Add AV1 codec configuration writing

HEIF_IMPLEMENTATION_PLAN.md

@@ -869,6 +869,7 @@ Encoder verification contract:
 
 ### 7. Write complete AVIF output
 
+- [~] The encoder-side AV1 codec configuration is now derived directly from the encoded sequence header and writes the fixed four-byte `av1C` record with empty `configOBUs`. The image payload retains the required sequence header, so the property introduces no sequence-header allocation, retention, or copy. Four production-header cases cover main, high, and professional profiles; 8-, 10-, and 12-bit precision; monochrome, 4:2:0, 4:2:2, and 4:4:4 sampling; exact fixed bytes; decoder reparsing; and header/property equivalence through direct net11 Release VSTest. Property-container emission and public AVIF activation remain open.
 - [ ] Write the correct AVIF file type, item information, locations, references, properties, AV1 configuration, dimensions, color, alpha, metadata, and media data.
 - [ ] Support single images, alpha auxiliary images, grids, multiple extents, and bounded image sequences in the final public scope.
 - [ ] Preserve ICC, Exif, and XMP according to encoder options.

src/ImageSharp/Formats/Heif/Av1/Av1CodecConfiguration.cs

@@ -12,6 +12,11 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1;
 /// </summary>
 internal sealed class Av1CodecConfiguration
 {
+    /// <summary>
+    /// The number of bytes in the fixed AV1 codec-configuration record.
+    /// </summary>
+    public const int FixedHeaderSize = 4;
+
     /// <summary>
     /// The optional sequence-header payload retained from the configuration open bitstream units.
     /// </summary>
@@ -40,7 +45,7 @@ internal sealed class Av1CodecConfiguration
     /// <param name="options">The general options governing metadata validation.</param>
     public Av1CodecConfiguration(Span<byte> boxBuffer, DecoderOptions options)
     {
-        if (boxBuffer.Length < 4)
+        if (boxBuffer.Length < FixedHeaderSize)
         {
             throw new InvalidImageContentException("The AV1 codec configuration is truncated.");
         }
@@ -83,7 +88,7 @@ internal sealed class Av1CodecConfiguration
 
         // The delay syntax is consumed to validate the fixed record, but it describes sample presentation and has
         // no meaning for the independently presented image item supported by this bounded container implementation.
-        ReadOnlySpan<byte> configObus = boxBuffer[4..];
+        ReadOnlySpan<byte> configObus = boxBuffer[FixedHeaderSize..];
         int sequenceHeaderCount = ScanObus(
             configObus,
             true,
@@ -114,6 +119,30 @@ internal sealed class Av1CodecConfiguration
         }
     }
 
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Av1CodecConfiguration"/> class from an encoded sequence header.
+    /// </summary>
+    /// <param name="sequenceHeader">The sequence header describing the encoded image item.</param>
+    public Av1CodecConfiguration(ObuSequenceHeader sequenceHeader)
+    {
+        ObuOperatingPoint operatingPoint = sequenceHeader.OperatingPoint[0];
+        ObuColorConfig colorConfig = sequenceHeader.ColorConfig;
+
+        this.SequenceProfile = (byte)sequenceHeader.SequenceProfile;
+        this.SequenceLevelIndex = (byte)operatingPoint.SequenceLevelIndex;
+        this.SequenceTier = operatingPoint.SequenceTier != 0;
+        this.HighBitDepth = colorConfig.BitDepth is Av1BitDepth.TenBit or Av1BitDepth.TwelveBit;
+        this.TwelveBit = colorConfig.BitDepth == Av1BitDepth.TwelveBit;
+        this.IsMonochrome = colorConfig.IsMonochrome;
+        this.ChromaSubsamplingX = colorConfig.SubSamplingX;
+        this.ChromaSubsamplingY = colorConfig.SubSamplingY;
+        this.ChromaSamplePosition = (byte)colorConfig.ChromaSamplePosition;
+        this.configSequenceHeader = [];
+        this.configSequenceHeaderExtension = -1;
+        this.configContentLightLevel = null;
+        this.configMasteringDisplayColorVolume = null;
+    }
+
     /// <summary>
     /// Gets the sequence profile declared for the coded image.
     /// </summary>
@@ -164,6 +193,27 @@ internal sealed class Av1CodecConfiguration
     /// </summary>
     public byte ChromaSamplePosition { get; }
 
+    /// <summary>
+    /// Writes the fixed AV1 codec-configuration record without optional configuration OBUs.
+    /// </summary>
+    /// <param name="destination">The destination receiving the four-byte record.</param>
+    public void WriteFixedHeader(Span<byte> destination)
+    {
+        // The image item payload already begins with its required sequence header. Keeping configOBUs empty avoids
+        // retaining or copying the same OBU into the configuration property.
+        destination[0] = 0x81;
+        destination[1] = (byte)((this.SequenceProfile << 5) | this.SequenceLevelIndex);
+        destination[2] = (byte)(
+            (this.SequenceTier ? 1 << 7 : 0)
+            | (this.HighBitDepth ? 1 << 6 : 0)
+            | (this.TwelveBit ? 1 << 5 : 0)
+            | (this.IsMonochrome ? 1 << 4 : 0)
+            | (this.ChromaSubsamplingX ? 1 << 3 : 0)
+            | (this.ChromaSubsamplingY ? 1 << 2 : 0)
+            | this.ChromaSamplePosition);
+        destination[3] = 0;
+    }
+
     /// <summary>
     /// Validates the AV1 image item OBU layout and metadata against its item properties and configuration record.
     /// </summary>

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1EncoderFrameTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Heif.Av1;
 using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
@@ -251,6 +252,38 @@ public class Av1EncoderFrameTests
         Assert.Equal(allocation.AllocationId, returned.AllocationId);
     }
 
+    [Theory]
+    [InlineData(EightBit, Yuv400, 0x1F, 0x1C)]
+    [InlineData(TenBit, Yuv420, 0x1F, 0x4C)]
+    [InlineData(TenBit, Yuv444, 0x3F, 0x40)]
+    [InlineData(TwelveBit, Yuv422, 0x5F, 0x68)]
+    public void CodecConfigurationWritesFixedHeaderFromEncodedSequenceHeader(
+        int bitDepthValue,
+        int colorFormatValue,
+        byte expectedProfileAndLevel,
+        byte expectedColorFlags)
+    {
+        Av1BitDepth bitDepth = (Av1BitDepth)bitDepthValue;
+        Av1ColorFormat colorFormat = (Av1ColorFormat)colorFormatValue;
+        using Image<Rgba32> source = new(8, 8);
+        using MemoryStream stream = new();
+        ObuSequenceHeader sequenceHeader = Av1FrameEncoder.Encode(
+            Configuration.Default,
+            source.Frames.RootFrame,
+            stream,
+            CreateColorConfig(bitDepth, colorFormat),
+            qIndex: 37);
+
+        Av1CodecConfiguration configuration = new(sequenceHeader);
+        byte[] fixedHeader = new byte[Av1CodecConfiguration.FixedHeaderSize];
+        configuration.WriteFixedHeader(fixedHeader);
+
+        Assert.Equal([0x81, expectedProfileAndLevel, expectedColorFlags, 0x00], fixedHeader);
+        Av1CodecConfiguration parsed = new(fixedHeader, new DecoderOptions());
+        Assert.True(configuration.HasMatchingImageConfiguration(parsed));
+        parsed.Validate(sequenceHeader);
+    }
+
     [Fact]
     public void ScreenContentDetectorMatchesLibaomFeatureThresholds()
     {