Add MipMapper tests

Author: Nominom

BCnEncTests/MipMapperTests.cs

@@ -0,0 +1,79 @@
+using System;
+using BCnEncoder.Shared;
+using BCnEncTests.Support;
+using CommunityToolkit.HighPerformance;
+using SixLabors.ImageSharp;
+using SixLabors.ImageSharp.Processing;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace BCnEncTests
+{
+	public class MipMapperTests
+	{
+		private readonly ITestOutputHelper output;
+
+		public MipMapperTests(ITestOutputHelper output) => this.output = output;
+
+		[Fact]
+		public void MipChainHasCorrectDimensions()
+		{
+			var image = ImageLoader.TestGradient1; // 512x416
+			var numMips = 0;
+			var chain = MipMapper.GenerateMipChain(image, ref numMips);
+
+			Assert.Equal(chain.Length, numMips);
+			Assert.True(numMips > 1);
+
+			for (var i = 0; i < numMips; i++)
+			{
+				Assert.Equal(Math.Max(1, image.Width  >> i), chain[i].Width);
+				Assert.Equal(Math.Max(1, image.Height >> i), chain[i].Height);
+			}
+
+			// Last level must be 1x1
+			Assert.Equal(1, chain[numMips - 1].Width);
+			Assert.Equal(1, chain[numMips - 1].Height);
+		}
+
+		/// <summary>
+		/// Compares each mip level produced by MipMapper against the equivalent
+		/// ImageSharp Box-filter resize (Compand=false so both operate in sRGB
+		/// byte space without gamma correction). The PSNR should be very high
+		/// because both algorithms perform the same 2x2 box average.
+		/// </summary>
+		[Theory]
+		[InlineData(1)]
+		[InlineData(2)]
+		[InlineData(3)]
+		[InlineData(4)]
+		public void MipLevelMatchesImageSharpBoxFilter(int mipLevel)
+		{
+			var image = ImageLoader.TestGradient1;
+			var numMips = mipLevel + 1;
+			var chain = MipMapper.GenerateMipChain(image, ref numMips);
+
+			var mipW = chain[mipLevel].Width;
+			var mipH = chain[mipLevel].Height;
+
+			// ImageSharp Box sampler + Compand=false: no gamma correction,
+			// averages pixel values in sRGB space — matches MipMapper exactly.
+			using var imageSharp = ImageLoader.LoadTestImageSharp("../../../testImages/test_gradient_1_512.jpg");
+			using var reference = imageSharp.Clone(x => x.Resize(new ResizeOptions
+			{
+				Size = new Size(mipW, mipH),
+				Sampler = KnownResamplers.Box,
+				Compand = false
+			}));
+
+			var mipColors = TestHelper.GetSinglePixelArrayAsColors(chain[mipLevel]);
+			var refColors  = TestHelper.GetSinglePixelArrayAsColors(reference);
+
+			var psnr = ImageQuality.PeakSignalToNoiseRatio(mipColors, refColors);
+			output.WriteLine($"Mip level {mipLevel} ({mipW}x{mipH}): PSNR vs ImageSharp Box = {psnr:F2} dB");
+
+			Assert.True(psnr > 40,
+				$"Mip level {mipLevel}: PSNR vs ImageSharp Box was {psnr:F2} dB (expected > 40)");
+		}
+	}
+}