Crossfade slideshow effect

[meesoft]

Summary by CodeRabbit

• New Features

  • Added Crossfade effect to video transformation and effects processing options.

• Bug Fixes

  • Improved null-safety validation across bitmap operations for enhanced application stability.

• Style

  • Enhanced PictureItemView component background rendering with transparent background support.

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[coderabbitai]

Walkthrough

Updates .gitignore to ignore publish.bat, removes null-forgiving operators from accumulator array accesses across bitmap operations, adds [MemberNotNull] attribute to PrepareFrame method, adds transparent background to PictureItemView XAML, and introduces Crossfade effect support to VideoTransformCommands with related nullability improvements.

Changes

Cohort / File(s)	Summary
Configuration .gitignore	Adds publish.bat to ignore list; TestResults/ entry re-added with line handling adjustment
Bitmap Operations – Null-Safety Cleanup PhotoLocator/BitmapOperations/AverageFramesOperation.cs, FadingAverageOperation.cs, FadingMaxOperation.cs, RollingAverageOperation.cs, TimeCompressionAverageOperation.cs	Removes null-forgiving operator (!) from _accumulatorPixels array accesses; assumes non-null at point of use
Bitmap Operations – Nullability Contract PhotoLocator/BitmapOperations/CombineFramesOperationBase.cs	Adds using System.Diagnostics.CodeAnalysis directive and [MemberNotNull(nameof(_accumulatorPixels))] attribute to PrepareFrame method to declare that _accumulatorPixels is guaranteed non-null after method returns
UI Styling PhotoLocator/PictureItemView.xaml	Adds Background="Transparent" attribute to root UserControl
Video Transform – Crossfade Effect & Nullability PhotoLocator/VideoTransformCommands.cs	Introduces Crossfade constant; adds [MemberNotNullWhen(true, nameof(_localContrastSetup))] to IsLocalContrastChecked property; updates scale and local-contrast processing logic to account for Crossfade; refactors property getter/setter to use backing field checks

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Possibly related PRs

• Reverse video effect #54: Modifies VideoTransformCommands.cs to add effect identifiers (Zoom/Reverse) and adjust scale-skipping logic based on active effects, directly related to Crossfade effect handling in this PR.
• Improve dark frame handling #53: Updates nullability annotations and accumulator handling in CombineFramesOperationBase and fading operation classes, aligning with the [MemberNotNull] contract introduced here.
• Time Compression Average #64: Adds IsResultReady checks and accumulator readiness validation in CombineFramesOperationBase, complementing the nullability guarantees established by the [MemberNotNull] attribute on PrepareFrame.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The pull request title 'Crossfade slideshow effect' directly reflects a major addition to the codebase: a new Crossfade feature with supporting infrastructure changes across multiple files.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch features/Crossfade

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@PhotoLocator/VideoTransformCommands.cs`:
- Around line 692-693: The Crossfade case is inconsistently handled: ProcessArgs
skips adding the scale filter for Crossfade but StabilizeArgs still adds scale,
causing mismatched vidstab transforms; update the logic in the StabilizeArgs
construction (the code path that contains filters.Add($"scale={ScaleTo}") and
related use of IsScaleChecked, SelectedEffect.Content, ZoomEffect and Crossfade)
to mirror ProcessArgs by skipping adding the scale filter when
SelectedEffect.Content == Crossfade (i.e., only append $"scale={ScaleTo}" when
IsScaleChecked is true AND SelectedEffect.Content is neither ZoomEffect nor
Crossfade).

[coderabbitai]

⚠️ Potential issue | 🟠 Major

Keep vidstab detect/transform scale filters consistent for Crossfade.
ProcessArgs now skips scale for Crossfade, but StabilizeArgs still applies it, which can yield incorrect transforms when stabilization is enabled.

🔧 Suggested fix

-                if (IsScaleChecked && SelectedEffect.Content != ZoomEffect)
+                if (IsScaleChecked && SelectedEffect.Content != ZoomEffect && SelectedEffect.Content != Crossfade)
                     filters.Add($"scale={ScaleTo}");

🤖 Prompt for AI Agents

In `@PhotoLocator/VideoTransformCommands.cs` around lines 692 - 693, The Crossfade
case is inconsistently handled: ProcessArgs skips adding the scale filter for
Crossfade but StabilizeArgs still adds scale, causing mismatched vidstab
transforms; update the logic in the StabilizeArgs construction (the code path
that contains filters.Add($"scale={ScaleTo}") and related use of IsScaleChecked,
SelectedEffect.Content, ZoomEffect and Crossfade) to mirror ProcessArgs by
skipping adding the scale filter when SelectedEffect.Content == Crossfade (i.e.,
only append $"scale={ScaleTo}" when IsScaleChecked is true AND
SelectedEffect.Content is neither ZoomEffect nor Crossfade).