Added SOF data precision comments, decoupled precision value from decoder core

src/ImageSharp/Formats/Jpeg/Components/Decoder/IRawJpegData.cs

@@ -26,11 +26,6 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         /// </summary>
         JpegColorSpace ColorSpace { get; }
 
-        /// <summary>
-        /// Gets the number of bits used for precision.
-        /// </summary>
-        int Precision { get; }
-
         /// <summary>
         /// Gets the components.
         /// </summary>
@@ -41,4 +36,4 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         /// </summary>
         Block8x8F[] QuantizationTables { get; }
     }
-}
+}

src/ImageSharp/Formats/Jpeg/Components/Decoder/JpegBlockPostProcessor.cs

@@ -38,11 +38,6 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         /// </summary>
         private Size subSamplingDivisors;
 
-        /// <summary>
-        /// Defines the maximum value derived from the bitdepth.
-        /// </summary>
-        private readonly int maximumValue;
-
         /// <summary>
         /// Initializes a new instance of the <see cref="JpegBlockPostProcessor"/> struct.
         /// </summary>
@@ -53,7 +48,6 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             int qtIndex = component.QuantizationTableIndex;
             this.DequantiazationTable = ZigZag.CreateDequantizationTable(ref decoder.QuantizationTables[qtIndex]);
             this.subSamplingDivisors = component.SubSamplingDivisors;
-            this.maximumValue = (int)MathF.Pow(2, decoder.Precision) - 1;
 
             this.SourceBlock = default;
             this.WorkspaceBlock1 = default;

src/ImageSharp/Formats/Jpeg/Components/Decoder/JpegComponentPostProcessor.cs

@@ -21,11 +21,18 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         /// </summary>
         private readonly Size blockAreaSize;
 
+        /// <summary>
+        /// Jpeg frame instance containing required decoding metadata.
+        /// </summary>
+        private readonly JpegFrame frame;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="JpegComponentPostProcessor"/> class.
         /// </summary>
-        public JpegComponentPostProcessor(MemoryAllocator memoryAllocator, IRawJpegData rawJpeg, Size postProcessorBufferSize, IJpegComponent component)
+        public JpegComponentPostProcessor(MemoryAllocator memoryAllocator, JpegFrame frame, IRawJpegData rawJpeg, Size postProcessorBufferSize, IJpegComponent component)
         {
+            this.frame = frame;
+
             this.Component = component;
             this.RawJpeg = rawJpeg;
             this.blockAreaSize = this.Component.SubSamplingDivisors * 8;
@@ -70,7 +77,9 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             Buffer2D<Block8x8> spectralBuffer = this.Component.SpectralBlocks;
 
             var blockPp = new JpegBlockPostProcessor(this.RawJpeg, this.Component);
-            float maximumValue = MathF.Pow(2, this.RawJpeg.Precision) - 1;
+
+            // TODO: this is a constant value for ALL components
+            float maximumValue = MathF.Pow(2, this.frame.Precision) - 1;
 
             int destAreaStride = this.ColorBuffer.Width;
 

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

@@ -78,7 +78,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             this.componentProcessors = new JpegComponentPostProcessor[frame.Components.Length];
             for (int i = 0; i < this.componentProcessors.Length; i++)
             {
-                this.componentProcessors[i] = new JpegComponentPostProcessor(allocator, jpegData, postProcessorBufferSize, frame.Components[i]);
+                this.componentProcessors[i] = new JpegComponentPostProcessor(allocator, frame, jpegData, postProcessorBufferSize, frame.Components[i]);
             }
 
             // single 'stride' rgba32 buffer for conversion between spectral and TPixel

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -30,7 +30,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// <summary>
         /// The only supported precision
         /// </summary>
-        private readonly int[] supportedPrecisions = { 8, 12 };
+        private readonly byte[] supportedPrecisions = { 8, 12 };
 
         /// <summary>
         /// The buffer used to temporarily store bytes read from the stream.
@@ -148,9 +148,6 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// <inheritdoc/>
         public JpegColorSpace ColorSpace { get; private set; }
 
-        /// <inheritdoc/>
-        public int Precision { get; private set; }
-
         /// <summary>
         /// Gets the components.
         /// </summary>
@@ -825,23 +822,24 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                 JpegThrowHelper.ThrowInvalidImageContentException("Multiple SOF markers. Only single frame jpegs supported.");
             }
 
-            // Read initial marker definitions.
+            // Read initial marker definitions
             const int length = 6;
             stream.Read(this.temp, 0, length);
 
-            // We only support 8-bit and 12-bit precision.
+            // 1 byte: Bits/sample precision
-            if (Array.IndexOf(this.supportedPrecisions, this.temp[0]) == -1)
+            byte precision = this.temp[0];
+
+            // Validity check: only 8-bit and 12-bit precisions are supported
+            if (Array.IndexOf(this.supportedPrecisions, precision) == -1)
             {
                 JpegThrowHelper.ThrowInvalidImageContentException("Only 8-Bit and 12-Bit precision supported.");
             }
 
-            this.Precision = this.temp[0];
-
             this.Frame = new JpegFrame
             {
                 Extended = frameMarker.Marker == JpegConstants.Markers.SOF1,
                 Progressive = frameMarker.Marker == JpegConstants.Markers.SOF2,
-                Precision = this.temp[0],
+                Precision = precision,
                 PixelHeight = (this.temp[1] << 8) | this.temp[2],
                 PixelWidth = (this.temp[3] << 8) | this.temp[4],
                 ComponentCount = this.temp[5]