Read all items from input stream

src/ImageSharp/Common/Helpers/DisposableDictionary.cs

@@ -0,0 +1,42 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Common.Helpers;
+
+/// <summary>
+/// Dictionary of <see cref="IDisposable"/> objects, which is itself <see cref="IDisposable"/>.
+/// </summary>
+/// <typeparam name="TKey">The type of the key.</typeparam>
+/// <typeparam name="TValue">Tye type of value, needs to implement <see cref="IDisposable"/>.</typeparam>
+public sealed class DisposableDictionary<TKey, TValue> : Dictionary<TKey, TValue>, IDisposable
+    where TKey : notnull
+    where TValue : IDisposable
+{
+    private bool disposedValue;
+
+    /// <inheritdoc />
+    public void Dispose()
+    {
+        // Do not change this code. Put cleanup code in 'Dispose(bool disposing)' method
+        this.Dispose(disposing: true);
+        GC.SuppressFinalize(this);
+    }
+
+    private void Dispose(bool disposing)
+    {
+        if (!this.disposedValue)
+        {
+            if (disposing)
+            {
+                foreach (KeyValuePair<TKey, TValue> pair in this)
+                {
+                    pair.Value?.Dispose();
+                }
+            }
+
+            this.Clear();
+            this.disposedValue = true;
+        }
+    }
+
+}

src/ImageSharp/Common/Helpers/DisposableList.cs

@@ -0,0 +1,40 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Common.Helpers;
+
+/// <summary>
+/// List of <see cref="IDisposable"/> objects, which is itself <see cref="IDisposable"/>.
+/// </summary>
+/// <typeparam name="TValue">Tye type of value, needs to implement <see cref="IDisposable"/>.</typeparam>
+public sealed class DisposableList<TValue> : List<TValue>, IDisposable
+    where TValue : IDisposable
+{
+    private bool disposedValue;
+
+    /// <inheritdoc />
+    public void Dispose()
+    {
+        // Do not change this code. Put cleanup code in 'Dispose(bool disposing)' method
+        this.Dispose(disposing: true);
+        GC.SuppressFinalize(this);
+    }
+
+    private void Dispose(bool disposing)
+    {
+        if (!this.disposedValue)
+        {
+            if (disposing)
+            {
+                foreach (TValue item in this)
+                {
+                    item?.Dispose();
+                }
+            }
+
+            this.Clear();
+            this.disposedValue = true;
+        }
+    }
+
+}

src/ImageSharp/Formats/Heif/Av1/Av1Decoder.cs

@@ -4,6 +4,7 @@
 using SixLabors.ImageSharp.Formats.Heif.Av1.OpenBitstreamUnit;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Pipeline;
 using SixLabors.ImageSharp.Formats.Heif.Av1.Tiling;
+using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp.Formats.Heif.Av1;
 
@@ -28,7 +29,8 @@ internal class Av1Decoder : IAv1TileReader
 
     public Av1FrameBuffer<byte>? FrameBuffer { get; private set; }
 
-    public void Decode(Span<byte> buffer)
+    public Image<TPixel> Decode<TPixel>(Span<byte> buffer)
+        where TPixel : unmanaged, IPixel<TPixel>
     {
         Av1BitStreamReader reader = new(buffer);
         this.obuReader.ReadAll(ref reader, buffer.Length, () => this, false);
@@ -40,6 +42,9 @@ internal class Av1Decoder : IAv1TileReader
         this.FrameBuffer = new(this.configuration, this.SequenceHeader, this.SequenceHeader.ColorConfig.GetColorFormat(), false);
         this.frameDecoder = new(this.SequenceHeader, this.FrameHeader, this.FrameInfo, this.FrameBuffer);
         this.frameDecoder.DecodeFrame();
+
+        // TODO: Implement returning proper pixels.
+        return new Image<TPixel>(1, 1);
     }
 
     public void ReadTile(Span<byte> tileData, int tileNum)

src/ImageSharp/Formats/Heif/Av1HeifItemDecoder.cs

@@ -0,0 +1,33 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Heif.Av1;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Heif;
+
+/// <summary>
+/// Decoder for a single <see cref="HeifItem"/> into a AVIF image.
+/// </summary>
+internal class Av1HeifItemDecoder<TPixel> : IHeifItemDecoder<TPixel>
+    where TPixel : unmanaged, IPixel<TPixel>
+{
+    /// <summary>
+    /// Gets the item type this decoder decodes, which is <see cref="Heif4CharCode.Av01"/>.
+    /// </summary>
+    public Heif4CharCode Type => Heif4CharCode.Av01;
+
+    /// <summary>
+    /// Gets the compression method this doceder uses, which is <see cref="HeifCompressionMethod.Av1"/>.
+    /// </summary>
+    public HeifCompressionMethod CompressionMethod => HeifCompressionMethod.Av1;
+
+    /// <summary>
+    /// Decode the specified item as AVIF.
+    /// </summary>
+    public Image<TPixel> DecodeItemData(Configuration configuration, HeifItem item, Span<byte> data)
+    {
+        Av1Decoder decoder = new(configuration);
+        return decoder.Decode<TPixel>(data);
+    }
+}

src/ImageSharp/Formats/Heif/HeifCompressionFactory.cs

@@ -0,0 +1,23 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Heif;
+
+/// <summary>
+/// Factory for item decoders inside the HEIF container format.
+/// </summary>
+internal class HeifCompressionFactory
+{
+    /// <summary>
+    /// Get a decoder implementation.
+    /// </summary>
+    public static IHeifItemDecoder<TPixel>? GetDecoder<TPixel>(Heif4CharCode type)
+        where TPixel : unmanaged, IPixel<TPixel> => type switch
+        {
+            Heif4CharCode.Jpeg => new JpegHeifItemDecoder<TPixel>(),
+            Heif4CharCode.Av01 => new Av1HeifItemDecoder<TPixel>(),
+            _ => null
+        };
+}

src/ImageSharp/Formats/Heif/HeifDecoderCore.cs

@@ -5,6 +5,7 @@ using System;
 using System.Buffers;
 using System.Buffers.Binary;
 using System.Text;
+using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Formats.Heif.Av1;
 using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.Memory;
@@ -573,7 +574,7 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
 
                 long extentOffset = ReadUIntVariable(boxBuffer, offsetSize, ref bytesRead);
                 long extentLength = ReadUIntVariable(boxBuffer, lengthSize, ref bytesRead);
-                HeifLocation loc = new(constructionMethod, baseOffset, dataReferenceIndex, extentOffset, extentLength, extentIndex);
+                HeifLocation loc = new(constructionMethod, baseOffset, extentOffset, extentLength);
                 item?.DataLocations.Add(loc);
             }
         }
@@ -629,41 +630,117 @@ internal sealed class HeifDecoderCore : ImageDecoderCore
         return result;
     }
 
-    private Image<TPixel> ParseMediaData<TPixel>(BufferedReadStream stream, long boxLength)
+    private Image<TPixel> ParseMediaData<TPixel>(Stream stream, long boxLength)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         EnsureBoxBoundary(boxLength, stream);
 
-        // FIXME: No specific decoding yet, so parse only a JPEG thumbnail.
-        HeifItemLink? thumbLink = this.itemLinks.FirstOrDefault(link => link.Type == Heif4CharCode.Thmb);
-        if (thumbLink == null)
+        IComparer<HeifLocation> comparer = new HeifLocationComparer(stream.Position, stream.Position);
+        SortedList<HeifLocation, HeifItem> locations = new(comparer);
+        foreach (HeifItem item in this.items)
         {
-            throw new NotImplementedException("No thumbnail found");
+            HeifLocation loc = item.DataLocations[0];
+            if (loc.Length != 0)
+            {
+                locations[loc] = item;
+            }
+        }
+
+        using DisposableDictionary<uint, IMemoryOwner<byte>> buffers = [];
+        foreach (HeifLocation loc in locations.Keys)
+        {
+            HeifItem item = locations[loc];
+            long streamPosition = loc.GetStreamPosition(stream.Position, stream.Position);
+            long dataLength = loc.Length;
+            stream.Skip((int)(streamPosition - stream.Position));
+            EnsureBoxBoundary(dataLength, stream);
+            buffers.Add(item.Id, this.ReadIntoBuffer(stream, dataLength));
         }
 
-        HeifItem? thumbItem = this.FindItemById(thumbLink.SourceId);
-        if (thumbItem == null || thumbItem.Type != Heif4CharCode.Jpeg)
+        HeifItem? rootItem = this.FindItemById(this.primaryItem);
+        if (rootItem == null)
         {
-            throw new NotImplementedException("No HVC decoding implemented yet");
+            throw new ImageFormatException("No primary HEIF item found.");
         }
 
-        long thumbPosition = thumbItem.DataLocations[0].GetStreamPosition(stream.Position, stream.Position);
-        long thumbLength = thumbItem.DataLocations[0].Length;
-        stream.Skip((int)(thumbPosition - stream.Position));
-        EnsureBoxBoundary(thumbLength, stream);
-        using IMemoryOwner<byte> thumbMemory = this.ReadIntoBuffer(stream, thumbLength);
-        Span<byte> thumbSpan = thumbMemory.GetSpan();
+        if (rootItem.Type == Heif4CharCode.Grid)
+        {
+            Image<TPixel> gridImage = this.DecodeGrid<TPixel>(rootItem, buffers);
+            if (gridImage.Width > 1 && gridImage.Height > 1)
+            {
+                return gridImage;
+            }
+
+            gridImage.Dispose();
+        }
 
+        IHeifItemDecoder<TPixel>? itemDecoder = HeifCompressionFactory.GetDecoder<TPixel>(rootItem.Type);
+        HeifItem itemToDecode = rootItem;
+        if (itemDecoder == null)
+        {
+            // FIXME: No specific decoding yet, so parse only a JPEG thumbnail.
+            HeifItemLink? thumbLink = this.itemLinks.FirstOrDefault(link => link.Type == Heif4CharCode.Thmb);
+            if (thumbLink == null)
+            {
+                throw new NotImplementedException("No thumbnail found");
+            }
+
+            HeifItem? thumbItem = this.FindItemById(thumbLink.SourceId);
+            if (thumbItem == null)
+            {
+                throw new NotImplementedException("No thumbnail defined");
+            }
+
+            itemDecoder = HeifCompressionFactory.GetDecoder<TPixel>(thumbItem.Type);
+            if (itemDecoder == null)
+            {
+                throw new NotImplementedException($"Don't know how to decode a thumbnail of type: {thumbItem.Type}.");
+            }
+
+            itemToDecode = thumbItem;
+        }
+
+        HeifMetadata meta = this.metadata.GetHeifMetadata();
+        meta.CompressionMethod = itemDecoder.CompressionMethod;
+
+        IMemoryOwner<byte> itemMemory = buffers[itemToDecode.Id];
+        return itemDecoder.DecodeItemData(this.configuration, itemToDecode, itemMemory.GetSpan());
+    }
+
+    private Image<TPixel> DecodeGrid<TPixel>(HeifItem gridItem, IDictionary<uint, IMemoryOwner<byte>> buffers)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        List<uint> linked = this.itemLinks.First(l => l.SourceId == gridItem.Id).DestinationIds;
         HeifMetadata meta = this.metadata.GetHeifMetadata();
-        meta.CompressionMethod = HeifCompressionMethod.LegacyJpeg;
+        using DisposableList<Image<TPixel>> gridTiles = [];
+        foreach (uint id in linked)
+        {
+            HeifItem? item = this.FindItemById(id);
+            if (item != null)
+            {
+                IHeifItemDecoder<TPixel>? decoder = HeifCompressionFactory.GetDecoder<TPixel>(item.Type);
+                if (decoder != null)
+                {
+                    meta.CompressionMethod = decoder.CompressionMethod;
+                    IMemoryOwner<byte> itemMemory = buffers[item.Id];
+                    gridTiles.Add(decoder.DecodeItemData(this.configuration, item, itemMemory.GetSpan()));
+                }
+            }
+        }
+
+        if (gridTiles.Count == 0)
+        {
+            return new Image<TPixel>(1, 1);
+        }
 
-        return Image.Load<TPixel>(thumbSpan);
+        // return CombineImageTiles(gridTiles);
+        return new Image<TPixel>(1, 1);
     }
 
-    private static void SkipBox(BufferedReadStream stream, long boxLength)
+    private static void SkipBox(Stream stream, long boxLength)
         => stream.Skip((int)boxLength);
 
-    private IMemoryOwner<byte> ReadIntoBuffer(BufferedReadStream stream, long length)
+    private IMemoryOwner<byte> ReadIntoBuffer(Stream stream, long length)
     {
         IMemoryOwner<byte> buffer = this.configuration.MemoryAllocator.Allocate<byte>((int)length);
         int bytesRead = stream.Read(buffer.GetSpan());

src/ImageSharp/Formats/Heif/HeifEncoderCore.cs

@@ -66,7 +66,7 @@ internal sealed class HeifEncoderCore
         where TPixel : unmanaged, IPixel<TPixel>
     {
         HeifItem primaryItem = new(Heif4CharCode.Jpeg, 1u);
-        primaryItem.DataLocations.Add(new HeifLocation(HeifLocationOffsetOrigin.ItemDataOffset, 0L, 0U, 0L, pixels.LongLength, 0U));
+        primaryItem.DataLocations.Add(new HeifLocation(HeifLocationOffsetOrigin.ItemDataOffset, 0L, 0L, pixels.LongLength));
         primaryItem.BitsPerPixel = 24;
         primaryItem.ChannelCount = 3;
         primaryItem.SetExtent(image.Size);

src/ImageSharp/Formats/Heif/HeifLocation.cs

@@ -6,23 +6,18 @@ namespace SixLabors.ImageSharp.Formats.Heif;
 /// <summary>
 /// Location within the file of an <see cref="HeifItem"/>.
 /// </summary>
-internal class HeifLocation(HeifLocationOffsetOrigin method, long baseOffset, uint dataReferenceIndex, long offset, long length, uint extentIndex)
+internal class HeifLocation(HeifLocationOffsetOrigin origin, long baseOffset, long offset, long length)
 {
     /// <summary>
     /// Gets the origin of the offsets in this location.
     /// </summary>
-    public HeifLocationOffsetOrigin Origin { get; } = method;
+    public HeifLocationOffsetOrigin Origin { get; } = origin;
 
     /// <summary>
     /// Gets the base offset of this location.
     /// </summary>
     public long BaseOffset { get; } = baseOffset;
 
-    /// <summary>
-    /// Gets the data reference index of this location.
-    /// </summary>
-    public uint DataReferenceInxdex { get; } = dataReferenceIndex;
-
     /// <summary>
     /// Gets the offset of this location.
     /// </summary>
@@ -33,11 +28,6 @@ internal class HeifLocation(HeifLocationOffsetOrigin method, long baseOffset, ui
     /// </summary>
     public long Length { get; } = length;
 
-    /// <summary>
-    /// Gets the extent index of this location.
-    /// </summary>
-    public uint ExtentInxdex { get; } = extentIndex;
-
     /// <summary>
     /// Gets the stream position of this location.
     /// </summary>
@@ -49,4 +39,21 @@ internal class HeifLocation(HeifLocationOffsetOrigin method, long baseOffset, ui
         HeifLocationOffsetOrigin.ItemDataOffset => positionOfMediaData + this.BaseOffset + this.Offset,
         _ => positionOfItem + this.BaseOffset + this.Offset
     };
+
+    public override int GetHashCode() => HashCode.Combine(this.Origin, this.Offset, this.Length, this.BaseOffset);
+
+    public override bool Equals(object? obj)
+    {
+        if (obj is not HeifLocation other)
+        {
+            return false;
+        }
+
+        if (this.Origin != other.Origin || this.Length != other.Length)
+        {
+            return false;
+        }
+
+        return this.Offset == other.Offset && this.BaseOffset == other.BaseOffset;
+    }
 }

src/ImageSharp/Formats/Heif/HeifLocationComparer.cs

@@ -0,0 +1,39 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Heif;
+
+internal class HeifLocationComparer : IComparer<HeifLocation>
+{
+    private readonly long positionOfMediaData;
+    private readonly long positionOfItem;
+
+    public HeifLocationComparer(long positionOfMediaData, long positionOfItem)
+    {
+        this.positionOfMediaData = positionOfMediaData;
+        this.positionOfItem = positionOfItem;
+    }
+
+    public int Compare(HeifLocation? x, HeifLocation? y)
+    {
+        if (x == null)
+        {
+            if (y == null)
+            {
+                return 0;
+            }
+
+            return 1;
+        }
+
+        if (y == null)
+        {
+            return -1;
+        }
+
+        long xPos = x.GetStreamPosition(this.positionOfMediaData, this.positionOfItem);
+        long yPos = y.GetStreamPosition(this.positionOfMediaData, this.positionOfItem);
+
+        return Math.Sign(xPos - yPos);
+    }
+}

src/ImageSharp/Formats/Heif/IHeifItemDecoder.cs

@@ -0,0 +1,33 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Heif;
+
+/// <summary>
+/// Decoder for a single <see cref="HeifItem"/>.
+/// </summary>
+/// <typeparam name="TPixel">The pixel type to use.</typeparam>
+internal interface IHeifItemDecoder<TPixel>
+    where TPixel : unmanaged, IPixel<TPixel>
+{
+    /// <summary>
+    /// Gets the type of item this decoder can decode.
+    /// </summary>
+    public Heif4CharCode Type { get; }
+
+    /// <summary>
+    /// Gets the <see cref="HeifCompressionMethod"/> tis decoder uses.
+    /// </summary>
+    public HeifCompressionMethod CompressionMethod { get; }
+
+    /// <summary>
+    /// Decode the specified item, given encoded data.
+    /// </summary>
+    /// <param name="configuration">The configuration to used.</param>
+    /// <param name="item">The item to decode.</param>
+    /// <param name="data">The encoded data.</param>
+    /// <returns>The decoded image.</returns>
+    public Image<TPixel> DecodeItemData(Configuration configuration, HeifItem item, Span<byte> data);
+}

src/ImageSharp/Formats/Heif/JpegHeifItemDecoder.cs

@@ -0,0 +1,32 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Heif;
+
+/// <summary>
+/// Decoder for a single <see cref="HeifItem"/> into a JPEG image.
+/// </summary>
+internal class JpegHeifItemDecoder<TPixel> : IHeifItemDecoder<TPixel>
+    where TPixel : unmanaged, IPixel<TPixel>
+{
+    /// <summary>
+    /// Gets the item type this decoder decodes, which is <see cref="Heif4CharCode.Jpeg"/>.
+    /// </summary>
+    public Heif4CharCode Type => Heif4CharCode.Jpeg;
+
+    /// <summary>
+    /// Gets the compression method this doceder uses, which is <see cref="HeifCompressionMethod.LegacyJpeg"/>.
+    /// </summary>
+    public HeifCompressionMethod CompressionMethod => HeifCompressionMethod.LegacyJpeg;
+
+    /// <summary>
+    /// Decode the specified item as JPEG.
+    /// </summary>
+    public Image<TPixel> DecodeItemData(Configuration configuration, HeifItem item, Span<byte> data)
+    {
+        Image<TPixel> image = Image.Load<TPixel>(data);
+        return image;
+    }
+}

tests/ImageSharp.Tests/Formats/Heif/HeifLocationTests.cs

@@ -0,0 +1,116 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Heif;
+
+namespace SixLabors.ImageSharp.Tests.Formats.Heif;
+
+[Trait("Format", "Heif")]
+public class HeifLocationTests
+{
+    [Fact]
+    public void CheckSameLocationFromDifferentOrigin()
+    {
+        // Arrange
+        const int dataPosition = 50;
+        const int itemPosition = 100;
+        HeifLocation fromItem = new(HeifLocationOffsetOrigin.ItemOffset, 0, 10, 42);
+        HeifLocation fromFile = new(HeifLocationOffsetOrigin.FileOffset, 0, 110, 42);
+        HeifLocation fromData = new(HeifLocationOffsetOrigin.ItemDataOffset, 0, 60, 42);
+
+        // Act
+        long itemActual = fromItem.GetStreamPosition(dataPosition, itemPosition);
+        long fileActual = fromFile.GetStreamPosition(dataPosition, itemPosition);
+        long dataActual = fromData.GetStreamPosition(dataPosition, itemPosition);
+
+        // Assert
+        Assert.Equal(110, itemActual);
+        Assert.Equal(110, fileActual);
+        Assert.Equal(110, dataActual);
+    }
+
+    [Fact]
+    public void CheckSameLocationFromDifferentOriginWithBaseOffset()
+    {
+        // Arrange
+        const int dataPosition = 50;
+        const int itemPosition = 100;
+        HeifLocation fromItem = new(HeifLocationOffsetOrigin.ItemOffset, 40, 10, 42);
+        HeifLocation fromFile = new(HeifLocationOffsetOrigin.FileOffset, 40, 110, 42);
+        HeifLocation fromData = new(HeifLocationOffsetOrigin.ItemDataOffset, 40, 60, 42);
+
+        // Act
+        long itemActual = fromItem.GetStreamPosition(dataPosition, itemPosition);
+        long fileActual = fromFile.GetStreamPosition(dataPosition, itemPosition);
+        long dataActual = fromData.GetStreamPosition(dataPosition, itemPosition);
+
+        // Assert
+        Assert.Equal(150, itemActual);
+        Assert.Equal(150, fileActual);
+        Assert.Equal(150, dataActual);
+    }
+
+    [Fact]
+    public void CheckComparerOnSameLocation()
+    {
+        // Arrange
+        const int dataPosition = 50;
+        const int itemPosition = 100;
+        HeifLocation fromItem = new(HeifLocationOffsetOrigin.ItemOffset, 0, 50, 42);
+        HeifLocation fromFile = new(HeifLocationOffsetOrigin.FileOffset, 30, 120, 42);
+        HeifLocation fromData = new(HeifLocationOffsetOrigin.ItemDataOffset, 40, 60, 42);
+        HeifLocationComparer comparer = new(dataPosition, itemPosition);
+
+        // Act
+        int item2Data = comparer.Compare(fromItem, fromData);
+        int item2File = comparer.Compare(fromItem, fromFile);
+        int file2Data = comparer.Compare(fromFile, fromData);
+        int data2File = comparer.Compare(fromData, fromFile);
+
+        // Assert
+        Assert.Equal(0, item2Data);
+        Assert.Equal(0, item2File);
+        Assert.Equal(0, file2Data);
+        Assert.Equal(0, data2File);
+    }
+
+    [Fact]
+    public void CheckComparerOnLowerLocation()
+    {
+        // Arrange
+        const int dataPosition = 50;
+        const int itemPosition = 100;
+        HeifLocation fromItem = new(HeifLocationOffsetOrigin.ItemOffset, 0, 50, 42);
+        HeifLocation fromFile = new(HeifLocationOffsetOrigin.FileOffset, 30, 150, 42);
+        HeifLocation fromData = new(HeifLocationOffsetOrigin.ItemDataOffset, 40, 80, 42);
+        HeifLocationComparer comparer = new(dataPosition, itemPosition);
+
+        // Act
+        int item2Data = comparer.Compare(fromItem, fromData);
+        int item2File = comparer.Compare(fromItem, fromFile);
+
+        // Assert
+        Assert.Equal(-1, item2Data);
+        Assert.Equal(-1, item2File);
+    }
+
+    [Fact]
+    public void CheckComparerOnHigherLocation()
+    {
+        // Arrange
+        const int dataPosition = 50;
+        const int itemPosition = 100;
+        HeifLocation fromItem = new(HeifLocationOffsetOrigin.ItemOffset, 10, 50, 42);
+        HeifLocation fromFile = new(HeifLocationOffsetOrigin.FileOffset, 30, 120, 42);
+        HeifLocation fromData = new(HeifLocationOffsetOrigin.ItemDataOffset, 40, 60, 42);
+        HeifLocationComparer comparer = new(dataPosition, itemPosition);
+
+        // Act
+        int item2Data = comparer.Compare(fromItem, fromData);
+        int item2File = comparer.Compare(fromItem, fromFile);
+
+        // Assert
+        Assert.Equal(1, item2Data);
+        Assert.Equal(1, item2File);
+    }
+}