Added comments to DQT marker parser, DQT exceptions now provide better info messages

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

@@ -32,7 +32,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
 
             if (quantizationTableIndex > 3)
             {
-                JpegThrowHelper.ThrowBadQuantizationTable();
+                JpegThrowHelper.ThrowBadQuantizationTableIndex(quantizationTableIndex);
             }
 
             this.QuantizationTableIndex = quantizationTableIndex;

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -755,26 +755,29 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         {
             while (remaining > 0)
             {
-                bool done = false;
+                // 1 byte: quantization table spec
-                remaining--;
+                // bit 0..3: table index (0..3)
+                // bit 4..7: table precision (0 = 8 bit, 1 = 16 bit)
                 int quantizationTableSpec = stream.ReadByte();
                 int tableIndex = quantizationTableSpec & 15;
+                int tablePrecision = quantizationTableSpec >> 4;
 
-                // Max index. 4 Tables max.
+                // Validate:
                 if (tableIndex > 3)
                 {
-                    JpegThrowHelper.ThrowBadQuantizationTable();
+                    JpegThrowHelper.ThrowBadQuantizationTableIndex(tableIndex);
                 }
 
-                switch (quantizationTableSpec >> 4)
+                remaining--;
+                switch (tablePrecision)
                 {
+                    // 8 bit values
                     case 0:
                     {
-                        // 8 bit values
+                        // Validate: 8 bit table needs exactly 64 bytes
                         if (remaining < 64)
                         {
-                            done = true;
+                            JpegThrowHelper.ThrowBadMarker(nameof(JpegConstants.Markers.DQT), remaining);
-                            break;
                         }
 
                         stream.Read(this.temp, 0, 64);
@@ -785,16 +788,17 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                         {
                             table[j] = this.temp[j];
                         }
+
+                        break;
                     }
 
-                    break;
+                    // 16 bit values
                     case 1:
                     {
-                        // 16 bit values
+                        // Validate: 16 bit table needs exactly 128 bytes
                         if (remaining < 128)
                         {
-                            done = true;
+                            JpegThrowHelper.ThrowBadMarker(nameof(JpegConstants.Markers.DQT), remaining);
-                            break;
                         }
 
                         stream.Read(this.temp, 0, 128);
@@ -805,26 +809,17 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                         {
                             table[j] = (this.temp[2 * j] << 8) | this.temp[(2 * j) + 1];
                         }
-                    }
 
-                    break;
+                        break;
+                    }
 
+                    // Unknown precision - error
                     default:
                     {
-                        JpegThrowHelper.ThrowBadQuantizationTable();
+                        JpegThrowHelper.ThrowBadQuantizationTablePrecision(tablePrecision);
                         break;
                     }
                 }
-
-                if (done)
-                {
-                    break;
-                }
-            }
-
-            if (remaining != 0)
-            {
-                JpegThrowHelper.ThrowBadMarker(nameof(JpegConstants.Markers.DQT), remaining);
             }
 
             this.Metadata.GetFormatMetadata(JpegFormat.Instance).Quality = QualityEvaluator.EstimateQuality(this.QuantizationTables);

src/ImageSharp/Formats/Jpeg/JpegThrowHelper.cs

@@ -36,7 +36,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         public static void ThrowBadMarker(string marker, int length) => throw new InvalidImageContentException($"Marker {marker} has bad length {length}.");
 
         [MethodImpl(InliningOptions.ColdPath)]
-        public static void ThrowBadQuantizationTable() => throw new InvalidImageContentException("Bad Quantization Table index.");
+        public static void ThrowBadQuantizationTableIndex(int index) => throw new InvalidImageContentException($"Bad Quantization Table index {index}.");
+
+        [MethodImpl(InliningOptions.ColdPath)]
+        public static void ThrowBadQuantizationTablePrecision(int precision) => throw new InvalidImageContentException($"Unknown Quantization Table precision {precision}.");
 
         [MethodImpl(InliningOptions.ColdPath)]
         public static void ThrowBadSampling() => throw new InvalidImageContentException("Bad sampling factor.");