllValid`; `GeometryProcessCase.AMosaicWhoseValidityIsAMaskIsRefusedByName`
Ha/OIII Resampling is disabled	Method is not Extract Ha/OIII.	Select that method if the acquisition supports it.	Method dependency handler
Output loses last row/column	Source dimension is odd.	Crop intentionally to even dimensions before splitting and account for the offset.	engine block split
Output has no astrometric solution	No output states a solution or keeps the mosaic's world-coordinate cards, which describe the mosaic's grid.	Plate-solve the derived output.	operation effects; `GeometryProcessCase.SplitAndMergeCfaCarryNoWorldCoordinateCards`
The busy warning	Another run holds the sequence.	Wait for it to land and run again.	`runToNewViews`

Run Image Calibration before splitting. Use CFA Integer Resample when you need a CFA-preserving integer geometry change, Debayer for full-color reconstruction, and Merge CFA only for the four positional planes. Derived Ha/OIII masters can feed narrowband composition only after their assumptions and geometry are validated.

References and provenance¶

Bayer pattern names and extraction behavior are defined by AfterNight's image engine and the support unit.
The Lanczos definition used when matching Ha/OIII dimensions is stated in the engine's narrowband extraction header.