Этап 4

Minint.Core/Services/Impl/Compositor.cs

@@ -0,0 +1,73 @@
+using Minint.Core.Models;
+
+namespace Minint.Core.Services.Impl;
+
+public sealed class Compositor : ICompositor
+{
+    /// <inheritdoc />
+    public uint[] Composite(MinintDocument document, int width, int height)
+    {
+        int pixelCount = width * height;
+        var result = new uint[pixelCount]; // starts as 0x00000000 (transparent black)
+
+        var palette = document.Palette;
+
+        foreach (var layer in document.Layers)
+        {
+            if (!layer.IsVisible)
+                continue;
+
+            byte layerOpacity = layer.Opacity;
+            if (layerOpacity == 0)
+                continue;
+
+            var pixels = layer.Pixels;
+            for (int i = 0; i < pixelCount; i++)
+            {
+                int idx = pixels[i];
+                if (idx == 0)
+                    continue; // transparent — skip
+
+                var src = palette[idx];
+
+                // Effective source alpha = palette alpha * layer opacity / 255
+                int srcA = src.A * layerOpacity / 255;
+                if (srcA == 0)
+                    continue;
+
+                if (srcA == 255)
+                {
+                    // Fully opaque — fast path, no blending needed
+                    result[i] = PackArgb(src.R, src.G, src.B, 255);
+                    continue;
+                }
+
+                // Standard "over" alpha compositing
+                uint dst = result[i];
+                int dstA = (int)(dst >> 24);
+                int dstR = (int)((dst >> 16) & 0xFF);
+                int dstG = (int)((dst >> 8) & 0xFF);
+                int dstB = (int)(dst & 0xFF);
+
+                int outA = srcA + dstA * (255 - srcA) / 255;
+                if (outA == 0)
+                    continue;
+
+                int outR = (src.R * srcA + dstR * dstA * (255 - srcA) / 255) / outA;
+                int outG = (src.G * srcA + dstG * dstA * (255 - srcA) / 255) / outA;
+                int outB = (src.B * srcA + dstB * dstA * (255 - srcA) / 255) / outA;
+
+                result[i] = PackArgb(
+                    (byte)Math.Min(outR, 255),
+                    (byte)Math.Min(outG, 255),
+                    (byte)Math.Min(outB, 255),
+                    (byte)Math.Min(outA, 255));
+            }
+        }
+
+        return result;
+    }
+
+    private static uint PackArgb(byte r, byte g, byte b, byte a) =>
+        (uint)(b | (g << 8) | (r << 16) | (a << 24));
+}