Handle default instances. #1583

src/ImageSharp/Processing/Processors/Dithering/ErrorDither.cs

@@ -95,6 +95,11 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             where TFrameQuantizer : struct, IQuantizer<TPixel>
             where TPixel : unmanaged, IPixel<TPixel>
         {
+            if (this == default)
+            {
+                ThrowDefaultInstance();
+            }
+
             int offsetY = bounds.Top;
             int offsetX = bounds.Left;
             float scale = quantizer.Options.DitherScale;
@@ -210,5 +215,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
         /// <inheritdoc/>
         public override int GetHashCode()
             => HashCode.Combine(this.offset, this.matrix);
+
+        [MethodImpl(InliningOptions.ColdPath)]
+        private static void ThrowDefaultInstance()
+            => throw new ImageProcessingException("Cannot use the default value type instance to dither.");
     }
 }

src/ImageSharp/Processing/Processors/Dithering/OrderedDither.cs

@@ -109,6 +109,11 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             where TFrameQuantizer : struct, IQuantizer<TPixel>
             where TPixel : unmanaged, IPixel<TPixel>
         {
+            if (this == default)
+            {
+                ThrowDefaultInstance();
+            }
+
             int spread = CalculatePaletteSpread(destination.Palette.Length);
             float scale = quantizer.Options.DitherScale;
 
@@ -201,5 +206,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
         [MethodImpl(InliningOptions.ShortMethod)]
         public override int GetHashCode()
             => HashCode.Combine(this.thresholdMatrix, this.modulusX, this.modulusY);
+
+        [MethodImpl(InliningOptions.ColdPath)]
+        private static void ThrowDefaultInstance()
+            => throw new ImageProcessingException("Cannot use the default value type instance to dither.");
     }
 }

tests/ImageSharp.Tests/Processing/Processors/Quantization/QuantizerTests.cs

@@ -5,6 +5,7 @@ using System;
 
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Processing.Processors.Dithering;
 using SixLabors.ImageSharp.Processing.Processors.Quantization;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
 using Xunit;
@@ -152,6 +153,13 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Quantization
             new WuQuantizer(OrderedDitherOptions),
         };
 
+        public static readonly TheoryData<IDither> DefaultInstanceDitherers
+            = new TheoryData<IDither>
+        {
+            default(ErrorDither),
+            default(OrderedDither)
+        };
+
         private static readonly ImageComparer ValidatorComparer = ImageComparer.TolerantPercentage(0.05F);
 
         [Theory]
@@ -217,5 +225,20 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Quantization
                 testOutputDetails: testOutputDetails,
                 appendPixelTypeToFileName: false);
         }
+
+        [Theory]
+        [MemberData(nameof(DefaultInstanceDitherers))]
+        public void ShouldThrowForDefaultDitherInstance(IDither dither)
+        {
+            void Command()
+            {
+                using var image = new Image<Rgba32>(10, 10);
+                var quantizer = new WebSafePaletteQuantizer();
+                quantizer.Options.Dither = dither;
+                image.Mutate(x => x.Quantize(quantizer));
+            }
+
+            Assert.Throws<ImageProcessingException>(Command);
+        }
     }
 }