(WIP) quality

src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs

@@ -102,18 +102,9 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             // Compute number of components based on color type in options.
             int componentCount = (this.colorType == JpegColorType.Luminance) ? 1 : 3;
 
-            // Initialize the quantization tables.
             // TODO: Right now encoder writes both quantization tables for grayscale images - we shouldn't do that
-            int lumaQuality = Numerics.Clamp(this.luminanceQuality ?? jpegMetadata.LuminanceQuality, 1, 100);
-            Block8x8F luminanceQuantTable = Quantization.ScaleLuminanceTable(lumaQuality);
-            Block8x8F chrominanceQuantTable = default;
-            if (componentCount > 1)
-            {
-                int chromaQuality = Numerics.Clamp(this.chrominanceQuality ?? jpegMetadata.ChrominanceQuality, 1, 100);
-                this.subsample ??= chromaQuality >= 91 ? JpegSubsample.Ratio444 : JpegSubsample.Ratio420;
-
-                chrominanceQuantTable = Quantization.ScaleChrominanceTable(chromaQuality);
-            }
+            // Initialize the quantization tables.
+            this.InitQuantizationTables(componentCount, jpegMetadata, out Block8x8F luminanceQuantTable, out Block8x8F chrominanceQuantTable);
 
             // Write the Start Of Image marker.
             this.WriteApplicationHeader(metadata);
@@ -651,5 +642,39 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             this.buffer[3] = (byte)(length & 0xff);
             this.outputStream.Write(this.buffer, 0, 4);
         }
+
+        /// <summary>
+        /// Initializes quntization tables.
+        /// </summary>
+        /// <param name="componentCount">Color components count.</param>
+        /// <param name="metadata">Jpeg metadata instance.</param>
+        /// <param name="luminanceQuantTable">Output luminance quantization table.</param>
+        /// <param name="chrominanceQuantTable">Output chrominance quantization table.</param>
+        private void InitQuantizationTables(int componentCount, JpegMetadata metadata, out Block8x8F luminanceQuantTable, out Block8x8F chrominanceQuantTable)
+        {
+            // We take quality values in a hierarchical order:
+            // 1. Check if encoder has set quality
+            // 2. Check if metadata has special table for encoding
+            // 3. Check if metadata has set quality
+            // 4. Take default quality value - 75
+            int lumaQuality = Numerics.Clamp(
+                this.luminanceQuality ?? metadata.LuminanceQuality ?? DefaultQualityValue,
+                min: 1,
+                max: 100);
+
+            luminanceQuantTable = Quantization.ScaleLuminanceTable(lumaQuality);
+            chrominanceQuantTable = default;
+            if (componentCount > 1)
+            {
+                int chromaQuality = Numerics.Clamp(
+                    this.chrominanceQuality ?? metadata.ChrominanceQuality ?? DefaultQualityValue,
+                    min: 1,
+                    max: 100);
+
+                this.subsample ??= chromaQuality >= 91 ? JpegSubsample.Ratio444 : JpegSubsample.Ratio420;
+
+                chrominanceQuantTable = Quantization.ScaleChrominanceTable(chromaQuality);
+            }
+        }
     }
 }

src/ImageSharp/Formats/Jpeg/JpegMetadata.cs

@@ -24,7 +24,6 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// <param name="other">The metadata to create an instance from.</param>
         private JpegMetadata(JpegMetadata other)
         {
-            this.Quality = other.Quality;
             this.ColorType = other.ColorType;
 
             this.LumaQuantizationTable = other.LumaQuantizationTable;
@@ -56,7 +55,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// This value might not be accurate if it was calculated during jpeg decoding
         /// with non-complient ITU quantization tables.
         /// </remarks>
-        public int LuminanceQuality { get; set; }
+        public int? LuminanceQuality { get; set; }
 
         /// <summary>
         /// Gets or sets the jpeg chrominance quality.
@@ -65,7 +64,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// This value might not be accurate if it was calculated during jpeg decoding
         /// with non-complient ITU quantization tables.
         /// </remarks>
-        public int ChrominanceQuality { get; set; }
+        public int? ChrominanceQuality { get; set; }
 
         /// <summary>
         /// Gets a value indicating whether jpeg luminance data was encoded using ITU complient quantization table.
@@ -82,7 +81,16 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// </summary>
         public int Quality
         {
-            get => (int)Math.Round((this.LuminanceQuality + this.ChrominanceQuality) / 2f);
+            [Obsolete("This accessor will soon be deprecated. Use LuminanceQuality and ChrominanceQuality getters instead.", error: false)]
+            get
+            {
+                const int defaultQuality = 75;
+
+                int lumaQuality = this.LuminanceQuality ?? defaultQuality;
+                int chromaQuality = this.LuminanceQuality ?? lumaQuality;
+                return (int)Math.Round((lumaQuality + chromaQuality) / 2f);
+            }
+
             set
             {
                 this.LuminanceQuality = value;