Introduce palette property

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -384,8 +384,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                     }
                     else
                     {
-                        int stride = this.currentScanline.Length();
+                        quantized.GetPixelRowSpan(row).CopyTo(this.currentScanline.GetSpan());
-                        quantized.GetPixelBufferSpan().Slice(row * stride, stride).CopyTo(this.currentScanline.GetSpan());
                     }
 
                     break;

src/ImageSharp/Processing/Processors/Dithering/PaletteDitherProcessor{TPixel}.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Buffers;
 using System.Runtime.CompilerServices;
-using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing.Processors.Quantization;
 
@@ -19,7 +18,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
     {
         private readonly DitherProcessor ditherProcessor;
         private readonly IDither dither;
-        private IMemoryOwner<TPixel> paletteMemory;
+        private IMemoryOwner<TPixel> paletteOwner;
         private bool isDisposed;
 
         /// <summary>
@@ -35,12 +34,12 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             this.dither = definition.Dither;
 
             ReadOnlySpan<Color> sourcePalette = definition.Palette.Span;
-            this.paletteMemory = this.Configuration.MemoryAllocator.Allocate<TPixel>(sourcePalette.Length);
+            this.paletteOwner = this.Configuration.MemoryAllocator.Allocate<TPixel>(sourcePalette.Length);
-            Color.ToPixel(this.Configuration, sourcePalette, this.paletteMemory.Memory.Span);
+            Color.ToPixel(this.Configuration, sourcePalette, this.paletteOwner.Memory.Span);
 
             this.ditherProcessor = new DitherProcessor(
                 this.Configuration,
-                this.paletteMemory.Memory,
+                this.paletteOwner.Memory,
                 definition.DitherScale);
         }
 
@@ -59,14 +58,13 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
                 return;
             }
 
+            this.isDisposed = true;
             if (disposing)
             {
-                this.paletteMemory?.Dispose();
+                this.paletteOwner.Dispose();
             }
 
-            this.paletteMemory = null;
+            this.paletteOwner = null;
-
-            this.isDisposed = true;
             base.Dispose(disposing);
         }
 

src/ImageSharp/Processing/Processors/Quantization/FrameQuantizerExtensions.cs → src/ImageSharp/Processing/Processors/Quantization/FrameQuantizerUtilities.cs

@@ -11,10 +11,28 @@ using SixLabors.ImageSharp.Processing.Processors.Dithering;
 namespace SixLabors.ImageSharp.Processing.Processors.Quantization
 {
     /// <summary>
-    /// Contains extension methods for frame quantizers.
+    /// Contains utility methods for <see cref="IFrameQuantizer{TPixel}"/> instances.
     /// </summary>
-    public static class FrameQuantizerExtensions
+    public static class FrameQuantizerUtilities
     {
+        /// <summary>
+        /// Helper method for throwing an exception when a frame quantizer palette has
+        /// been requested but not built yet.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="palette">The frame quantizer palette.</param>
+        /// <exception cref="InvalidOperationException">
+        /// The palette has not been built via <see cref="IFrameQuantizer{TPixel}.BuildPalette(ImageFrame{TPixel}, Rectangle)"/>
+        /// </exception>
+        public static void CheckPaletteState<TPixel>(in ReadOnlyMemory<TPixel> palette)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            if (palette.Equals(default))
+            {
+                throw new InvalidOperationException("Frame Quantizer palette has not been built.");
+            }
+        }
+
         /// <summary>
         /// Quantizes an image frame and return the resulting output pixels.
         /// </summary>
@@ -37,8 +55,13 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
             var interest = Rectangle.Intersect(source.Bounds(), bounds);
 
             // Collect the palette. Required before the second pass runs.
-            ReadOnlyMemory<TPixel> palette = quantizer.BuildPalette(source, interest);
+            quantizer.BuildPalette(source, interest);
-            var destination = new IndexedImageFrame<TPixel>(quantizer.Configuration, interest.Width, interest.Height, palette);
+
+            var destination = new IndexedImageFrame<TPixel>(
+                quantizer.Configuration,
+                interest.Width,
+                interest.Height,
+                quantizer.Palette);
 
             if (quantizer.Options.Dither is null)
             {
@@ -67,7 +90,12 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
 
             if (dither is null)
             {
-                var operation = new RowIntervalOperation<TFrameQuantizer, TPixel>(ref quantizer, source, destination, bounds);
+                var operation = new RowIntervalOperation<TFrameQuantizer, TPixel>(
+                    ref quantizer,
+                    source,
+                    destination,
+                    bounds);
+
                 ParallelRowIterator.IterateRowIntervals(
                     quantizer.Configuration,
                     bounds,

src/ImageSharp/Processing/Processors/Quantization/IFrameQuantizer{TPixel}.cs

@@ -23,13 +23,20 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         /// </summary>
         QuantizerOptions Options { get; }
 
+        /// <summary>
+        /// Gets the quantized color palette.
+        /// </summary>
+        /// <exception cref="InvalidOperationException">
+        /// The palette has not been built via <see cref="BuildPalette(ImageFrame{TPixel}, Rectangle)"/>.
+        /// </exception>
+        ReadOnlyMemory<TPixel> Palette { get; }
+
         /// <summary>
         /// Builds the quantized palette from the given image frame and bounds.
         /// </summary>
         /// <param name="source">The source image frame.</param>
         /// <param name="bounds">The region of interest bounds.</param>
-        /// <returns>The <see cref="ReadOnlyMemory{TPixel}"/> palette.</returns>
+        void BuildPalette(ImageFrame<TPixel> source, Rectangle bounds);
-        ReadOnlyMemory<TPixel> BuildPalette(ImageFrame<TPixel> source, Rectangle bounds);
 
         /// <summary>
         /// Quantizes an image frame and return the resulting output pixels.

src/ImageSharp/Processing/Processors/Quantization/OctreeFrameQuantizer{TPixel}.cs

@@ -23,6 +23,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         private readonly int maxColors;
         private readonly Octree octree;
         private IMemoryOwner<TPixel> paletteOwner;
+        private ReadOnlyMemory<TPixel> palette;
         private EuclideanPixelMap<TPixel> pixelMap;
         private readonly bool isDithering;
         private bool isDisposed;
@@ -44,6 +45,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
             this.maxColors = this.Options.MaxColors;
             this.octree = new Octree(ImageMaths.GetBitsNeededForColorDepth(this.maxColors).Clamp(1, 8));
             this.paletteOwner = configuration.MemoryAllocator.Allocate<TPixel>(this.maxColors, AllocationOptions.Clean);
+            this.palette = default;
             this.pixelMap = default;
             this.isDithering = !(this.Options.Dither is null);
             this.isDisposed = false;
@@ -56,13 +58,18 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         public QuantizerOptions Options { get; }
 
         /// <inheritdoc/>
-        [MethodImpl(InliningOptions.ShortMethod)]
+        public ReadOnlyMemory<TPixel> Palette
-        public readonly IndexedImageFrame<TPixel> QuantizeFrame(ImageFrame<TPixel> source, Rectangle bounds)
+        {
-            => FrameQuantizerExtensions.QuantizeFrame(ref Unsafe.AsRef(this), source, bounds);
+            get
+            {
+                FrameQuantizerUtilities.CheckPaletteState(in this.palette);
+                return this.palette;
+            }
+        }
 
         /// <inheritdoc/>
         [MethodImpl(InliningOptions.ShortMethod)]
-        public ReadOnlyMemory<TPixel> BuildPalette(ImageFrame<TPixel> source, Rectangle bounds)
+        public void BuildPalette(ImageFrame<TPixel> source, Rectangle bounds)
         {
             using IMemoryOwner<Rgba32> buffer = this.Configuration.MemoryAllocator.Allocate<Rgba32>(bounds.Width);
             Span<Rgba32> bufferSpan = buffer.GetSpan();
@@ -90,9 +97,14 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
             ReadOnlyMemory<TPixel> result = this.paletteOwner.Memory.Slice(0, Math.Min(paletteIndex + 2, QuantizerConstants.MaxColors));
             this.pixelMap = new EuclideanPixelMap<TPixel>(this.Configuration, result);
 
-            return result;
+            this.palette = result;
         }
 
+        /// <inheritdoc/>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public readonly IndexedImageFrame<TPixel> QuantizeFrame(ImageFrame<TPixel> source, Rectangle bounds)
+            => FrameQuantizerUtilities.QuantizeFrame(ref Unsafe.AsRef(this), source, bounds);
+
         /// <inheritdoc/>
         [MethodImpl(InliningOptions.ShortMethod)]
         public readonly byte GetQuantizedColor(TPixel color, out TPixel match)

src/ImageSharp/Processing/Processors/Quantization/PaletteFrameQuantizer{TPixel}.cs

@@ -43,15 +43,19 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         /// <inheritdoc/>
         public QuantizerOptions Options { get; }
 
+        /// <inheritdoc/>
+        public ReadOnlyMemory<TPixel> Palette => this.pixelMap.Palette;
+
         /// <inheritdoc/>
         [MethodImpl(InliningOptions.ShortMethod)]
         public readonly IndexedImageFrame<TPixel> QuantizeFrame(ImageFrame<TPixel> source, Rectangle bounds)
-            => FrameQuantizerExtensions.QuantizeFrame(ref Unsafe.AsRef(this), source, bounds);
+            => FrameQuantizerUtilities.QuantizeFrame(ref Unsafe.AsRef(this), source, bounds);
 
         /// <inheritdoc/>
         [MethodImpl(InliningOptions.ShortMethod)]
-        public readonly ReadOnlyMemory<TPixel> BuildPalette(ImageFrame<TPixel> source, Rectangle bounds)
+        public void BuildPalette(ImageFrame<TPixel> source, Rectangle bounds)
-            => this.pixelMap.Palette;
+        {
+        }
 
         /// <inheritdoc/>
         [MethodImpl(InliningOptions.ShortMethod)]

src/ImageSharp/Processing/Processors/Quantization/WuFrameQuantizer{TPixel}.cs

@@ -69,6 +69,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         private IMemoryOwner<Moment> momentsOwner;
         private IMemoryOwner<byte> tagsOwner;
         private IMemoryOwner<TPixel> paletteOwner;
+        private ReadOnlyMemory<TPixel> palette;
         private int maxColors;
         private readonly Box[] colorCube;
         private EuclideanPixelMap<TPixel> pixelMap;
@@ -93,6 +94,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
             this.momentsOwner = this.memoryAllocator.Allocate<Moment>(TableLength, AllocationOptions.Clean);
             this.tagsOwner = this.memoryAllocator.Allocate<byte>(TableLength, AllocationOptions.Clean);
             this.paletteOwner = this.memoryAllocator.Allocate<TPixel>(this.maxColors, AllocationOptions.Clean);
+            this.palette = default;
             this.colorCube = new Box[this.maxColors];
             this.isDisposed = false;
             this.pixelMap = default;
@@ -106,12 +108,17 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         public QuantizerOptions Options { get; }
 
         /// <inheritdoc/>
-        [MethodImpl(InliningOptions.ShortMethod)]
+        public ReadOnlyMemory<TPixel> Palette
-        public readonly IndexedImageFrame<TPixel> QuantizeFrame(ImageFrame<TPixel> source, Rectangle bounds)
+        {
-            => FrameQuantizerExtensions.QuantizeFrame(ref Unsafe.AsRef(this), source, bounds);
+            get
+            {
+                FrameQuantizerUtilities.CheckPaletteState(in this.palette);
+                return this.palette;
+            }
+        }
 
         /// <inheritdoc/>
-        public ReadOnlyMemory<TPixel> BuildPalette(ImageFrame<TPixel> source, Rectangle bounds)
+        public void BuildPalette(ImageFrame<TPixel> source, Rectangle bounds)
         {
             this.Build3DHistogram(source, bounds);
             this.Get3DMoments(this.memoryAllocator);
@@ -134,9 +141,14 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
 
             ReadOnlyMemory<TPixel> result = this.paletteOwner.Memory.Slice(0, this.maxColors);
             this.pixelMap = new EuclideanPixelMap<TPixel>(this.Configuration, result);
-            return result;
+            this.palette = result;
         }
 
+        /// <inheritdoc/>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public readonly IndexedImageFrame<TPixel> QuantizeFrame(ImageFrame<TPixel> source, Rectangle bounds)
+            => FrameQuantizerUtilities.QuantizeFrame(ref Unsafe.AsRef(this), source, bounds);
+
         /// <inheritdoc/>
         public readonly byte GetQuantizedColor(TPixel color, out TPixel match)
         {