fix(hide): read the preview's pixels at 8 bits a channel

src/Filee.App/Services/HideService.cs

@@ -78,7 +78,7 @@ public static Bitmap Cover(MagickImage preview, HideStyle style, double size)
     /// <summary>The picture as a bitmap the window draws.</summary>
     public static Bitmap ToBitmap(IMagickImage<byte> image)
     {
-        var pixels = image.ToByteArray(MagickFormat.Bgra);
+        var pixels = PixelsOf(image);
         var handle = GCHandle.Alloc(pixels, GCHandleType.Pinned);
         try
         {
@@ -90,4 +90,19 @@ public static Bitmap ToBitmap(IMagickImage<byte> image)
             handle.Free();
         }
     }
+
+    /// <summary>
+    /// The picture's pixels as BGRA, four bytes each, for <see cref="ToBitmap"/>: read from its pixels, 8 bits a
+    /// channel whatever depth it was stored with. Written out as "BGRA" a 1-bit or palette PNG kept its depth, an
+    /// eighth of the bytes, and the bitmap read past their end.
+    /// </summary>
+    internal static byte[] PixelsOf(IMagickImage<byte> image)
+    {
+        using var pixels = image.GetPixels();
+        var bytes = pixels.ToByteArray(PixelMapping.BGRA)
+                    ?? throw new InvalidOperationException("The picture's pixels could not be read.");
+        if (bytes.Length != (long)image.Width * image.Height * 4)
+            throw new InvalidOperationException($"Expected {image.Width * image.Height * 4} bytes of pixels, got {bytes.Length}.");
+        return bytes;
+    }
 }

tests/Filee.App.Tests/HidePreviewPixelsTests.cs

@@ -0,0 +1,39 @@
+// The Pixelate / blur window's preview of a picture stored with fewer than 8 bits a channel: a 1-bit grey or palette
+// PNG (a flat screenshot, a diagram). Its pixels come out at 8 bits a channel, four bytes each, in their real colours;
+// reading them as the picture's own depth gave an eighth of the bytes, and the window then read past their end.
+
+using Filee.App.Services;
+using ImageMagick;
+using ImageMagick.Drawing;
+
+namespace Filee.App.Tests;
+
+public class HidePreviewPixelsTests
+{
+    private static MagickImage Saved(MagickImage image)
+    {
+        var path = Path.Combine(Directory.CreateTempSubdirectory("filee-depth-").FullName, "picture.png");
+        image.Write(path, MagickFormat.Png);
+        return new MagickImage(path);
+    }
+
+    [Fact]
+    public void A_one_bit_and_a_palette_png_give_four_bytes_a_pixel_in_their_colours()
+    {
+        using var white = Saved(new MagickImage(MagickColors.White, 40, 30));
+        using var drawing = new MagickImage(MagickColors.White, 40, 30);
+        new Drawables().FillColor(new MagickColor("#3E63DD")).Rectangle(0, 0, 9, 9).Draw(drawing);
+        using var palette = Saved(drawing);
+        Assert.True(white.Depth < 8 || palette.Depth < 8, "the pictures should be stored with fewer than 8 bits");
+
+        var whitePixels = HideService.PixelsOf(white);
+        var palettePixels = HideService.PixelsOf(palette);
+
+        Assert.Equal(40 * 30 * 4, whitePixels.Length);
+        Assert.Equal(40 * 30 * 4, palettePixels.Length);
+        Assert.Equal(new byte[] { 255, 255, 255, 255 }, whitePixels[..4]);
+        // BGRA: the blue square in the top left corner, white beside it.
+        Assert.Equal(new byte[] { 0xDD, 0x63, 0x3E, 255 }, palettePixels[..4]);
+        Assert.Equal(new byte[] { 255, 255, 255, 255 }, palettePixels[(20 * 4)..(20 * 4 + 4)]);
+    }
+}