Adjust Brightness/Contrast

Purpose and expected result

Adjust Brightness/Contrast first adds a fixed normalized offset, clamps it, then expands or compresses values around a 0.5 pivot and clamps again. Use it for a small display-tone correction after the main stretch, not as exposure control or a substitute for a purpose-built stretch.

The process can transform stored RGB channels together, one stored channel, or one converted color-space component before reconstructing RGB.

At a glance

Item	Current behavior
Menu path	Process > Adjustments > Brightness/Contrast
Enabled when	A grayscale/RGB still or displayable sequence is active, this process is not already attached, and no attached process holds exclusive access
Inputs	One grayscale/RGB still or sequence in any sample type, read in normalized units (in a stored channel a sample holding NaN passes through unchanged and an infinity ends at 0 or 1); optional active mask; RGB exposes 21 channel/component choices
Outputs	Same geometry/pixel type in every channel mode; converted component modes reconstruct RGB; successful sequence Apply uses a temporary replacement sequence; snapshot creates a still
Recommended image state	Nonlinear display data after the main stretch; every Apply result is recorded nonlinear
Preview support	Exact (runs Apply's own program), histogram, original/result switch, Preview area, snapshot, fixed-linear display
Sequence support	Temporary replacement sequence with progress and Cancel; automatic CFA debayer may change decoded frame format
Mask support	Active mask post-blends source/result; white applies, black protects, gray blends, inversion reverses weights
External dependencies	None; bundled engine

Before you start

Perform calibration, integration, background correction, color calibration, and the intended primary stretch first. Duplicate the target. Strong Brightness or Contrast clips information and cannot be undone after saving.

Component selections are powerful but literal: applying a scalar brightness/ contrast curve to encoded hue or chroma coordinates does not preserve circular hue relationships or perceptual uniformity. Prefer a purpose-built color tool unless you have a specific component-space reason.

Procedure

1. Duplicate the target and inspect any active mask/inversion.
2. Open Process > Adjustments > Brightness/Contrast.
3. Leave Channel at RGB (or disabled Grayscale) for a conventional adjustment. Choose a component only deliberately.
4. Start near Brightness -10..10 and Contrast -10..10. Tune Brightness first because its clamp occurs before Contrast.
5. Inspect black/white endpoint counts, background, bright stars, noise, color balance, and mask edges. Use Preview area for a representative region only.
6. Compare with Switch image, then Apply to the duplicate. Preview runs the same program as Apply, so the whole-image Preview shows the values Apply writes.
7. Verify Image History, processing state, and metadata before saving.

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 Preview	None	Binning follows Plot resolution	Shows source or mask-aware Preview and approximate endpoint counts	Show histogram controls visibility	Recalculation adds work; endpoints bin-dependent	Display only	Recomputed
Left-drag in histogram	Graph gesture	No readout	None	Hold left/move	Shows normalized coordinates	Histogram shown	Diagnostic only	Display only	Ends on release
Middle-drag in histogram	Graph gesture	Fitted/unpanned	None	Hold middle/drag	Pans graph	Histogram shown	No output effect	Display only	Until reset/close
Ctrl+mouse wheel in histogram	Graph gesture	Fitted size	None	Up 1.20x; down 0.8x	Zooms around cursor	Interactive graph	Rendering only	Display only	Window lifetime
Zoom in	Toolbar action	Fitted	None	Each 1.30x	Enlarges plot	Disabled with hidden histogram	Rendering only	Display only	Window lifetime
Zoom out	Toolbar action	Fitted	None	Each 0.70x	Shrinks plot	Disabled with hidden histogram	Rendering only	Display only	Window lifetime
Zoom reset	Toolbar action	—	None	One action	Fits/resets graph	Disabled with hidden histogram	No output effect	Display only	Immediate
Show grid	Toggle	Checked	None	Checked/unchecked	Shows/hides grid	Disabled with hidden histogram	Display aid	Display only	Window lifetime
Show clipping info	Toggle	Checked	None	Checked/unchecked	Shows/hides endpoint-bin counts	Disabled with hidden histogram	Estimate, not exact float clipping	Display only	Window lifetime
Absolute scales	Toggle	Unchecked	None	Checked/unchecked	Normalizes each displayed channel to its own peak	Disabled with hidden histogram	Can conceal relative channel strength	Display only	Window lifetime
Draw histogram areas	Toggle	Unchecked	None	Checked/unchecked	Filled-area versus line graph	Disabled with hidden histogram	Overlap can obscure channels	Display only	Window lifetime
Show histogram	Toggle	Checked	None	Checked/unchecked	Hides/clears or reloads graph/toolbar	Idle	Saves graph work only	Display only	Window lifetime
Plot resolution	Combo, bins	8-bit (256)	None	6-bit (64) through 16-bit (64K) plus 100/1000/10000 levels	Refines graph/endpoint estimate	Disabled with hidden histogram	More graph memory/rendering; output unchanged	Display only	Window lifetime
Brightness	Integer/slider, code-value units	0	None	-150..150, step 1; offset B/255 = about -0.5882..+0.5882	Adds offset, then clamps before Contrast	Idle; Apply disabled only when both controls zero	Moves every selected scalar equally; early clipping permanently flattens ends	Preview/Apply; selected channel/component; mask post-blend; still/sequence	Captured; Reset/success 0
Contrast	Integer/slider, mapped strength	0	None	-100..100, step 1; factor 259(C+255)/(255(259-C)) ≈ 0.4385..2.2677	Scales around 0.5 after Brightness/clamp; negative compresses, positive expands	Idle	Positive clips shadows/highlights; negative flattens toward mid-gray	Preview/Apply; selected channel/component; mask post-blend; still/sequence	Captured; Reset/success 0
Channel	Combo	RGB for RGB; Grayscale and disabled for mono	Reset/success returns RGB for RGB	RGB; Red; Green; Blue; CIE L, a, b, c, h*; CIE X, Y, Z; HSL Hue/Saturation/Lightness; HSV Hue/Saturation/Value; YCbCr Luma/Blue Chroma/Red Chroma	Chooses stored planes or converts, transforms one normalized component, and reconstructs RGB	Component list only for RGB	Conversion/reconstruction cost; reconstruction saturates an integer image at its range but can leave a floating one outside 0..1; hue components are treated linearly, not circularly	Preview/Apply, selected component; still/sequence	Window lifetime until Reset/success
Target Image	Grayscale/RGB selector	Attached/active target	None	Eligible still/sequence	Refreshes source/channel/histogram/Preview	Supported view only	Selected target is replaced	Source and active mask	Target lifecycle
Preview	Checkbox	Checked	None	Checked/unchecked	Creates/destroys async fixed-linear Preview	Not locked while Apply runs	Runs Apply's own program, so it shows the values Apply writes	Preview/current displayed frame	Window lifetime
Preview pane	Viewer	Current result	None	Viewer zoom/pan/channel controls	Shows mask-aware result	Exists with Preview	Same values as Apply; inside a Preview area, Apply's result for an image of that area	Preview only	Window lifetime
Switch image	Preview action	Result shown	None	Click, Space, right-click, or Ctrl+Shift+O	Toggles result/original	Preview exists	Comparison only	Display only	Preview lifetime
Preview area	Preview checkbox	Unchecked	None	Checked/unchecked; move/resize overlay	Processes selected area and composites it over the original	Preview exists	Reduces Preview work; does not limit Apply	Preview/snapshot; mask cropped consistently	Window lifetime
Take snapshot	Preview action	None	None	Click or Ctrl+Shift+K	Creates <target> - Adjust Brightness And Contrast - SNAPSHOT	Preview exists	Allocates an auxiliary still of the Preview result; respects Preview area/mask; not given Apply's nonlinear record	Auxiliary still	Creates a view
Apply	Button/action	Disabled at B=0,C=0	None	One action when nonidentity	Captures parameters, channel, target, and mask, then runs the fused program	While it runs, target, channel, both values, Apply, Reset, and Close are disabled, with or without Preview	Destructive, clips twice; reconstruction from a component can saturate an integer image again	Still/sequence replacement; mask applies	Success records history/resets values/channel
Reset	Button/action	Enabled idle	None	One action	Sets both values 0; RGB channel returns RGB	Does not change target/Preview/histogram	No image change	Configuration only	Immediate
Close	Button/action	Enabled idle	None	One action	Closes without applying current values	Disabled while Apply runs	Closing the window another way during a run hides it at once; it closes when the run has landed, and a still Apply in place still lands. Closing the image view stops the run, which then publishes nothing	Window/execution	Values not saved
Sequence progress / Cancel	Shared progress/Cancel	Sequence Apply	None	Per-frame progress/Cancel	Writes temporary sequence; target replaced only after success	Decoded frames and selected component must be supported	Decode/encode/conversion/temp disk; the engine operation observes Cancel	Sequence Apply; same params/channel/mask	Cancel discards temp; success resets

How the algorithm works

For normalized selected value x, Brightness B, and Contrast C:

b = B / 255
k = 259(C + 255) / (255(259 - C))
u = clamp(x + b, 0, 1)
y = clamp(k(u - 0.5) + 0.5, 0, 1)

Order matters: Brightness is applied and clipped before Contrast. The contrast pivot is exactly 0.5; with no brightness, x=0.5 stays 0.5 for every C. Apply runs the four steps as one fused pointwise program of the image engine over the selected samples, with b and k computed in single precision. At C=0 the factor is exactly 1 and the contrast steps are skipped; at B=0 and C=0 nothing is done.

x is the sample in normalized units: an unsigned integer divided by its largest value, a signed integer counted from its smallest value to its largest, and a floating sample as it is. Floating samples are worked where they are, in their own precision, and a value outside 0..1 is brought into it by the first clamp. Integer samples are converted into a Float32 working copy (Float64 for 32-bit integers) and narrowed back to the source type at the nearest code, ties to even.

Preview runs the same program over the same samples, so it shows the values Apply writes.

For RGB, RGB transforms all stored planes; Red/Green/Blue transform one stored plane and preserve the others. Other selections convert to the named space, transform one component with the same scalar equation, then reconstruct RGB. The conversion reads the stored red, green, and blue as sRGB-encoded unless the image states a linear RGB color space, and hands the component over in the unit interval (CIE L*a*b* and L*C*h packed into it, XYZ divided by the white point, hues in turns) as a Float32 working plane (Float64 for a 64-bit float image). Reconstruction saturates an integer image at its range but does not clamp a floating one, which can keep values outside 0..1. CIE components are reconstructed through the exact inverse of the XYZ matrix and YCbCr through coefficients derived from the BT.601 weights.

Parameter interactions and starting points

Combination	Conservative start	Too weak	Too strong	Adjustment order
Brightness + Contrast	B=-10..10, then C=-10..10	Tone remains flat/dim	Ends clip after one or both stages	Set offset first, then separation
Positive Brightness + Contrast	Watch highlights before raising C	Highlights little changed	Brightness clips them before C	Reduce B, then C
Negative Brightness + positive Contrast	Watch background	Background remains high	Shadows collapse to zero	Reduce negative B or C
Component channel + RGB	Start with RGB	Desired component unchanged	Hue/gamut artifacts	Use purpose-built color process if possible
Mask + tonal change	Inspect mask first	Region barely changes	Visible mask boundary	Feather/reduce upstream

Inputs, outputs, and metadata

• Apply preserves geometry and pixel type in every channel mode, and the result keeps the source's FITS cards, astrometric solution, and acquisition record, converted component modes included.
• Every Apply result is recorded nonlinear on the processing-state cards a saved file carries: ANLIN nonlinear, ANBASIS the basis of a stretched image (display-rgb for color and monochrome for grayscale, unless the source's cards already recorded a basis: a recorded monochrome is kept, and a recorded linear, camera, or display RGB becomes display-rgb), ANASRC process, and ANOP brightness-contrast. A linear encoding the source claimed is no longer claimed, and the image lineage gains an entry naming an::executePointwiseProgram with both values. The record is kept in every channel mode. Preview and snapshot results are not recorded.
• A pixel that the image's own validity marks as having no data is left as it was.
• A sequence Apply processes each frame as Apply processes a still image of it and writes the frames to a temporary sequence in the source's container; the target is replaced only after every frame succeeds.
• Active mask blending happens after the local result and can restore protected source samples. Preview area affects Preview/snapshot only.
• History uses Adjust Brightness And Contrast and records both numbers, for example Adjust brightness to 40 and contrast to 25 in channel: RGB; the in channel: label (any Channel choice) is added only for a color target. It does not record the mask, clipping, or component-space reconstruction details. Undo restores the previous image.

Performance

Apply and Preview run the same fused program, linear in selected pixels/frames, on the operation's threads. An integer image is converted into a floating working copy and back; component modes convert to a working plane and reconstruct RGB; and Apply normally makes one further full-image copy to write the nonlinear record onto the result. Preview cost also includes mask blend and histogram. Sequences also require temporary output storage. No production timing or peak-memory run occurred.

Worked example

For x=0.25, Brightness +25 gives u=0.25+25/255≈0.348039. Contrast +50 gives k≈1.482222, so y≈1.482222(0.348039-0.5)+0.5≈0.274760. The same Contrast leaves u=0.5 unchanged.

At the control limits, Contrast factors are approximately 0.438528 at -100 and 2.267727 at +100. Brightness +150 adds about 0.588235, so any input above about 0.411765 clips before Contrast. These checkpoints verify the current scalar mapping, not a recommended astronomical setting.

Warnings and limitations

• Brightness clips before Contrast; later lowering Contrast cannot recover samples already clamped by Brightness.
• Integer results are rounded to the nearest code, ties to even.
• Component modes treat hue coordinates as ordinary linear values and can break wrap continuity. Reconstruction to RGB saturates an integer image at its range; a floating image can keep values outside 0..1.
• This is not exposure: the current Exposure Correction multiplies samples by its mapped scalar (and is itself not standard 2^stops). Brightness adds B/255; Contrast then pivots around 0.5.
• This is not a stretch: Histogram/GHT/ArcSinh provide deliberate nonlinear transfer shapes, black/mid/white controls, and better diagnostic feedback.
• A still Apply shows no progress window and has no Cancel button; closing the process window hides it until the Apply lands, and closing the image view stops it. A stopped or refused Apply publishes nothing and leaves the image and its history as they were.

Troubleshooting

Symptom or exact error	Verified cause/validation path	Recovery	Evidence source
Apply disabled	Both values are 0	Set B or C nonzero	AdjustBrightnessAndContrastProcess::settingsUpdatedImpl
Highlights stay flat after lowering Contrast	Brightness clipped before Contrast	Reset/start from duplicate; lower Brightness	AdjustBrightnessAndContrastProcess::executeTypedProcessIteration (an::buildBrightnessProgram, an::buildClampProgram)
Mid-gray moves unexpectedly	Brightness changes value before the 0.5 pivot	Tune Brightness first or use B=0	AdjustBrightnessAndContrastProcess::executeTypedProcessIteration (an::buildContrastProgram)
Preview and Apply differ	Target, mask/inversion, Channel, or sequence frame changed after the last Preview; a Preview area shows only Apply's result for that area	Wait for the latest Preview; compare with Preview area off	AdjustBrightnessAndContrastProcess::executeTypedProcessImpl (one body for Preview and Apply); an::ui::cropProcessRegion, an::ui::composeProcessRegion
Floating values above 1 or below 0 end at 1 or 0	The Brightness step holds its result to 0..1 before Contrast	Expected; rescale the data upstream if its range must be kept	AdjustBrightnessAndContrastProcess::executeTypedProcessIteration (an::buildClampProgram)
Color shifts in CIE/HSL/HSV/YCbCr mode	Scalar curve changed one encoded component; reconstruction saturated an integer image or left a floating color outside 0..1	Use RGB or purpose-built color process	an::ui::applyProcessChannelScope
Saved image is marked nonlinear	Every Apply records its result nonlinear	Expected; return to the pre-Apply history step for linear work	process_utils::recordNonlinearResult
Sequence unchanged after Cancel	Frames are written to a temporary sequence published only on success; the engine operation observes Cancel	Expected; rerun and let every frame finish	ImageRTPreviewControl::runOverSequence; process_utils::runNormalizedChain

Related workflow steps

Use Histogram Transformation, Generalized Hyperbolic Transformation, or ArcSinh Stretch for the primary stretch. Compare Exposure Correction for its project-specific multiplicative mapping, and Gamma Correction for a power curve. Use Adjust HSL or Adjust LCh for named color coordinates. Verify Image History and Image Statistics after Apply.

References and provenance

• AfterNight source is authoritative for the exact transfer, 0.5 pivot, parameter ranges, double clipping, normalized working units and rounding, channel conversions, metadata/state behavior, masks, and sequences.
• The worked values were calculated directly from the source equation; no third-party brightness/contrast convention is assumed.