Animation Editor

Animation Editor — Location & Layout

The Animation Editor is the desktop tool that lets users edit .achx animation chain files (frames, regions, shapes, onion-skinning, preview playback). It is being rewritten on top of Avalonia and lives inside this repository at:

tools/AnimationEditorAvalonia/

The legacy WinForms version (FlatRedBall.AnimationEditorForms) lives in the separate FlatRedBall (FRB1) repo at FRBDK/FlatRedBall.AnimationEditorForms/. Do not edit it for FRB2 issues — that codebase is being replaced. Issues filed in vchelaru/FlatRedBall2 always refer to the Avalonia version.

For writing tests against the editor — headless Avalonia, service wiring, the [AvaloniaFact] deadlock pitfall — see the animation-editor-testing skill. For generating headless documentation screenshots of the UI, see the animation-editor-screenshots skill. For WASM/?demo= visual proof in the browser host, see animation-editor-browser-verify.

.achx is a general-purpose format — the editor authors, runtimes interpret

.achx is not an FRB2 file. It is a general-purpose animation/atlas format consumed by several runtimes that each render it their own way: Gum (across its Skia, raylib, and sokol.net backends), MonoGame/KNI/FNA, FRB1 (custom-shader rendering), and FRB2 (SpriteBatch). The editor authors the format; each runtime decides what to do with the data. This frames every feature decision here:

• A field the editor exposes does not obligate any runtime to apply it. Store the data in the format; whether a given runtime renders it is that runtime's choice. Do not gate adding a frame field on FRB2 (or any single runtime) implementing it — e.g. per-frame Red/Green/Blue are authored and stored for game code to consume, while FRB2's SpriteBatch path never applies them itself.
• The preview is a reference rendering, not a per-runtime contract. The bottom panel renders with SkiaSharp (PreviewControl, SKCanvas/SKColorFilter in DrawFrameCore), so it will diverge from what a MonoGame/FNA/FRB1 runtime produces for the same file. That divergence is inherent to a general-purpose tool and is not a bug — pick a sensible canonical interpretation. "The preview might not match a runtime" is never a reason to withhold an authoring feature.

Negative R/G/B is allowed — Multiply clamps it, Add uses it as subtract

Red/Green/Blue are plain int? (AnimationChain.Common/AnimationFrameSave.cs) with no engine-side range check, and the FRB2 runtime never applies these fields at all (see above), so the only place a range matters is the preview. The inspector's PropRed/PropGreen/PropBlue NumericUpDowns (MainWindow.axaml) allow -255..255; PropAlpha stays 0..255 since alpha is a separate straight-opacity value, not part of a ColorOperation. In FrameColorFilter.Create (AnimationEditor.Views/FrameColorFilter.cs), Add's Skia color-matrix offset is signed, so a negative value subtracts and clamps at the final pixel for free. Multiply packs the channel into a byte for SKColor, so it explicitly Math.Clamps to 0 first — an unclamped negative int→byte cast wraps (-10 becomes 246) instead of darkening. Keep that clamp if you touch this method.

Project layout

tools/AnimationEditorAvalonia/
├── AnimationEditorAvalonia.slnx
├── docs/
│   ├── DEVELOPMENT.md            ← read first when starting work
│   └── FEATURE_COVERAGE_REPORT.md
├── src/
│   ├── AnimationEditor.App/      ← Avalonia host: MainWindow.axaml(.cs), Models/, Services/, Settings/, and App-only Controls/ (e.g. FilesPanelControl)
│   ├── AnimationEditor.Views/    ← the SkiaSharp controls (App and Browser both consume it)
│   │   └── Controls/
│   │       ├── WireframeControl.cs, TextureViewport.cs    ← top panel (texture + frame regions)
│   │       ├── PreviewControl.cs, PngPreviewControl.cs    ← bottom panel (playback) + PNG diff viewer
│   │       └── ZoomControl.axaml(.cs), IZoomTarget.cs     ← reusable zoom widget (see "Two-panel mental model")
│   ├── AnimationEditor.Core/     ← UI-independent logic (no SkiaSharp)
│   │   ├── CommandsAndState/     ← AppState, AppCommands, ApplicationEvents
│   │   ├── Data/, IO/, Rendering/, ViewModels/
│   │   └── ProjectManager.cs, SelectedState.cs
│   └── AnimationEditor.Browser/  ← WASM (BlazorGL/KNI) head
└── tests/
    ├── AnimationEditor.App.Tests/    ← headless Avalonia; covers App + Views
    └── AnimationEditor.Core.Tests/   ← pure logic

Controls that physically live in AnimationEditor.Views still use the namespace AnimationEditor.App.Controls (folder ≠ namespace) — locate them by type name, not by namespace path.

Build

dotnet build tools/AnimationEditorAvalonia/AnimationEditorAvalonia.slnx

Test commands and headless-test discipline live in the animation-editor-testing skill.

Two-panel mental model

• Wireframe (top) — the texture editor. User loads a sprite sheet, draws/edits frame regions on it. State: pan, zoom, selected frame, snap-to-grid.
• Preview (bottom) — the animation player. Plays the selected AnimationChain at runtime speed; supports onion skin and origin guides. State: pan, zoom, playback timer, speed multiplier.

All three zoom surfaces — wireframe toolbar, preview toolbar, and the PNG diff bar — mount the same reusable ZoomControl (AnimationEditor.Views/Controls/), the [−][editable %][+] widget. Wire it in code with zoomControl.Attach(target), where target is an IZoomTarget (exposes live Zoom, SetZoomPercent, ZoomChanged, WheelZoomPresets); Attach installs the wheel presets, follows ZoomChanged to display the live percent, and routes edits/steps back into the target. The suppression flag that breaks the echo loop lives inside ZoomControl — callers don't manage it.

Landmine — the zoom hosts share no base class. IZoomTarget exists only because TextureViewport (wireframe + PNG viewer) and PreviewControl are unrelated types. To share any other viewport behavior across both, extend IZoomTarget (or add a sibling interface); there is no common base to hang it on.

Scan for an existing control before adding one to a second surface; extract on the second copy. ZoomControl exists because the widget was first duplicated as raw XAML plus per-host event wiring across three toolbars. When a control and its wiring would be copied a second time, factor it into a reusable UserControl — duplicated markup and its feedback-loop plumbing drift apart otherwise. (Testing an extracted UserControl has a namescope gotcha — see animation-editor-testing.)

Rendering & performance

Both panels render through a SkiaSharp ICustomDrawOperation on Avalonia's render thread (top: WireframeControl.DrawOp.Render; bottom: PreviewControl.DrawFrameCore). lease.GrContext != null means the GPU (ANGLE) path; null means CPU (software) — the two behave differently, so always know which you're on before reasoning about cost.

A "used to be smooth, now it's slow" report is a git signal, not an architecture signal. Before theorizing about the pipeline, git log the render files — a recent commit that changed how an image is drawn is far more often the cause than a long-standing pattern suddenly biting. Chasing the architecture first wastes rounds.

Measure before guessing. An on-canvas draw-time overlay (rolling ms/frame + a GPU/CPU tag) toggles with F3 (DiagnosticsEnabled on each control, rendered by DrawTimeOverlay). Turn it on first: the ms reading plus the GPU/CPU tag localize the cost and rule out whole categories of hypothesis immediately.

Landmine — a raster SKImage re-uploads to the GPU every frame. An SKImage from SKImage.FromBitmap is CPU-resident; on the GPU path Skia re-uploads the visible source region on each draw, so cost scales inversely with zoom — zoomed out is slower, which misdirects toward mipmaps/filtering. Fix: let Skia keep the texture cached by raising the GPU resource-cache budget once per lease (GRContext.SetResourceCacheLimit), sized to hold the image. Do not hand-manage a GPU copy via SKImage.ToTextureImage held across frames — opening a menu/popup purges the GRContext, leaving that cached texture dangling so it draws nothing (blank/flicker of only the image, while vector draws in the same pass survive). Skia's own cache re-uploads correctly after a purge; a hand-held texture does not.

Cross-platform path operations — use FilePath, not System.IO.Path

Never use System.IO.Path.GetFileName, Path.GetDirectoryName, or Path.Combine on paths stored in ProjectManager.FileName or any user-supplied path. These methods are OS-native: on Linux they only recognise / as a separator, so a Windows-authored C:\foo\bar.achx path would be returned whole by Path.GetFileName.

FilePath (AnimationEditor.Core.Paths.FilePath) normalises both \ and / regardless of host OS. Use its properties instead:

Tests that exercise path logic must use Windows-style backslash literals (e.g. @"C:\projects\MyAnim.achx") to prove the cross-platform handling works — not Path.Combine, which would only exercise the current OS's separator.

Tree reorder — chains and frames; shape order is fixed

Drag-and-drop tree reorder covers chains and frames (pure resolvers ChainDropResolver / FrameDropResolver, wired in MainWindow). Do not add shape DnD reorder: collision shapes in .achx keep a fixed list order for FRB1 runtime compatibility — order is meaningful to legacy consumers, not a cosmetic tree sort. Menu/Alt+Arrow shape reorder exists in AppCommands.MoveShape today; treat new reorder UX as chain/frame-only unless an issue explicitly revisits shape ordering across runtimes.

Grid mode: double-click resizes; click/drag only repositions

Grid-mode click-to-place and handle-drag preserve the frame's existing size — only double-click resizes it to the full grid cell (GridPlacementCalculator.SnapToCell, called only from WireframeControl.SnapSelectedFrameToGridCell). This is deliberate: a fresh PNG drop creates one frame sized to the whole sheet, and double-click-to-carve- a-cell is how that gets sized down without dragging edge handles by hand. Don't collapse the two gestures onto one shared size-preserving helper again — see GridPlacementCalculator's doc comment for why that was tried and reverted.