KTX2 textures with single MipMap fail to decode

src/ImageSharp.Textures/Formats/Ktx2/Ktx2DecoderCore.cs

@@ -69,7 +69,8 @@ public Texture DecodeTexture(Stream stream)
             int width = (int)this.ktxHeader.PixelWidth;
             int height = (int)this.ktxHeader.PixelHeight;
 
-            var levelIndices = new LevelIndex[this.ktxHeader.LevelCount];
+            // Zero levels means only the base level is present, so we must read at least 1.
+            LevelIndex[] levelIndices = new LevelIndex[Math.Max(1, this.ktxHeader.LevelCount)];
             for (int i = 0; i < levelIndices.Length; i++)
             {
                 stream.Read(this.buffer, 0, 24);

tests/ImageSharp.Textures.Tests/Formats/Ktx2/Ktx2DecoderFlatTests.cs

@@ -0,0 +1,32 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.ComponentModel;
+using SixLabors.ImageSharp.Textures.Formats.Ktx2;
+using SixLabors.ImageSharp.Textures.Tests.Enums;
+using SixLabors.ImageSharp.Textures.Tests.TestUtilities.Attributes;
+using SixLabors.ImageSharp.Textures.Tests.TestUtilities.TextureProviders;
+using SixLabors.ImageSharp.Textures.TextureFormats;
+
+namespace SixLabors.ImageSharp.Textures.Tests.Formats.Ktx2;
+
+[Trait("Format", "Ktx2")]
+public class Ktx2DecoderFlatTests
+{
+    private static readonly Ktx2Decoder Ktx2Decoder = new();
+
+    [Theory]
+    [Description("Ensure that a single mipmap level does not result in an empty mipmap collection")]
+    [WithFile(TestTextureFormat.Ktx2, TestTextureType.Flat, TestTextureTool.ToKtx, TestImages.Ktx2.Rgba32SrgbMips)]
+    public void Ktx2Decoder_LevelCountZero_DecodesBaseLevelMipMap(TestTextureProvider provider)
+    {
+        using Texture texture = provider.GetTexture(Ktx2Decoder);
+        provider.SaveTextures(texture);
+
+        FlatTexture flatTexture = texture as FlatTexture;
+        Assert.NotNull(flatTexture);
+        Assert.Single(flatTexture.MipMaps);
+        Assert.Equal(256, flatTexture.MipMaps[0].GetImage().Width);
+        Assert.Equal(256, flatTexture.MipMaps[0].GetImage().Height);
+    }
+}

tests/ImageSharp.Textures.Tests/TestImages.cs

@@ -12,5 +12,10 @@ public static class Ktx
         {
             public const string Rgba = "rgba8888.ktx";
         }
+
+        public static class Ktx2
+        {
+            public const string Rgba32SrgbMips = "rgba32-srgb-mips.ktx2";
+        }
     }
 }