feat(avalonia): draw plates in the saved colour scheme

The Avalonia app applies the colour scheme saved as ActiveColorScheme in
the shared user-settings file when the main window opens, and recolours a
nest's drawings from it when opening the nest, as WinForms does.

The plate canvas draws the background, sheet, edge spacing, etch, part
outlines and selection from the active scheme through SkiaSchemePalette,
a cache of Skia colours rebuilt when the scheme's colours change, instead
of hard-coded colours. Part outlines follow the scheme's outline colour
or the fill darkened as WinForms darkens it. There is no Options screen
to change the scheme yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Avalonia/Controls/PlateCanvas.cs

@@ -16,18 +16,13 @@ using Vector = OpenNest.Geometry.Vector;
 namespace OpenNest.Avalonia.Controls
 {
     /// <summary>
-    /// Draws one plate with Skia: pans with a left or middle drag, zooms at the cursor with the
-    /// wheel, fits on a double click, and highlights the part under the cursor.
+    /// Draws one plate with Skia in the colours of <see cref="Scheme"/>: pans with a left or
+    /// middle drag, zooms at the cursor with the wheel, fits on a double click, and highlights the
+    /// part under the cursor.
     /// </summary>
     public sealed class PlateCanvas : Control
     {
-        private static readonly SKColor BackgroundColor = new(169, 169, 169);
-        private static readonly SKColor PlateFillColor = new(245, 245, 245);
-        private static readonly SKColor PlateOutlineColor = new(128, 128, 128);
-        private static readonly SKColor EdgeSpacingColor = new(180, 180, 180);
         private static readonly SKColor LeadColor = new(255, 0, 0);
-        private static readonly SKColor EtchColor = new(0, 150, 60);
-        private static readonly SKColor SelectedColor = new(150, 200, 255, 90 * 255 / 100);
 
         // Paths are built once per program and only read afterwards, so the render thread may draw
         // them while the UI thread builds others. Keyed by reference: a rotation replaces the program.
@@ -52,6 +47,9 @@ namespace OpenNest.Avalonia.Controls
             }
         }
 
+        /// <summary>The colours drawn with: the application's active scheme unless set.</summary>
+        public ColorScheme Scheme { get; set; } = ColorScheme.Default;
+
         public Part HoveredPart { get; private set; }
 
         public Part SelectedPart { get; private set; }
@@ -131,21 +129,23 @@ namespace OpenNest.Avalonia.Controls
             if (fitPending && Fit())
                 global::Avalonia.Threading.Dispatcher.UIThread.Post(() => ViewChanged?.Invoke(this, EventArgs.Empty));
 
+            var palette = SkiaSchemePalette.For(Scheme ?? ColorScheme.Default);
             var items = new List<PartItem>();
             if (plate != null)
             {
                 for (var i = 0; i < plate.Parts.Count; i++)
                 {
                     var part = plate.Parts[i];
+                    var color = ToSkColor(part.BaseDrawing.Color);
                     items.Add(new PartItem(PathsFor(part), part.Location, part.BoundingBox,
-                        ToSkColor(part.BaseDrawing.Color), (i + 1).ToString(),
+                        color, palette.PartOutline(color), (i + 1).ToString(),
                         ReferenceEquals(part, SelectedPart), ReferenceEquals(part, HoveredPart),
                         part.BaseDrawing.IsCutOff, part.HasManualLeadIns));
                 }
             }
 
             context.Custom(new PlateDrawOperation(new Rect(Bounds.Size), plate == null ? null : PlateRect(plate),
-                plate?.EdgeSpacing, WorldMatrix, items));
+                plate?.EdgeSpacing, WorldMatrix, items, palette));
         }
 
         private static SKColor ToSkColor(System.Drawing.Color color) =>
@@ -257,7 +257,7 @@ namespace OpenNest.Avalonia.Controls
         }
 
         private sealed record PartItem(ProgramPaths Paths, Vector Location, Box Bounds, SKColor Color,
-            string Label, bool Selected, bool Hovered, bool IsCutOff, bool ShowLeads);
+            SKColor Outline, string Label, bool Selected, bool Hovered, bool IsCutOff, bool ShowLeads);
 
         private sealed class PlateDrawOperation : ICustomDrawOperation
         {
@@ -265,10 +265,12 @@ namespace OpenNest.Avalonia.Controls
             private readonly Spacing? edgeSpacing;
             private readonly SKMatrix matrix;
             private readonly List<PartItem> items;
+            private readonly SkiaSchemePalette palette;
 
             public PlateDrawOperation(Rect bounds, Box plateRect, Spacing? edgeSpacing, SKMatrix matrix,
-                List<PartItem> items)
+                List<PartItem> items, SkiaSchemePalette palette)
             {
+                this.palette = palette;
                 Bounds = bounds;
                 this.plateRect = plateRect;
                 this.edgeSpacing = edgeSpacing;
@@ -295,7 +297,7 @@ namespace OpenNest.Avalonia.Controls
                 var canvas = api.SkCanvas;
                 canvas.Save();
                 canvas.ClipRect(new SKRect(0, 0, (float)Bounds.Width, (float)Bounds.Height));
-                canvas.Clear(BackgroundColor);
+                canvas.Clear(palette.Background);
 
                 if (plateRect != null)
                     Draw(canvas);
@@ -315,16 +317,16 @@ namespace OpenNest.Avalonia.Controls
 
                 var plate = new SKRect((float)plateRect.Left, (float)plateRect.Bottom,
                     (float)plateRect.Right, (float)plateRect.Top);
-                fill.Color = PlateFillColor;
+                fill.Color = palette.PlateFill;
                 canvas.DrawRect(plate, fill);
                 if (edgeSpacing is { } spacing)
                 {
-                    stroke.Color = EdgeSpacingColor;
+                    stroke.Color = palette.EdgeSpacing;
                     canvas.DrawRect(new SKRect(plate.Left + (float)spacing.Left,
                         plate.Top + (float)spacing.Bottom, plate.Right - (float)spacing.Right,
                         plate.Bottom - (float)spacing.Top), stroke);
                 }
-                stroke.Color = PlateOutlineColor;
+                stroke.Color = palette.PlateOutline;
                 canvas.DrawRect(plate, stroke);
 
                 var view = canvas.LocalClipBounds;
@@ -337,13 +339,14 @@ namespace OpenNest.Avalonia.Controls
                     canvas.Save();
                     canvas.Translate((float)item.Location.X, (float)item.Location.Y);
 
-                    fill.Color = item.Selected ? SelectedColor
+                    fill.Color = item.Selected ? palette.SelectedFill
                         : item.Hovered ? Lighten(item.Color) : item.Color;
                     canvas.DrawPath(item.Paths.Fill, fill);
 
-                    stroke.Color = EtchColor;
+                    stroke.Color = palette.Etch;
                     canvas.DrawPath(item.Paths.Etch, stroke);
-                    stroke.Color = item.Selected ? new SKColor(30, 90, 200) : Darken(item.Color);
+                    // Keep real cuts visible even where an etch overlaps them.
+                    stroke.Color = item.Selected ? palette.SelectedOutline : item.Outline;
                     canvas.DrawPath(item.Paths.Cut, stroke);
                     // As on the desktop, only manually placed lead-ins are shown.
                     if (item.ShowLeads)
@@ -377,9 +380,6 @@ namespace OpenNest.Avalonia.Controls
                 }
             }
 
-            private static SKColor Darken(SKColor c) =>
-                new((byte)(c.Red * 0.55), (byte)(c.Green * 0.55), (byte)(c.Blue * 0.55), 255);
-
             private static SKColor Lighten(SKColor c) =>
                 new((byte)(c.Red + (255 - c.Red) / 2), (byte)(c.Green + (255 - c.Green) / 2),
                     (byte)(c.Blue + (255 - c.Blue) / 2), c.Alpha);

OpenNest.Avalonia/MainWindow.axaml.cs

@@ -50,6 +50,8 @@ namespace OpenNest.Avalonia
             var engineWarning = EngineSelection.LoadSavedSelection();
             if (!string.IsNullOrEmpty(engineWarning))
                 notice = engineWarning;
+            // The colour scheme chosen in the WinForms app's options, from the shared user settings.
+            ColorSchemeRegistry.ApplySaved(Data.UserSettingsFile.Read(Data.UserSettingsFile.DefaultPath));
             tabs.Changed += (_, _) => ShowActiveTab();
             KeyBindings.Add(new KeyBinding
             {
@@ -99,6 +101,8 @@ namespace OpenNest.Avalonia
                     var reader = new NestReader(file);
                     var document = new Document { Nest = reader.Read() };
                     warnings = reader.Warnings;
+                    // The view follows the application's scheme, not the one the file was saved under.
+                    ColorSchemeRegistry.RecolorDrawings(document.Nest);
                     return document;
                 });
                 if (!views.ContainsKey(tab))

OpenNest.Avalonia/Rendering/SkiaSchemePalette.cs

@@ -0,0 +1,79 @@
+using System.Collections.Concurrent;
+using System.Runtime.CompilerServices;
+using SkiaSharp;
+using DrawingColor = System.Drawing.Color;
+
+namespace OpenNest.Avalonia.Rendering
+{
+    /// <summary>
+    /// A colour scheme's colours as Skia colours, for the plate canvas. Immutable, so the render
+    /// thread may read one while the UI thread changes the scheme; <see cref="For"/> returns the
+    /// cached palette until the scheme's colours change.
+    /// </summary>
+    internal sealed class SkiaSchemePalette
+    {
+        // The WinForms view's translucent selection fill, used when the scheme sets none.
+        private static readonly DrawingColor LegacySelectedColor = DrawingColor.FromArgb(90, 150, 200, 255);
+
+        private static readonly ConditionalWeakTable<ColorScheme, SkiaSchemePalette> Cache = new();
+
+        private readonly DrawingColor partOutline;
+        private readonly ConcurrentDictionary<SKColor, SKColor> outlines = new();
+
+        private SkiaSchemePalette(ColorScheme scheme)
+        {
+            partOutline = scheme.PartOutlineColor;
+            Version = scheme.Version;
+            Background = ToSk(scheme.BackgroundColor);
+            PlateFill = ToSk(scheme.LayoutFillColor);
+            PlateOutline = ToSk(scheme.LayoutOutlineColor);
+            EdgeSpacing = ToSk(scheme.EdgeSpacingColor);
+            Etch = ToSk(scheme.EtchColor);
+            var selected = scheme.SelectedPartColor.IsEmpty ? LegacySelectedColor : scheme.SelectedPartColor;
+            SelectedFill = ToSk(selected);
+            // As WinForms: the scheme's outline, else the legacy selection fill darkened.
+            SelectedOutline = ToSk(scheme.PartOutlineColor.IsEmpty
+                ? ColorShade.Dark(LegacySelectedColor)
+                : scheme.PartOutlineColor);
+        }
+
+        private int Version { get; }
+
+        public SKColor Background { get; }
+
+        public SKColor PlateFill { get; }
+
+        public SKColor PlateOutline { get; }
+
+        public SKColor EdgeSpacing { get; }
+
+        public SKColor Etch { get; }
+
+        public SKColor SelectedFill { get; }
+
+        public SKColor SelectedOutline { get; }
+
+        /// <summary>The palette for <paramref name="scheme"/>'s current colours.</summary>
+        public static SkiaSchemePalette For(ColorScheme scheme)
+        {
+            lock (Cache)
+            {
+                if (!Cache.TryGetValue(scheme, out var palette) || palette.Version != scheme.Version)
+                {
+                    palette = new SkiaSchemePalette(scheme);
+                    Cache.AddOrUpdate(scheme, palette);
+                }
+
+                return palette;
+            }
+        }
+
+        /// <summary>A part's outline for its fill: the scheme's outline colour, or the fill darkened.</summary>
+        public SKColor PartOutline(SKColor fill) =>
+            outlines.GetOrAdd(fill, f => ToSk(partOutline.IsEmpty
+                ? ColorShade.Dark(DrawingColor.FromArgb(f.Alpha, f.Red, f.Green, f.Blue))
+                : partOutline));
+
+        public static SKColor ToSk(DrawingColor color) => new(color.R, color.G, color.B, color.A);
+    }
+}

docs/nesting-view-colors.md

@@ -63,6 +63,18 @@ fill hue falls in the etch green band. Selected rendering has separate bitmap te
 Custom drawing colors are not automatically contrast-corrected. Existing scheme
 names, saved choices, custom palettes and geometry remain unchanged.
 
+## Shared code and the Avalonia app
+
+The schemes, their JSON format, the `Schemes/` folder and the recolor rule live in
+`OpenNest.Desktop` (`ColorScheme`, `ColorSchemeRegistry`, `ColorSchemeSerializer`,
+and `ColorShade`, a port of `ControlPaint.Dark` for the legacy part outline), so
+both desktop apps share them; `Schemes/*.json` in that project copies beside both
+apps. WinForms draws with per-thread GDI+ pens from `GdiSchemeTools`; the Avalonia
+plate canvas draws with Skia colours from `SkiaSchemePalette`. The Avalonia app
+applies the scheme saved as `ActiveColorScheme` in the shared user-settings file
+when it starts, and recolors a nest's drawings when it opens one; it has no
+Options screen to change the scheme yet.
+
 ## Verification
 
 On Windows, run: