Merge branch 'feature/fixAdaptiveHistIssue' into js/update-submodule

src/ImageSharp.Drawing/Processing/PathGradientBrush.cs

@@ -0,0 +1,286 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Numerics;
+
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.Primitives;
+using SixLabors.Shapes;
+
+namespace SixLabors.ImageSharp.Processing
+{
+    /// <summary>
+    /// Provides an implementation of a brush for painting gradients between multiple color positions in 2D coordinates.
+    /// It works similarly with the class in System.Drawing.Drawing2D of the same name.
+    /// </summary>
+    public sealed class PathGradientBrush : IBrush
+    {
+        private readonly Polygon path;
+
+        private readonly IList<Edge> edges;
+
+        private readonly Color centerColor;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PathGradientBrush"/> class.
+        /// </summary>
+        /// <param name="lines">Line segments of a polygon that represents the gradient area.</param>
+        /// <param name="colors">Array of colors that correspond to each point in the polygon.</param>
+        /// <param name="centerColor">Color at the center of the gradient area to which the other colors converge.</param>
+        public PathGradientBrush(ILineSegment[] lines, Color[] colors, Color centerColor)
+        {
+            if (lines == null)
+            {
+                throw new ArgumentNullException(nameof(lines));
+            }
+
+            if (lines.Length < 3)
+            {
+                throw new ArgumentOutOfRangeException(
+                    nameof(lines),
+                    "There must be at least 3 lines to construct a path gradient brush.");
+            }
+
+            if (colors == null)
+            {
+                throw new ArgumentNullException(nameof(colors));
+            }
+
+            if (!colors.Any())
+            {
+                throw new ArgumentOutOfRangeException(
+                    nameof(colors),
+                    "One or more color is needed to construct a path gradient brush.");
+            }
+
+            this.path = new Polygon(lines);
+            this.centerColor = centerColor;
+
+            Color ColorAt(int index) => colors[index % colors.Length];
+
+            this.edges = this.path.LineSegments.Select(s => new Path(s))
+                .Select((path, i) => new Edge(path, ColorAt(i), ColorAt(i + 1))).ToList();
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PathGradientBrush"/> class.
+        /// </summary>
+        /// <param name="lines">Line segments of a polygon that represents the gradient area.</param>
+        /// <param name="colors">Array of colors that correspond to each point in the polygon.</param>
+        public PathGradientBrush(ILineSegment[] lines, Color[] colors)
+            : this(lines, colors, CalculateCenterColor(colors))
+        {
+        }
+
+        /// <inheritdoc />
+        public BrushApplicator<TPixel> CreateApplicator<TPixel>(
+            ImageFrame<TPixel> source,
+            RectangleF region,
+            GraphicsOptions options)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            return new PathGradientBrushApplicator<TPixel>(source, this.path, this.edges, this.centerColor, options);
+        }
+
+        private static Color CalculateCenterColor(Color[] colors)
+        {
+            if (colors == null)
+            {
+                throw new ArgumentNullException(nameof(colors));
+            }
+
+            if (!colors.Any())
+            {
+                throw new ArgumentOutOfRangeException(
+                    nameof(colors),
+                    "One or more color is needed to construct a path gradient brush.");
+            }
+
+            return new Color(colors.Select(c => c.ToVector4()).Aggregate((p1, p2) => p1 + p2) / colors.Length);
+        }
+
+        private static float DistanceBetween(PointF p1, PointF p2) => ((Vector2)(p2 - p1)).Length();
+
+        private struct Intersection
+        {
+            public Intersection(PointF point, float distance)
+            {
+                this.Point = point;
+                this.Distance = distance;
+            }
+
+            public PointF Point { get; }
+
+            public float Distance { get; }
+        }
+
+        /// <summary>
+        /// An edge of the polygon that represents the gradient area.
+        /// </summary>
+        private class Edge
+        {
+            private readonly Path path;
+
+            private readonly float length;
+
+            private readonly PointF[] buffer;
+
+            public Edge(Path path, Color startColor, Color endColor)
+            {
+                this.path = path;
+
+                Vector2[] points = path.LineSegments.SelectMany(s => s.Flatten()).Select(p => (Vector2)p).ToArray();
+
+                this.Start = points.First();
+                this.StartColor = startColor.ToVector4();
+
+                this.End = points.Last();
+                this.EndColor = endColor.ToVector4();
+
+                this.length = DistanceBetween(this.End, this.Start);
+                this.buffer = new PointF[this.path.MaxIntersections];
+            }
+
+            public PointF Start { get; }
+
+            public Vector4 StartColor { get; }
+
+            public PointF End { get; }
+
+            public Vector4 EndColor { get; }
+
+            public Intersection? FindIntersection(PointF start, PointF end)
+            {
+                int intersections = this.path.FindIntersections(start, end, this.buffer);
+
+                if (intersections == 0)
+                {
+                    return null;
+                }
+
+                return this.buffer.Take(intersections)
+                    .Select(p => new Intersection(point: p, distance: ((Vector2)(p - start)).LengthSquared()))
+                    .Aggregate((min, current) => min.Distance > current.Distance ? current : min);
+            }
+
+            public Vector4 ColorAt(float distance)
+            {
+                float ratio = this.length > 0 ? distance / this.length : 0;
+
+                return Vector4.Lerp(this.StartColor, this.EndColor, ratio);
+            }
+
+            public Vector4 ColorAt(PointF point) => this.ColorAt(DistanceBetween(point, this.Start));
+        }
+
+        /// <summary>
+        /// The path gradient brush applicator.
+        /// </summary>
+        private class PathGradientBrushApplicator<TPixel> : BrushApplicator<TPixel>
+            where TPixel : struct, IPixel<TPixel>
+        {
+            private readonly Path path;
+
+            private readonly PointF center;
+
+            private readonly Vector4 centerColor;
+
+            private readonly float maxDistance;
+
+            private readonly IList<Edge> edges;
+
+            /// <summary>
+            /// Initializes a new instance of the <see cref="PathGradientBrushApplicator{TPixel}"/> class.
+            /// </summary>
+            /// <param name="source">The source image.</param>
+            /// <param name="path">A polygon that represents the gradient area.</param>
+            /// <param name="edges">Edges of the polygon.</param>
+            /// <param name="centerColor">Color at the center of the gradient area to which the other colors converge.</param>
+            /// <param name="options">The options.</param>
+            public PathGradientBrushApplicator(
+                ImageFrame<TPixel> source,
+                Path path,
+                IList<Edge> edges,
+                Color centerColor,
+                GraphicsOptions options)
+                : base(source, options)
+            {
+                this.path = path;
+                this.edges = edges;
+
+                PointF[] points = path.LineSegments.Select(s => s.EndPoint).ToArray();
+
+                this.center = points.Aggregate((p1, p2) => p1 + p2) / points.Length;
+                this.centerColor = centerColor.ToVector4();
+
+                this.maxDistance = points.Select(p => (Vector2)(p - this.center)).Select(d => d.Length()).Max();
+            }
+
+            /// <inheritdoc />
+            internal override TPixel this[int x, int y]
+            {
+                get
+                {
+                    var point = new PointF(x, y);
+
+                    if (point == this.center)
+                    {
+                        return new Color(this.centerColor).ToPixel<TPixel>();
+                    }
+
+                    if (!this.path.Contains(point))
+                    {
+                        return Color.Transparent.ToPixel<TPixel>();
+                    }
+
+                    Vector2 direction = Vector2.Normalize(point - this.center);
+
+                    PointF end = point + (PointF)(direction * this.maxDistance);
+
+                    (Edge edge, Intersection? info) = this.FindIntersection(point, end);
+
+                    PointF intersection = info.Value.Point;
+
+                    Vector4 edgeColor = edge.ColorAt(intersection);
+
+                    float length = DistanceBetween(intersection, this.center);
+                    float ratio = length > 0 ? DistanceBetween(intersection, point) / length : 0;
+
+                    Vector4 color = Vector4.Lerp(edgeColor, this.centerColor, ratio);
+
+                    return new Color(color).ToPixel<TPixel>();
+                }
+            }
+
+            private (Edge edge, Intersection? info) FindIntersection(PointF start, PointF end)
+            {
+                (Edge edge, Intersection? info) closest = default;
+
+                foreach (Edge edge in this.edges)
+                {
+                    Intersection? intersection = edge.FindIntersection(start, end);
+
+                    if (!intersection.HasValue)
+                    {
+                        continue;
+                    }
+
+                    if (closest.info == null || closest.info.Value.Distance > intersection.Value.Distance)
+                    {
+                        closest = (edge, intersection);
+                    }
+                }
+
+                return closest;
+            }
+
+            /// <inheritdoc />
+            public override void Dispose()
+            {
+            }
+        }
+    }
+}

src/ImageSharp/Processing/Processors/Normalization/AdaptiveHistogramEqualizationProcessor{TPixel}.cs

@@ -73,10 +73,12 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
 
                 var tileYStartPositions = new List<(int y, int cdfY)>();
                 int cdfY = 0;
-                for (int y = halfTileHeight; y < sourceHeight - halfTileHeight; y += tileHeight)
+                int yStart = halfTileHeight;
+                for (int tile = 0; tile < tileCount - 1; tile++)
                 {
-                    tileYStartPositions.Add((y, cdfY));
+                    tileYStartPositions.Add((yStart, cdfY));
                     cdfY++;
+                    yStart += tileHeight;
                 }
 
                 Parallel.For(
@@ -92,7 +94,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                             ref TPixel sourceBase = ref source.GetPixelReference(0, 0);
 
                             int cdfX = 0;
-                            for (int x = halfTileWidth; x < sourceWidth - halfTileWidth; x += tileWidth)
+                            int x = halfTileWidth;
+                            for (int tile = 0; tile < tileCount - 1; tile++)
                             {
                                 int tileY = 0;
                                 int yEnd = Math.Min(y + tileHeight, sourceHeight);
@@ -125,24 +128,25 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                                 }
 
                                 cdfX++;
+                                x += tileWidth;
                             }
                         });
 
                 ref TPixel pixelsBase = ref source.GetPixelReference(0, 0);
 
                 // Fix left column
-                ProcessBorderColumn(ref pixelsBase, cdfData, 0, sourceWidth, sourceHeight, tileWidth, tileHeight, xStart: 0, xEnd: halfTileWidth, luminanceLevels);
+                ProcessBorderColumn(ref pixelsBase, cdfData, 0, sourceWidth, sourceHeight, this.Tiles, tileHeight, xStart: 0, xEnd: halfTileWidth, luminanceLevels);
 
                 // Fix right column
                 int rightBorderStartX = ((this.Tiles - 1) * tileWidth) + halfTileWidth;
-                ProcessBorderColumn(ref pixelsBase, cdfData, this.Tiles - 1, sourceWidth, sourceHeight, tileWidth, tileHeight, xStart: rightBorderStartX, xEnd: sourceWidth, luminanceLevels);
+                ProcessBorderColumn(ref pixelsBase, cdfData, this.Tiles - 1, sourceWidth, sourceHeight, this.Tiles, tileHeight, xStart: rightBorderStartX, xEnd: sourceWidth, luminanceLevels);
 
                 // Fix top row
-                ProcessBorderRow(ref pixelsBase, cdfData, 0, sourceWidth, tileWidth, yStart: 0, yEnd: halfTileHeight, luminanceLevels);
+                ProcessBorderRow(ref pixelsBase, cdfData, 0, sourceWidth, this.Tiles, tileWidth, yStart: 0, yEnd: halfTileHeight, luminanceLevels);
 
                 // Fix bottom row
                 int bottomBorderStartY = ((this.Tiles - 1) * tileHeight) + halfTileHeight;
-                ProcessBorderRow(ref pixelsBase, cdfData, this.Tiles - 1, sourceWidth, tileWidth, yStart: bottomBorderStartY, yEnd: sourceHeight, luminanceLevels);
+                ProcessBorderRow(ref pixelsBase, cdfData, this.Tiles - 1, sourceWidth, this.Tiles, tileWidth, yStart: bottomBorderStartY, yEnd: sourceHeight, luminanceLevels);
 
                 // Left top corner
                 ProcessCornerTile(ref pixelsBase, cdfData, sourceWidth, 0, 0, xStart: 0, xEnd: halfTileWidth, yStart: 0, yEnd: halfTileHeight, luminanceLevels);
@@ -206,7 +210,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         /// <param name="cdfX">The X index of the lookup table to use.</param>
         /// <param name="sourceWidth">The source image width.</param>
         /// <param name="sourceHeight">The source image height.</param>
-        /// <param name="tileWidth">The width of a tile.</param>
+        /// <param name="tileCount">The number of vertical tiles.</param>
         /// <param name="tileHeight">The height of a tile.</param>
         /// <param name="xStart">X start position in the image.</param>
         /// <param name="xEnd">X end position of the image.</param>
@@ -220,7 +224,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             int cdfX,
             int sourceWidth,
             int sourceHeight,
-            int tileWidth,
+            int tileCount,
             int tileHeight,
             int xStart,
             int xEnd,
@@ -229,7 +233,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             int halfTileHeight = tileHeight / 2;
 
             int cdfY = 0;
-            for (int y = halfTileHeight; y < sourceHeight - halfTileHeight; y += tileHeight)
+            int y = halfTileHeight;
+            for (int tile = 0; tile < tileCount - 1; tile++)
             {
                 int yLimit = Math.Min(y + tileHeight, sourceHeight - 1);
                 int tileY = 0;
@@ -247,6 +252,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                 }
 
                 cdfY++;
+                y += tileHeight;
             }
         }
 
@@ -257,6 +263,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         /// <param name="cdfData">The pre-computed lookup tables to remap the grey values for each tiles.</param>
         /// <param name="cdfY">The Y index of the lookup table to use.</param>
         /// <param name="sourceWidth">The source image width.</param>
+        /// <param name="tileCount">The number of horizontal tiles.</param>
         /// <param name="tileWidth">The width of a tile.</param>
         /// <param name="yStart">Y start position in the image.</param>
         /// <param name="yEnd">Y end position of the image.</param>
@@ -269,6 +276,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             CdfTileData cdfData,
             int cdfY,
             int sourceWidth,
+            int tileCount,
             int tileWidth,
             int yStart,
             int yEnd,
@@ -277,7 +285,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
             int halfTileWidth = tileWidth / 2;
 
             int cdfX = 0;
-            for (int x = halfTileWidth; x < sourceWidth - halfTileWidth; x += tileWidth)
+            int x = halfTileWidth;
+            for (int tile = 0; tile < tileCount - 1; tile++)
             {
                 for (int dy = yStart; dy < yEnd; dy++)
                 {
@@ -294,6 +303,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
                 }
 
                 cdfX++;
+                x += tileWidth;
             }
         }
 

src/ImageSharp/Processing/Processors/Normalization/HistogramEqualizationOptions.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 namespace SixLabors.ImageSharp.Processing.Processors.Normalization
@@ -25,7 +25,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Normalization
         public int LuminanceLevels { get; set; } = 256;
 
         /// <summary>
-        /// Gets or sets a value indicating whether to clip the histogram bins at a specific value. Defaults to false.
+        /// Gets or sets a value indicating whether to clip the histogram bins at a specific value.
+        /// It is recommended to use clipping when the AdaptiveTileInterpolation method is used, to suppress artifacts which can occur on the borders of the tiles.
+        /// Defaults to false.
         /// </summary>
         public bool ClipHistogram { get; set; } = false;
 

tests/ImageSharp.Tests/Drawing/FillPathGradientBrushTests.cs

@@ -0,0 +1,209 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
+using SixLabors.Primitives;
+using SixLabors.Shapes;
+
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Drawing
+{
+    [GroupOutput("Drawing/GradientBrushes")]
+    public class FillPathGradientBrushTests
+    {
+        public static ImageComparer TolerantComparer = ImageComparer.TolerantPercentage(0.01f);
+
+        [Theory]
+        [WithBlankImages(10, 10, PixelTypes.Rgba32)]
+        public void FillRectangleWithDifferentColors<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            provider.VerifyOperation(
+                TolerantComparer,
+                image =>
+                {
+                    ILineSegment[] path =
+                    {
+                        new LinearLineSegment(new PointF(0, 0), new PointF(10, 0)),
+                        new LinearLineSegment(new PointF(10, 0), new PointF(10, 10)),
+                        new LinearLineSegment(new PointF(10, 10), new PointF(0, 10)),
+                        new LinearLineSegment(new PointF(0, 10), new PointF(0, 0))
+                    };
+
+                    Color[] colors = { Color.Black, Color.Red, Color.Yellow, Color.Green };
+
+                    var brush = new PathGradientBrush(path, colors);
+
+                    image.Mutate(x => x.Fill(brush));
+                    image.DebugSave(provider, appendPixelTypeToFileName: false, appendSourceFileOrDescription: false);
+                });
+        }
+
+        [Theory]
+        [WithBlankImages(10, 10, PixelTypes.Rgba32)]
+        public void FillTriangleWithDifferentColors<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            provider.VerifyOperation(
+                TolerantComparer,
+                image =>
+                {
+                    ILineSegment[] path =
+                    {
+                        new LinearLineSegment(new PointF(5, 0), new PointF(10, 10)),
+                        new LinearLineSegment(new PointF(10, 10), new PointF(0, 10)),
+                        new LinearLineSegment(new PointF(0, 10), new PointF(5, 0))
+                    };
+
+                    Color[] colors = { Color.Red, Color.Green, Color.Blue };
+
+                    var brush = new PathGradientBrush(path, colors);
+
+                    image.Mutate(x => x.Fill(brush));
+                    image.DebugSave(provider, appendPixelTypeToFileName: false, appendSourceFileOrDescription: false);
+                });
+        }
+
+        [Theory]
+        [WithBlankImages(10, 10, PixelTypes.Rgba32)]
+        public void FillRectangleWithSingleColor<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage())
+            {
+                ILineSegment[] path =
+                {
+                    new LinearLineSegment(new PointF(0, 0), new PointF(10, 0)),
+                    new LinearLineSegment(new PointF(10, 0), new PointF(10, 10)),
+                    new LinearLineSegment(new PointF(10, 10), new PointF(0, 10)),
+                    new LinearLineSegment(new PointF(0, 10), new PointF(0, 0))
+                };
+
+                Color[] colors = { Color.Red };
+
+                var brush = new PathGradientBrush(path, colors);
+
+                image.Mutate(x => x.Fill(brush));
+
+                image.ComparePixelBufferTo(Color.Red);
+            }
+        }
+
+        [Theory]
+        [WithBlankImages(10, 10, PixelTypes.Rgba32)]
+        public void ShouldRotateTheColorsWhenThereAreMorePoints<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            provider.VerifyOperation(
+                TolerantComparer,
+                image =>
+                {
+                    ILineSegment[] path =
+                    {
+                        new LinearLineSegment(new PointF(0, 0), new PointF(10, 0)),
+                        new LinearLineSegment(new PointF(10, 0), new PointF(10, 10)),
+                        new LinearLineSegment(new PointF(10, 10), new PointF(0, 10)),
+                        new LinearLineSegment(new PointF(0, 10), new PointF(0, 0))
+                    };
+
+                    Color[] colors = { Color.Red, Color.Yellow };
+
+                    var brush = new PathGradientBrush(path, colors);
+
+                    image.Mutate(x => x.Fill(brush));
+                    image.DebugSave(provider, appendPixelTypeToFileName: false, appendSourceFileOrDescription: false);
+                });
+        }
+
+        [Theory]
+        [WithBlankImages(10, 10, PixelTypes.Rgba32)]
+        public void FillWithCustomCenterColor<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            provider.VerifyOperation(
+                TolerantComparer,
+                image =>
+                {
+                    ILineSegment[] path =
+                    {
+                        new LinearLineSegment(new PointF(0, 0), new PointF(10, 0)),
+                        new LinearLineSegment(new PointF(10, 0), new PointF(10, 10)),
+                        new LinearLineSegment(new PointF(10, 10), new PointF(0, 10)),
+                        new LinearLineSegment(new PointF(0, 10), new PointF(0, 0))
+                    };
+
+                    Color[] colors = { Color.Black, Color.Red, Color.Yellow, Color.Green };
+
+                    var brush = new PathGradientBrush(path, colors, Color.White);
+
+                    image.Mutate(x => x.Fill(brush));
+                    image.DebugSave(provider, appendPixelTypeToFileName: false, appendSourceFileOrDescription: false);
+                });
+        }
+
+        [Fact]
+        public void ShouldThrowArgumentNullExceptionWhenLinesAreNull()
+        {
+            Color[] colors = { Color.Black, Color.Red, Color.Yellow, Color.Green };
+
+            PathGradientBrush Create() => new PathGradientBrush(null, colors, Color.White);
+
+            Assert.Throws<ArgumentNullException>(Create);
+        }
+
+        [Fact]
+        public void ShouldThrowArgumentOutOfRangeExceptionWhenLessThan3LinesAreGiven()
+        {
+            ILineSegment[] path =
+            {
+                new LinearLineSegment(new PointF(0, 0), new PointF(10, 0)),
+                new LinearLineSegment(new PointF(10, 0), new PointF(10, 10))
+            };
+
+            Color[] colors = { Color.Black, Color.Red, Color.Yellow, Color.Green };
+
+            PathGradientBrush Create() => new PathGradientBrush(path, colors, Color.White);
+
+            Assert.Throws<ArgumentOutOfRangeException>(Create);
+        }
+
+        [Fact]
+        public void ShouldThrowArgumentNullExceptionWhenColorsAreNull()
+        {
+            ILineSegment[] path =
+            {
+                new LinearLineSegment(new PointF(0, 0), new PointF(10, 0)),
+                new LinearLineSegment(new PointF(10, 0), new PointF(10, 10)),
+                new LinearLineSegment(new PointF(10, 10), new PointF(0, 10)),
+                new LinearLineSegment(new PointF(0, 10), new PointF(0, 0))
+            };
+
+            PathGradientBrush Create() => new PathGradientBrush(path, null, Color.White);
+
+            Assert.Throws<ArgumentNullException>(Create);
+        }
+
+        [Fact]
+        public void ShouldThrowArgumentOutOfRangeExceptionWhenEmptyColorArrayIsGiven()
+        {
+            ILineSegment[] path =
+            {
+                new LinearLineSegment(new PointF(0, 0), new PointF(10, 0)),
+                new LinearLineSegment(new PointF(10, 0), new PointF(10, 10)),
+                new LinearLineSegment(new PointF(10, 10), new PointF(0, 10)),
+                new LinearLineSegment(new PointF(0, 10), new PointF(0, 0))
+            };
+
+            var colors = new Color[0];
+
+            PathGradientBrush Create() => new PathGradientBrush(path, colors, Color.White);
+
+            Assert.Throws<ArgumentOutOfRangeException>(Create);
+        }
+    }
+}

tests/ImageSharp.Tests/Processing/Normalization/HistogramEqualizationTests.cs

@@ -9,6 +9,7 @@ using Xunit;
 
 namespace SixLabors.ImageSharp.Tests.Processing.Normalization
 {
+    // ReSharper disable InconsistentNaming
     public class HistogramEqualizationTests
     {
         private static readonly ImageComparer ValidatorComparer = ImageComparer.TolerantPercentage(0.0456F);
@@ -113,5 +114,30 @@ namespace SixLabors.ImageSharp.Tests.Processing.Normalization
                 image.CompareToReferenceOutput(ValidatorComparer, provider);
             }
         }
+
+        /// <summary>
+        /// This is regression test for a bug with the calculation of the y-start positions,
+        /// where it could happen that one too much start position was calculated in some cases.
+        /// See: https://github.com/SixLabors/ImageSharp/pull/984
+        /// </summary>
+        [Theory]
+        [WithTestPatternImages(110, 110, PixelTypes.Rgba32)]
+        [WithTestPatternImages(170, 170, PixelTypes.Rgba32)]
+        public void Issue984<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage())
+            {
+                var options = new HistogramEqualizationOptions()
+                              {
+                                  Method = HistogramEqualizationMethod.AdaptiveTileInterpolation,
+                                  LuminanceLevels = 256,
+                                  ClipHistogram = true,
+                                  NumberOfTiles = 10
+                              };
+                image.Mutate(x => x.HistogramEqualization(options));
+                image.DebugSave(provider);
+            }
+        }
     }
 }

tests/Images/External

@@ -1 +1 @@
-Subproject commit 36f39bc624f8a49caf512077bf70cab30c2e5fb4
+Subproject commit 99a2bc523cd4eb00e37af20d1b2088fa11564c57