Reduce an animated input to its first frame [patch]

IconHelper.Test/ProcessImageTests.cs

@@ -376,4 +376,26 @@ public void PreservesTransparencyOfTheSourceArtwork()
 		Assert.AreEqual(0, image[15, 15].A, "The transparent notch should survive processing.");
 		Assert.AreEqual(255, image[2, 2].A, "The surrounding artwork should remain opaque.");
 	}
+
+	[TestMethod]
+	public void ReducesAnAnimatedInputToItsFirstFrame()
+	{
+		// An animated GIF, APNG or WebP decodes to one frame per animation step. Only the root frame
+		// used to get the coverage pass, so the rest were written out as an animated PNG of raw
+		// greyscale, cropped to the root frame's bounds.
+		using Image<Rgba32> image = TestImages.Blank(64, 64);
+		TestImages.FillRect(image, 20, 20, 20, 20, OpaqueWhite);
+		using (Image<Rgba32> second = TestImages.Blank(64, 64))
+		{
+			TestImages.FillRect(second, 5, 5, 55, 55, OpaqueWhite);
+			image.Frames.AddFrame(second.Frames.RootFrame);
+		}
+
+		IconHelper.ProcessImage(image, Color.FromBytes(255, 0, 0), 32, 0);
+
+		Assert.AreEqual(1, image.Frames.Count, "The output should be a single static frame.");
+		Assert.AreEqual(20, image.Width, "The bounds should come from the first frame's artwork.");
+		Assert.AreEqual(new Rgba32(255, 0, 0, 255), image[10, 10]);
+		Assert.AreEqual(new Rgba32(255, 0, 0, 255), image[0, 0]);
+	}
 }

IconHelper/IconHelper.cs

@@ -159,12 +159,21 @@ internal static BatchResult ProcessDirectory(Arguments args, Color color)
 	/// Reduces an icon to a coverage mask: every pixel carries the flat target colour and the shape
 	/// lives entirely in the alpha channel. The result is trimmed of its transparent margins, centred
 	/// on a square canvas and scaled down to at most <paramref name="size"/> pixels.
+	/// An animated input is reduced to its first frame, since the output is a static icon.
 	/// The image is mutated in place.
 	/// </summary>
 	internal static void ProcessImage(Image<Rgba32> image, Color color, int size, int padding)
 	{
 		Ensure.NotNull(image);
 
+		// An animated GIF, APNG or WebP loads with one frame per step. Mutate applies to every frame,
+		// but ProcessPixelRows below reads and writes the root frame alone, so the other frames would
+		// skip the coverage pass and be saved as an animated PNG of raw greyscale. Drop them first.
+		while (image.Frames.Count > 1)
+		{
+			image.Frames.RemoveFrame(image.Frames.Count - 1);
+		}
+
 		// The semantic Color stores linear channels as doubles. Encode to sRGB bytes once here rather
 		// than per pixel, both for speed and so the pass below stays plain byte arithmetic.
 		(byte colorR, byte colorG, byte colorB, byte _) = color.ToBytes();