Added comments, docs and improved estimation performances with intrinsics (not tested)

src/ImageSharp/Formats/Jpeg/Components/Block8x8F.cs

@@ -830,5 +830,46 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
                 d.V7R.W = this.V7R.W;
             }
         }
+
+        /// <summary>
+        /// Compares entire 8x8 block to a single scalar value.
+        /// </summary>
+        /// <param name="value">Value to compare to.</param>
+        public bool EqualsToScalar(int value)
+        {
+#if SUPPORTS_RUNTIME_INTRINSICS
+            if (Avx2.IsSupported)
+            {
+                const int equalityMask = unchecked((int)0b1111_1111_1111_1111_1111_1111_1111_1111);
+
+                var targetVector = Vector256.Create(value);
+                ref Vector256<float> blockStride = ref this.V0;
+
+                for (int i = 0; i < RowCount; i++)
+                {
+                    Vector256<int> areEqual = Avx2.CompareEqual(Avx.ConvertToVector256Int32WithTruncation(Unsafe.Add(ref this.V0, i)), targetVector);
+                    if (Avx2.MoveMask(areEqual.AsByte()) != equalityMask)
+                    {
+                        return false;
+                    }
+                }
+
+                return true;
+            }
+#endif
+            {
+                ref float scalars = ref Unsafe.As<Block8x8F, float>(ref this);
+
+                for (int i = 0; i < Size; i++)
+                {
+                    if ((int)Unsafe.Add(ref scalars, i) != value)
+                    {
+                        return false;
+                    }
+                }
+
+                return true;
+            }
+        }
     }
 }

src/ImageSharp/Formats/Jpeg/Components/Quantization.cs

@@ -69,37 +69,44 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
         // https://github.com/ImpulseAdventure/JPEGsnoop/blob/9732ee0961f100eb69bbff4a0c47438d5997abee/source/JfifDecode.cpp#L4570-L4694
         public static void EstimateQuality(ref Block8x8F table, ReadOnlySpan<byte> target, out double quality, out double variance)
         {
-            // This method can be SIMD'ified if standard table is injected as Block8x8F
+            // This method can be SIMD'ified if standard table is injected as Block8x8F.
-            // Or when we go to full-int16 spectral code implementation and inject both tables as Block8x8
+            // Or when we go to full-int16 spectral code implementation and inject both tables as Block8x8.
             double comparePercent;
             double sumPercent = 0;
             double sumPercentSqr = 0;
 
-            bool allOnes = true;
+            // Corner case - all 1's => 100 quality
+            // It would fail to deduce using algorithm below without this check
+            if (table.EqualsToScalar(1))
+            {
+                // While this is a 100% to be 100 quality, any given table can be scaled to all 1's.
+                // According to jpeg creators, top of the line quality is 99, 100 is just a technical 'limit'.
+                quality = 100;
+                variance = 0;
+                return;
+            }
 
             for (int i = 0; i < Block8x8F.Size; i++)
             {
                 float coeff = table[i];
                 int coeffInteger = (int)coeff;
 
-                // coefficients are actually int16 casted to float numbers so there's no truncating error
+                // Coefficients are actually int16 casted to float numbers so there's no truncating error.
                 if (coeffInteger != 0)
                 {
                     comparePercent = 100.0 * (table[i] / target[i]);
                 }
                 else
                 {
+                    // No 'valid' quantization table should contain zero at any position
+                    // while this is okay to decode with, it will throw DivideByZeroException at encoding proces stage.
+                    // Not sure what to do here, we can't throw as this technically correct
+                    // but this will screw up the encoder.
                     comparePercent = 999.99;
                 }
 
                 sumPercent += comparePercent;
                 sumPercentSqr += comparePercent * comparePercent;
-
-                // Check just in case entire table are ones (Quality 100)
-                if (coeffInteger != 1)
-                {
-                    allOnes = false;
-                }
             }
 
             // Perform some statistical analysis of the quality factor
@@ -111,11 +118,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
             variance = sumPercentSqr - (sumPercent * sumPercent);
 
             // Generate the equivalent IJQ "quality" factor
-            if (allOnes)
+            if (sumPercent <= 100.0)
-            {
-                quality = 100;
-            }
-            else if (sumPercent <= 100.0)
             {
                 quality = (200 - sumPercent) / 2;
             }

src/ImageSharp/Formats/Jpeg/JpegMetadata.cs

@@ -46,8 +46,11 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         {
             this.Quality = other.Quality;
             this.ColorType = other.ColorType;
+
             this.lumaQuantizationTable = other.lumaQuantizationTable;
             this.chromaQuantizationTable = other.chromaQuantizationTable;
+            this.LumaQuality = other.LumaQuality;
+            this.ChromaQuality = other.ChromaQuality;
         }
 
         /// <summary>
@@ -64,6 +67,11 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             }
         }
 
+        /// <summary>
+        /// Gets a value indicating whether jpeg was encoded using ITU section spec K.1 quantization tables
+        /// </summary>
+        public bool ItuSpecQuantization => !this.lumaQuantizationTable.HasValue && !this.chromaQuantizationTable.HasValue;
+
         /// <summary>
         /// Gets or sets the encoded quality.
         /// </summary>