Add Mean16x4 sse tests

src/ImageSharp/Formats/Webp/Lossy/LossyUtils.cs

@@ -15,7 +15,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
     internal static unsafe class LossyUtils
     {
 #if SUPPORTS_RUNTIME_INTRINSICS
-        private static readonly Vector128<byte> Mean16x4Mask = Vector128.Create(0x00ff).AsByte();
+        private static readonly Vector128<byte> Mean16x4Mask = Vector128.Create((short)0x00ff).AsByte();
 #endif
 
         [MethodImpl(InliningOptions.ShortMethod)]

tests/ImageSharp.Tests/Formats/WebP/LossyUtilsTests.cs

@@ -0,0 +1,49 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System.Linq;
+using SixLabors.ImageSharp.Formats.Webp.Lossy;
+using SixLabors.ImageSharp.Tests.TestUtilities;
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Formats.WebP
+{
+    [Trait("Format", "Webp")]
+    public class LossyUtilsTests
+    {
+        private static void RunMean16x4Test()
+        {
+            // arrange
+            byte[] input =
+            {
+                154, 145, 102, 115, 127, 129, 126, 125, 126, 120, 133, 152, 157, 153, 119, 94, 104, 116, 111, 113,
+                113, 109, 105, 124, 173, 175, 177, 170, 175, 172, 166, 164, 151, 141, 99, 114, 125, 126, 135, 150,
+                133, 115, 127, 149, 141, 168, 100, 54, 110, 117, 115, 116, 119, 115, 117, 130, 174, 174, 174, 157,
+                146, 171, 166, 158, 117, 140, 96, 111, 119, 119, 136, 171, 188, 134, 121, 126, 136, 119, 59, 77,
+                109, 115, 113, 120, 120, 117, 128, 115, 174, 173, 173, 161, 152, 148, 153, 162, 105, 140, 96, 114,
+                115, 122, 141, 173, 190, 190, 142, 106, 151, 78, 66, 141, 110, 117, 123, 136, 118, 124, 127, 114,
+                173, 175, 166, 155, 155, 159, 159, 158
+            };
+            uint[] dc = new uint[4];
+            ushort[] tmp = new ushort[8];
+            uint[] expectedDc = { 1940, 2139, 2252, 1813 };
+
+            // act
+            LossyUtils.Mean16x4(input, dc, tmp);
+
+            // assert
+            Assert.True(dc.SequenceEqual(expectedDc));
+        }
+
+        [Fact]
+        public void Mean16x4_Works() => RunMean16x4Test();
+
+#if SUPPORTS_RUNTIME_INTRINSICS
+        [Fact]
+        public void Mean16x4_WithHardwareIntrinsics_Works() => FeatureTestRunner.RunWithHwIntrinsicsFeature(RunMean16x4Test, HwIntrinsics.AllowAll);
+
+        [Fact]
+        public void Mean16x4_WithoutSSE2_Works() => FeatureTestRunner.RunWithHwIntrinsicsFeature(RunMean16x4Test, HwIntrinsics.DisableSSE2);
+#endif
+    }
+}