3.3 IImagePipeline + ImagePipeline

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

IMPLEMENTATION.md

@@ -103,7 +103,7 @@ Each type lives in `src/FrameProcessor/Domain/`. Tests in `tests/FrameProcessor.
 ### [x] 3.2 `DitheringRegistry`
 - `IReadOnlyDictionary<DitherAlgorithm, IDither>` mapping each enum to the corresponding ImageSharp `KnownDitherings.*`.
 
-### [ ] 3.3 `IImagePipeline` + `ImagePipeline`
+### [x] 3.3 `IImagePipeline` + `ImagePipeline`
 - Single method: `byte[] Process(Stream input, Frame frame)`.
 - Steps (`SPEC.md` §3.2): decode → resize (cover/center-crop to oriented resolution) → dither against display palette → remap pixels display→device → rotate 90° CW if portrait → encode indexed PNG (`PngColorType.Palette`, smallest bit depth fitting the palette, `PaletteQuantizer(deviceColors)`).
 - Output dimensions always match the frame's *declared* `width × height` regardless of orientation.

src/FrameProcessor/ImagePipeline/IImagePipeline.cs

@@ -0,0 +1,13 @@
+using FrameProcessor.Domain;
+
+namespace FrameProcessor.ImagePipeline;
+
+/// <summary>
+/// Converts an arbitrary input image into the indexed-PNG bytes a frame's firmware
+/// expects: resized to the frame's resolution, dithered against the frame's display
+/// palette, then remapped to the device-color palette.
+/// </summary>
+public interface IImagePipeline
+{
+    byte[] Process(Stream input, Frame frame);
+}

src/FrameProcessor/ImagePipeline/ImagePipeline.cs

@@ -0,0 +1,83 @@
+using FrameProcessor.Domain;
+using SixLabors.ImageSharp;
+using SixLabors.ImageSharp.Formats.Png;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Processing.Processors.Quantization;
+
+namespace FrameProcessor.ImagePipeline;
+
+public sealed class ImagePipeline : IImagePipeline
+{
+    public byte[] Process(Stream input, Frame frame)
+    {
+        ArgumentNullException.ThrowIfNull(input);
+        ArgumentNullException.ThrowIfNull(frame);
+
+        var dither = DitheringRegistry.All[frame.Dithering];
+        var displayPalette = PaletteFactory.BuildDisplay(frame.Palette);
+        var devicePalette = PaletteFactory.BuildDevice(frame.Palette);
+        var oriented = frame.Resolution.ForOrientation(frame.Orientation);
+
+        using var image = Image.Load<Rgba32>(input);
+
+        image.Mutate(ctx => ctx
+            .Resize(new ResizeOptions
+            {
+                Size = new Size(oriented.Width, oriented.Height),
+                Mode = ResizeMode.Crop,
+                Position = AnchorPositionMode.Center,
+            })
+            .Dither(dither, displayPalette));
+
+        RemapDisplayToDevice(image, frame.Palette);
+
+        if (frame.Orientation == Orientation.Portrait)
+        {
+            image.Mutate(ctx => ctx.Rotate(RotateMode.Rotate90));
+        }
+
+        var encoder = new PngEncoder
+        {
+            ColorType = PngColorType.Palette,
+            BitDepth = SmallestBitDepthFor(frame.Palette.Count),
+            Quantizer = new PaletteQuantizer(devicePalette),
+        };
+
+        using var output = new MemoryStream();
+        image.SaveAsPng(output, encoder);
+        return output.ToArray();
+    }
+
+    private static void RemapDisplayToDevice(Image<Rgba32> image, IReadOnlyList<PaletteEntry> palette)
+    {
+        var map = new Dictionary<Rgba32, Rgba32>(palette.Count);
+        foreach (var entry in palette)
+        {
+            map[entry.DisplayColor.ToPixel<Rgba32>()] = entry.DeviceColor.ToPixel<Rgba32>();
+        }
+
+        image.ProcessPixelRows(accessor =>
+        {
+            for (var y = 0; y < accessor.Height; y++)
+            {
+                var row = accessor.GetRowSpan(y);
+                for (var x = 0; x < row.Length; x++)
+                {
+                    if (map.TryGetValue(row[x], out var mapped))
+                    {
+                        row[x] = mapped;
+                    }
+                }
+            }
+        });
+    }
+
+    private static PngBitDepth SmallestBitDepthFor(int paletteCount) => paletteCount switch
+    {
+        <= 2 => PngBitDepth.Bit1,
+        <= 4 => PngBitDepth.Bit2,
+        <= 16 => PngBitDepth.Bit4,
+        _ => PngBitDepth.Bit8,
+    };
+}