Basement for spectral tests

src/ImageSharp/Formats/Jpeg/Components/Decoder/SpectralConverter{TPixel}.cs

@@ -32,10 +32,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
 
         private int pixelRowCounter;
 
-        public SpectralConverter(Configuration configuration, CancellationToken ct)
+        public SpectralConverter(Configuration configuration, CancellationToken cancellationToken)
         {
             this.configuration = configuration;
-            this.cancellationToken = ct;
+            this.cancellationToken = cancellationToken;
         }
 
         private bool Converted => this.pixelRowCounter >= this.pixelBuffer.Height;
@@ -90,10 +90,6 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
 
         public override void ConvertStrideBaseline()
         {
-            // Convert next pixel stride using single spectral `stride'
-            // Note that zero passing eliminates the need of virtual call from JpegComponentPostProcessor
-            this.ConvertNextStride(spectralStep: 0);
-
             // Clear spectral stride - this is VERY important as jpeg possibly won't fill entire buffer each stride
             // Which leads to decoding artifacts
             // Note that this code clears all buffers of the post processors, it's their responsibility to allocate only single stride
@@ -101,6 +97,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             {
                 cpp.ClearSpectralBuffers();
             }
+
+            // Convert next pixel stride using single spectral `stride'
+            // Note that zero passing eliminates the need of virtual call from JpegComponentPostProcessor
+            this.ConvertNextStride(spectralStep: 0);
         }
 
         public override void Dispose()

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -220,9 +220,9 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         {
             using var spectralConverter = new SpectralConverter<TPixel>(this.Configuration, cancellationToken);
 
-            this.scanDecoder = new HuffmanScanDecoder(stream, spectralConverter, cancellationToken);
+            var scanDecoder = new HuffmanScanDecoder(stream, spectralConverter, cancellationToken);
 
-            this.ParseStream(stream, cancellationToken: cancellationToken);
+            this.ParseStream(stream, scanDecoder, cancellationToken);
             this.InitExifProfile();
             this.InitIccProfile();
             this.InitIptcProfile();
@@ -234,7 +234,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// <inheritdoc/>
         public IImageInfo Identify(BufferedReadStream stream, CancellationToken cancellationToken)
         {
-            this.ParseStream(stream, true, cancellationToken);
+            this.ParseStream(stream, scanDecoder: null, cancellationToken);
             this.InitExifProfile();
             this.InitIccProfile();
             this.InitIptcProfile();
@@ -244,13 +244,17 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         }
 
         /// <summary>
-        /// Parses the input stream for file markers
+        /// Parses the input stream for file markers.
         /// </summary>
-        /// <param name="stream">The input stream</param>
+        /// <param name="stream">The input stream.</param>
-        /// <param name="metadataOnly">Whether to decode metadata only.</param>
+        /// <param name="scanDecoder">Scan decoder used exclusively to decode SOS marker.</param>
-        /// <param name="cancellationToken">The token to monitor cancellation.</param>
+        /// <param name="ct">The token to monitor cancellation.</param>
-        private void ParseStream(BufferedReadStream stream, bool metadataOnly = false, CancellationToken cancellationToken = default)
+        internal void ParseStream(BufferedReadStream stream, HuffmanScanDecoder scanDecoder, CancellationToken ct)
         {
+            bool metadataOnly = scanDecoder == null;
+
+            this.scanDecoder = scanDecoder;
+
             this.Metadata = new ImageMetadata();
 
             // Check for the Start Of Image marker.
@@ -279,7 +283,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             while (fileMarker.Marker != JpegConstants.Markers.EOI
                 || (fileMarker.Marker == JpegConstants.Markers.EOI && fileMarker.Invalid))
             {
-                cancellationToken.ThrowIfCancellationRequested();
+                ct.ThrowIfCancellationRequested();
 
                 if (!fileMarker.Invalid)
                 {
@@ -297,7 +301,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                         case JpegConstants.Markers.SOS:
                             if (!metadataOnly)
                             {
-                                this.ProcessStartOfScanMarker(stream, cancellationToken);
+                                this.ProcessStartOfScanMarker(stream, ct);
                                 break;
                             }
                             else
@@ -1030,6 +1034,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             }
 
             int selectorsCount = stream.ReadByte();
+            this.Frame.MultiScan = this.Frame.ComponentCount != selectorsCount;
             for (int i = 0; i < selectorsCount; i++)
             {
                 int componentIndex = -1;

tests/ImageSharp.Tests/Formats/Jpg/SpectralJpegTests.cs

@@ -6,6 +6,7 @@ using System.IO;
 using System.Linq;
 
 using SixLabors.ImageSharp.Formats.Jpeg;
+using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder;
 using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Tests.Formats.Jpg.Utils;
@@ -76,6 +77,10 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
 
             using var ms = new MemoryStream(sourceBytes);
             using var bufferedStream = new BufferedReadStream(Configuration.Default, ms);
+            using var spectralConverter = new SpectralConverter<TPixel>(Configuration.Default, cancellationToken: default);
+
+            var scanDecoder = new HuffmanScanDecoder(bufferedStream, spectralConverter, cancellationToken: default);
+
             using Image<Rgba32> image = decoder.Decode<Rgba32>(bufferedStream, cancellationToken: default);
 
             var imageSharpData = LibJpegTools.SpectralData.LoadFromImageSharpDecoder(decoder);
@@ -126,5 +131,31 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
 
             Assert.True(totalDifference < tolerance);
         }
+
+        private class DebugSpectralConverter<TPixel> : SpectralConverter
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            private readonly SpectralConverter<TPixel> converter;
+
+            public DebugSpectralConverter(SpectralConverter<TPixel> converter)
+            {
+                this.converter = converter;
+            }
+
+            public override void ConvertStrideBaseline()
+            {
+                this.converter.ConvertStrideBaseline();
+            }
+
+            public override void Dispose()
+            {
+                this.converter?.Dispose();
+            }
+
+            public override void InjectFrameData(JpegFrame frame, IRawJpegData jpegData)
+            {
+                this.converter.InjectFrameData(frame, jpegData);
+            }
+        }
     }
 }