Fix sampling offset

src/ImageSharp/Processing/Processors/Transforms/AffineProcessor.cs

@@ -29,7 +29,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
         }
 
         /// <summary>
-        /// Gets or sets a value indicating whether to expand the canvas to fit the skewed image.
+        /// Gets or sets a value indicating whether to expand the canvas to fit the transformed image.
         /// </summary>
         public bool Expand { get; set; } = true;
 
@@ -96,33 +96,58 @@ namespace SixLabors.ImageSharp.Processing.Processors
         /// <param name="source">The source image</param>
         /// <param name="maxX">The maximum x value</param>
         /// <param name="maxY">The maximum y value</param>
+        /// <param name="radius">The radius of the current sampling window</param>
         /// <param name="windowX">The horizontal weights</param>
         /// <param name="windowY">The vertical weights</param>
         /// <param name="point">The transformed position</param>
         /// <returns>The <see cref="Vector4"/></returns>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        protected Vector4 ComputeWeightedSumAtPosition(ImageFrame<TPixel> source, int maxX, int maxY, ref WeightsWindow windowX, ref WeightsWindow windowY, ref Point point)
+        protected Vector4 ComputeWeightedSumAtPosition(ImageFrame<TPixel> source, int maxX, int maxY, int radius, ref WeightsWindow windowX, ref WeightsWindow windowY, ref Point point)
         {
             ref float horizontalValues = ref windowX.GetStartReference();
             ref float verticalValues = ref windowY.GetStartReference();
-            int xLength = windowX.Length;
+
-            int yLength = windowY.Length;
+            int left = point.X - radius;
+            int right = point.X + radius;
+            int top = point.Y - radius;
+            int bottom = point.Y + radius;
+
+            // Faster than clamping + we know we are only looking in one direction
+            if (left < 0)
+            {
+                left = 0;
+            }
+
+            if (top < 0)
+            {
+                top = 0;
+            }
+
+            if (right > maxX)
+            {
+                right = maxX;
+            }
+
+            if (bottom > maxY)
+            {
+                bottom = maxY;
+            }
+
             Vector4 result = Vector4.Zero;
 
-            for (int y = 0; y < yLength; y++)
+            // We calculate our sample by iterating up-down/left-right from our transformed point.
+            // Ignoring the weight of outlying pixels is better for shape preservation on transforms such as skew with samplers that use larger radii.
+            // We don't offset our window index so that the weight compensates for the missing values
+            for (int y = top, yl = 0; y <= bottom; y++, yl++)
             {
-                float yweight = Unsafe.Add(ref verticalValues, y);
+                float yweight = Unsafe.Add(ref verticalValues, yl);
-                int offsetY = y + point.Y;
-                offsetY = offsetY.Clamp(0, maxY);
 
-                for (int x = 0; x < xLength; x++)
+                for (int x = left, xl = 0; x <= right; x++, xl++)
                 {
-                    float xweight = Unsafe.Add(ref horizontalValues, x);
+                    float xweight = Unsafe.Add(ref horizontalValues, xl);
-                    int offsetX = x + point.X;
-                    offsetX = offsetX.Clamp(0, maxX);
                     float weight = yweight * xweight;
 
-                    result += source[offsetX, offsetY].ToVector4() * weight;
+                    result += source[x, y].ToVector4() * weight;
                 }
             }
 

src/ImageSharp/Processing/Processors/Transforms/RotateProcessor.cs

@@ -111,6 +111,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
 
             int maxX = source.Width - 1;
             int maxY = source.Height - 1;
+            int radius = Math.Max((int)this.Sampler.Radius, 1);
 
             Parallel.For(
                 0,
@@ -127,7 +128,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                                 WeightsWindow windowX = this.HorizontalWeights.Weights[transformedPoint.X];
                                 WeightsWindow windowY = this.VerticalWeights.Weights[transformedPoint.Y];
 
-                                Vector4 dXY = this.ComputeWeightedSumAtPosition(source, maxX, maxY, ref windowX, ref windowY, ref transformedPoint);
+                                Vector4 dXY = this.ComputeWeightedSumAtPosition(source, maxX, maxY, radius, ref windowX, ref windowY, ref transformedPoint);
                                 ref TPixel dest = ref destRow[x];
                                 dest.PackFromVector4(dXY);
                             }

src/ImageSharp/Processing/Processors/Transforms/SkewProcessor.cs

@@ -83,6 +83,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
 
             int maxX = source.Width - 1;
             int maxY = source.Height - 1;
+            int radius = Math.Max((int)this.Sampler.Radius, 1);
 
             Parallel.For(
                 0,
@@ -99,7 +100,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                                 WeightsWindow windowX = this.HorizontalWeights.Weights[transformedPoint.X];
                                 WeightsWindow windowY = this.VerticalWeights.Weights[transformedPoint.Y];
 
-                                Vector4 dXY = this.ComputeWeightedSumAtPosition(source, maxX, maxY, ref windowX, ref windowY, ref transformedPoint);
+                                Vector4 dXY = this.ComputeWeightedSumAtPosition(source, maxX, maxY, radius, ref windowX, ref windowY, ref transformedPoint);
                                 ref TPixel dest = ref destRow[x];
                                 dest.PackFromVector4(dXY);
                             }

tests/ImageSharp.Tests/Processing/Processors/Transforms/RotateTests.cs

@@ -29,7 +29,8 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Transforms
             RotateType.Rotate270
         };
 
-        public static readonly TheoryData<string> ResmplerNames = new TheoryData<string>
+        public static readonly TheoryData<string> ResamplerNames
+            = new TheoryData<string>
         {
             nameof(KnownResamplers.Bicubic),
             nameof(KnownResamplers.Box),
@@ -62,8 +63,8 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Transforms
         }
 
         [Theory]
-        [WithTestPatternImages(nameof(ResmplerNames), 100, 50, DefaultPixelType)]
+        [WithTestPatternImages(nameof(ResamplerNames), 100, 50, DefaultPixelType)]
-        [WithTestPatternImages(nameof(ResmplerNames), 50, 100, DefaultPixelType)]
+        [WithTestPatternImages(nameof(ResamplerNames), 50, 100, DefaultPixelType)]
         public void RotateWithSampler<TPixel>(TestImageProvider<TPixel> provider, string resamplerName)
             where TPixel : struct, IPixel<TPixel>
         {
@@ -98,7 +99,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Transforms
                 throw new Exception("Invalid property name!");
             }
 
-            return (IResampler) property.GetValue(null);
+            return (IResampler)property.GetValue(null);
         }
     }
 }

tests/ImageSharp.Tests/Processing/Processors/Transforms/SkewTest.cs

@@ -6,6 +6,10 @@ using Xunit;
 
 namespace SixLabors.ImageSharp.Tests.Processing.Processors.Transforms
 {
+    using System;
+    using System.Collections.Generic;
+    using System.Reflection;
+
     using SixLabors.ImageSharp.Processing;
 
     public class SkewTest : FileTestBase
@@ -17,6 +21,26 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Transforms
             { -20, -10 }
         };
 
+        public static readonly List<string> ResamplerNames
+            = new List<string>
+        {
+            nameof(KnownResamplers.Bicubic),
+            nameof(KnownResamplers.Box),
+            nameof(KnownResamplers.CatmullRom),
+            nameof(KnownResamplers.Hermite),
+            nameof(KnownResamplers.Lanczos2),
+            nameof(KnownResamplers.Lanczos3),
+            nameof(KnownResamplers.Lanczos5),
+            nameof(KnownResamplers.Lanczos8),
+            nameof(KnownResamplers.MitchellNetravali),
+            nameof(KnownResamplers.NearestNeighbor),
+            nameof(KnownResamplers.Robidoux),
+            nameof(KnownResamplers.RobidouxSharp),
+            nameof(KnownResamplers.Spline),
+            nameof(KnownResamplers.Triangle),
+            nameof(KnownResamplers.Welch),
+        };
+
         [Theory]
         [WithFileCollection(nameof(DefaultFiles), nameof(SkewValues), DefaultPixelType)]
         public void ImageShouldSkew<TPixel>(TestImageProvider<TPixel> provider, float x, float y)
@@ -34,11 +58,27 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Transforms
         public void ImageShouldSkewWithSampler<TPixel>(TestImageProvider<TPixel> provider, float x, float y)
             where TPixel : struct, IPixel<TPixel>
         {
+            foreach (string resamplerName in ResamplerNames)
+            {
+                IResampler sampler = GetResampler(resamplerName);
                 using (Image<TPixel> image = provider.GetImage())
                 {
-                image.Mutate(i => i.Skew(x, y, KnownResamplers.Triangle));
+                    image.Mutate(i => i.Skew(x, y, sampler));
-                image.DebugSave(provider, string.Join("_", x, y, "triangle"));
+                    image.DebugSave(provider, string.Join("_", x, y, resamplerName));
+                }
+            }
         }
+
+        private static IResampler GetResampler(string name)
+        {
+            PropertyInfo property = typeof(KnownResamplers).GetTypeInfo().GetProperty(name);
+
+            if (property == null)
+            {
+                throw new Exception("Invalid property name!");
+            }
+
+            return (IResampler)property.GetValue(null);
         }
     }
 }