Document Layout Analysis - IPageSegmenter, Docstrum

- Create a TextBlock class
- Creates IPageSegmenter
- Add other useful distances: angle, etc.
- Update RecursiveXYCut
 - With IPageSegmenter and TextBlock
 - Make XYNode and XYLeaf internal
- Optimise (faster) NearestNeighbourWordExtractor and isolate the clustering algorithms for use outside of this class
- Implement a Docstrum inspired page segmentation algorithm

src/UglyToad.PdfPig.Tests/PublicApiScannerTests.cs

@@ -54,17 +54,19 @@
                 "UglyToad.PdfPig.Content.PageSize",
                 "UglyToad.PdfPig.Content.Word",
                 "UglyToad.PdfPig.Content.TextLine",
+                "UglyToad.PdfPig.Content.TextBlock",
                 "UglyToad.PdfPig.Content.TextDirection",
                 "UglyToad.PdfPig.Core.TransformationMatrix",
                 "UglyToad.PdfPig.CrossReference.CrossReferenceTable",
                 "UglyToad.PdfPig.CrossReference.CrossReferenceType",
                 "UglyToad.PdfPig.CrossReference.TrailerDictionary",
+                "UglyToad.PdfPig.DocumentLayoutAnalysis.ClusteringAlgorithms",
                 "UglyToad.PdfPig.DocumentLayoutAnalysis.Distances",
+                "UglyToad.PdfPig.DocumentLayoutAnalysis.DocstrumBB",
+                "UglyToad.PdfPig.DocumentLayoutAnalysis.IPageSegmenter",
                 "UglyToad.PdfPig.DocumentLayoutAnalysis.MathExtensions",
                 "UglyToad.PdfPig.DocumentLayoutAnalysis.NearestNeighbourWordExtractor",
                 "UglyToad.PdfPig.DocumentLayoutAnalysis.RecursiveXYCut",
-                "UglyToad.PdfPig.DocumentLayoutAnalysis.XYNode",
-                "UglyToad.PdfPig.DocumentLayoutAnalysis.XYLeaf",
                 "UglyToad.PdfPig.DocumentLayoutAnalysis.TextEdgesExtractor",
                 "UglyToad.PdfPig.DocumentLayoutAnalysis.EdgeType",
                 "UglyToad.PdfPig.Exceptions.PdfDocumentEncryptedException",

src/UglyToad.PdfPig/Content/TextBlock.cs

@@ -0,0 +1,68 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using UglyToad.PdfPig.Geometry;
+
+namespace UglyToad.PdfPig.Content
+{
+    /// <summary>
+    /// A block of text.
+    /// </summary>
+    public class TextBlock
+    {
+        /// <summary>
+        /// The text of the block.
+        /// </summary>
+        public string Text { get; }
+
+        /// <summary>
+        /// The text direction of the block.
+        /// </summary>
+        public TextDirection TextDirection { get; }
+
+        /// <summary>
+        /// The rectangle completely containing the block.
+        /// </summary>
+        public PdfRectangle BoundingBox { get; }
+
+        /// <summary>
+        /// The text lines contained in the block.
+        /// </summary>
+        public IReadOnlyList<TextLine> TextLines { get; }
+
+        /// <summary>
+        /// Create a new <see cref="TextBlock"/>.
+        /// </summary>
+        /// <param name="lines"></param>
+        public TextBlock(IReadOnlyList<TextLine> lines)
+        {
+            if (lines == null)
+            {
+                throw new ArgumentNullException(nameof(lines));
+            }
+
+            if (lines.Count == 0)
+            {
+                throw new ArgumentException("Empty lines provided.", nameof(lines));
+            }
+
+            TextLines = lines;
+
+            Text = string.Join(" ", lines.Select(x => x.Text));
+
+            var minX = lines.Min(x => x.BoundingBox.Left);
+            var minY = lines.Min(x => x.BoundingBox.Bottom);
+            var maxX = lines.Max(x => x.BoundingBox.Right);
+            var maxY = lines.Max(x => x.BoundingBox.Top);
+            BoundingBox = new PdfRectangle(minX, minY, maxX, maxY);
+
+            TextDirection = lines[0].TextDirection;
+        }
+
+        /// <inheritdoc />
+        public override string ToString()
+        {
+            return Text;
+        }
+    }
+}

src/UglyToad.PdfPig/DocumentLayoutAnalysis/ClusteringAlgorithms.cs

@@ -0,0 +1,164 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using UglyToad.PdfPig.Geometry;
+
+namespace UglyToad.PdfPig.DocumentLayoutAnalysis
+{
+    /// <summary>
+    /// Clustering Algorithms.
+    /// </summary>
+    internal class ClusteringAlgorithms
+    {
+        /// <summary>
+        /// Algorithm to group elements via transitive closure, using nearest neighbours and maximum distance.
+        /// https://en.wikipedia.org/wiki/Transitive_closure
+        /// </summary>
+        /// <typeparam name="T">Letter, Word, TextLine, etc.</typeparam>
+        /// <param name="elements">Array of elements to group.</param>
+        /// <param name="distMeasure">The distance measure between two points.</param>
+        /// <param name="maxDistanceFunction">The function that determines the distance between to points in the same cluster.</param>
+        /// <param name="pivotPoint">The pivot's point to use.</param>
+        /// <param name="candidatesPoint">The candidates to pair point to use.</param>
+        /// <param name="filterPivot">Filter to apply to the pivot point.</param>
+        /// <param name="filterFinal">Filter to apply to both the pivot and the paired point.</param>
+        internal static IEnumerable<HashSet<int>> SimpleTransitiveClosure<T>(T[] elements,
+            Func<PdfPoint, PdfPoint, double> distMeasure,
+            Func<T, T, double> maxDistanceFunction,
+            Func<T, PdfPoint> pivotPoint, Func<T, PdfPoint> candidatesPoint,
+            Func<T, bool> filterPivot, Func<T, T, bool> filterFinal)
+        {
+            /*************************************************************************************
+             * Algorithm steps
+             * 1. Find nearest neighbours indexes (done in parallel)
+             *  Iterate every point (pivot) and put its nearest neighbour's index in an array
+             *  e.g. if nearest neighbour of point i is point j, then indexes[i] = j.
+             *  Only conciders a neighbour if it is within the maximum distance. 
+             *  If not within the maximum distance, index will be set to -1.
+             *  NB: Given the possible asymmetry in the relationship, it is possible 
+             *  that if indexes[i] = j then indexes[j] != i.
+             *  
+             * 2. Group indexes
+             *  Group indexes if share neighbours in common - Transitive closure
+             *  e.g. if we have indexes[i] = j, indexes[j] = k, indexes[m] = n and indexes[n] = -1
+             *  (i,j,k) will form a group and (m,n) will form another group.
+             *  
+             * 3. Merge groups that have indexes in common - If any
+             *  If there are group with indexes in common, merge them.
+             *  (Could be improved and put in step 2)
+             *************************************************************************************/
+
+            int[] indexes = Enumerable.Repeat((int)-1, elements.Length).ToArray();
+            var candidatesPoints = elements.Select(x => candidatesPoint(x)).ToList();
+
+            // 1. Find nearest neighbours indexes
+            Parallel.For(0, elements.Length, e =>
+            {
+                var pivot = elements[e];
+
+                if (filterPivot(pivot))
+                {
+                    int index = pivotPoint(pivot).FindIndexNearest(candidatesPoints, distMeasure, out double dist);
+                    var paired = elements[index];
+
+                    if (filterFinal(pivot, paired) && dist < maxDistanceFunction(pivot, paired))
+                    {
+                        indexes[e] = index;
+                    }
+                }
+            });
+
+            // 2. Group indexes
+            List<HashSet<int>> groupedIndexes = new List<HashSet<int>>();
+            HashSet<int> indexDone = new HashSet<int>();
+
+            for (int e = 0; e < elements.Length; e++)
+            {
+                int index = indexes[e];
+
+                if (index == -1) // This element is not connected
+                {
+                    // Check if another element index is connected to this element (nb: distance measure is asymetric)
+                    if (!indexes.Contains(e))
+                    {
+                        // If no other element is connected to this element, add it as a standalone element
+                        groupedIndexes.Add(new HashSet<int>() { e });
+                        indexDone.Add(e);
+                    }
+                    continue;
+                }
+
+                bool isDoneC = indexDone.Contains(e);
+                bool isDoneI = indexDone.Contains(index);
+                if (isDoneC || isDoneI)
+                {
+                    if (isDoneC && !isDoneI)
+                    {
+                        foreach (var pair in groupedIndexes.Where(x => x.Contains(e)))
+                        {
+                            pair.Add(index);
+                        }
+                        indexDone.Add(index);
+                    }
+                    else if (!isDoneC && isDoneI)
+                    {
+                        foreach (var pair in groupedIndexes.Where(x => x.Contains(index)))
+                        {
+                            pair.Add(e);
+                        }
+                        indexDone.Add(e);
+                    }
+                    else // isDoneC && isDoneI
+                    {
+                        foreach (var pair in groupedIndexes.Where(x => x.Contains(index)))
+                        {
+                            if (!pair.Contains(e)) pair.Add(e);
+                        }
+
+                        foreach (var pair in groupedIndexes.Where(x => x.Contains(e)))
+                        {
+                            if (!pair.Contains(index)) pair.Add(index);
+                        }
+                    }
+                }
+                else
+                {
+                    groupedIndexes.Add(new HashSet<int>() { e, index });
+                    indexDone.Add(e);
+                    indexDone.Add(index);
+                }
+            }
+
+            // Check that all elements are done
+            if (elements.Length != indexDone.Count)
+            {
+                throw new Exception("ClusteringAlgorithms.GetNNGroupedIndexes(): Some elements were not done.");
+            }
+
+            // 3. Merge groups that have indexes in common
+            // Check if duplicates (if duplicates, then same index in different groups)
+            if (indexDone.Count != groupedIndexes.SelectMany(x => x).Count())
+            {
+                for (int e = 0; e < elements.Length; e++)
+                {
+                    List<HashSet<int>> candidates = groupedIndexes.Where(x => x.Contains(e)).ToList();
+                    int count = candidates.Count();
+                    if (count < 2) continue; // Only one group with this index
+
+                    HashSet<int> merged = candidates.First();
+                    groupedIndexes.Remove(merged);
+                    for (int i = 1; i < count; i++)
+                    {
+                        var current = candidates.ElementAt(i);
+                        merged.UnionWith(current);
+                        groupedIndexes.Remove(current);
+                    }
+                    groupedIndexes.Add(merged);
+                }
+            }
+
+            return groupedIndexes;
+        }
+    }
+}

src/UglyToad.PdfPig/DocumentLayoutAnalysis/Distances.cs

@@ -47,6 +47,39 @@ namespace UglyToad.PdfPig.DocumentLayoutAnalysis
             return (double)(Math.Abs(point1.X - point2.X) + Math.Abs(point1.Y - point2.Y));
         }
 
+        /// <summary>
+        /// The angle in degrees between the horizontal axis and the line between two points.
+        /// </summary>
+        /// <param name="point1">The first point.</param>
+        /// <param name="point2">The second point.</param>
+        /// <returns></returns>
+        public static double Angle(PdfPoint point1, PdfPoint point2)
+        {
+            return Math.Atan2((float)(point2.Y - point1.Y), (float)(point2.X - point1.X)) * 180.0 / Math.PI;
+        }
+
+        /// <summary>
+        /// The absolute distance between the Y coordinates of two points.
+        /// </summary>
+        /// <param name="point1">The first point.</param>
+        /// <param name="point2">The second point.</param>
+        /// <returns></returns>
+        public static double Vertical(PdfPoint point1, PdfPoint point2)
+        {
+            return Math.Abs((double)(point2.Y - point1.Y));
+        }
+
+        /// <summary>
+        /// The absolute distance between the X coordinates of two points.
+        /// </summary>
+        /// <param name="point1">The first point.</param>
+        /// <param name="point2">The second point.</param>
+        /// <returns></returns>
+        public static double Horizontal(PdfPoint point1, PdfPoint point2)
+        {
+            return Math.Abs((double)(point2.X - point1.X));
+        }
+
         /// <summary>
         /// Find the nearest point.
         /// </summary>

src/UglyToad.PdfPig/DocumentLayoutAnalysis/DocstrumBB.cs

@@ -0,0 +1,212 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using UglyToad.PdfPig.Content;
+using UglyToad.PdfPig.Geometry;
+
+namespace UglyToad.PdfPig.DocumentLayoutAnalysis
+{
+    /// <summary>
+    /// The Docstrum algorithm is a bottom-up page segmentation technique based on nearest-neighborhood 
+    /// clustering of connected components extracted from the document. 
+    /// This implementation leverages bounding boxes and does not exactly replicates the original algorithm.
+    /// <para>See 'The document spectrum for page layout analysis.' by L. O’Gorman.</para>
+    /// </summary>
+    public class DocstrumBB : IPageSegmenter
+    {
+        /// <summary>
+        /// Create an instance of Docstrum for bounding boxes page segmenter, <see cref="DocstrumBB"/>.
+        /// </summary>
+        public static DocstrumBB Instance { get; } = new DocstrumBB();
+
+        /// <summary>
+        /// Get the blocks.
+        /// <para>Uses wlAngleLB = -30, wlAngleUB = 30, blAngleLB = -135, blAngleUB = -45, blMulti = 1.3.</para>
+        /// </summary>
+        /// <param name="pageWords"></param>
+        /// <returns></returns>
+        public IReadOnlyList<TextBlock> GetBlocks(IEnumerable<Word> pageWords)
+        {
+            return GetBlocks(pageWords, -30, 30, -135, -45, 1.3);
+        }
+
+        /// <summary>
+        /// Get the blocks. See original paper for more information.
+        /// </summary>
+        /// <param name="pageWords"></param>
+        /// <param name="wlAngleLB">Within-line lower bound angle.</param>
+        /// <param name="wlAngleUB">Within-line upper bound angle.</param>
+        /// <param name="blAngleLB">Between-line lower bound angle.</param>
+        /// <param name="blAngleUB">Between-line upper bound angle.</param>
+        /// <param name="blMultiplier">Multiplier that gives the maximum perpendicular distance between 
+        /// text lines for blocking. Maximum distance will be this number times the between-line 
+        /// distance found by the analysis.</param>
+        /// <returns></returns>
+        public IReadOnlyList<TextBlock> GetBlocks(IEnumerable<Word> pageWords, double wlAngleLB, double wlAngleUB,
+            double blAngleLB, double blAngleUB, double blMultiplier)
+        {
+            var pageWordsArr = pageWords.Where(w => !string.IsNullOrWhiteSpace(w.Text)).ToArray(); // remove white spaces
+
+            var withinLineDistList = new ConcurrentBag<double[]>();
+            var betweenLineDistList = new ConcurrentBag<double[]>();
+
+            // 1. Estimate in line and between line spacing
+            Parallel.For(0, pageWordsArr.Length, i =>
+            {
+                var word = pageWordsArr[i];
+
+                // Within-line distance
+                var pointWL = GetNearestPointData(pageWordsArr, word,
+                    bb => bb.BottomRight, bb => bb.BottomRight,
+                    bb => bb.BottomLeft, bb => bb.BottomLeft,
+                    wlAngleLB, wlAngleUB, Distances.Horizontal);
+                if (pointWL != null) withinLineDistList.Add(pointWL);
+
+                // Between-line distance
+                var pointBL = GetNearestPointData(pageWordsArr, word,
+                    bb => bb.BottomLeft, bb => bb.Centroid,
+                    bb => bb.TopLeft, bb => bb.Centroid,
+                    blAngleLB, blAngleUB, Distances.Vertical);
+                if (pointBL != null) betweenLineDistList.Add(pointBL);
+            });
+
+            double withinLineDistance = GetPeakAverageDistance(withinLineDistList);
+            double betweenLineDistance = GetPeakAverageDistance(betweenLineDistList);
+
+            // 2. Find lines of text
+            double maxDistWL = Math.Min(3 * withinLineDistance, Math.Sqrt(2) * betweenLineDistance);
+            var lines = GetLines(pageWordsArr, maxDistWL).ToArray();
+
+            // 3. Find blocks of text
+            double maxDistBL = blMultiplier * betweenLineDistance;
+            return GetLinesGroups(lines, maxDistBL).ToList();
+        }
+
+        /// <summary>
+        /// Get information on the nearest point, filtered for angle.
+        /// </summary>
+        /// <param name="words"></param>
+        /// <param name="pivot"></param>
+        /// <param name="funcPivotDist"></param>
+        /// <param name="funcPivotAngle"></param>
+        /// <param name="funcPointsDist"></param>
+        /// <param name="funcPointsAngle"></param>
+        /// <param name="angleStart"></param>
+        /// <param name="angleEnd"></param>
+        /// <param name="finalDistMEasure"></param>
+        /// <returns></returns>
+        private double[] GetNearestPointData(Word[] words, Word pivot, Func<PdfRectangle,
+            PdfPoint> funcPivotDist, Func<PdfRectangle, PdfPoint> funcPivotAngle,
+            Func<PdfRectangle, PdfPoint> funcPointsDist, Func<PdfRectangle, PdfPoint> funcPointsAngle,
+            double angleStart, double angleEnd,
+            Func<PdfPoint, PdfPoint, double> finalDistMEasure)
+        {
+            var pointR = funcPivotDist(pivot.BoundingBox);
+            var filtered = words.Where(w =>
+            {
+                var angleWL = Distances.Angle(funcPivotAngle(pivot.BoundingBox), funcPointsAngle(w.BoundingBox));
+                return (angleWL >= angleStart && angleWL <= angleEnd);
+            }).ToList();
+            filtered.Remove(pivot); // remove itself
+
+            if (filtered.Count > 0)
+            {
+                int index = pointR.FindIndexNearest(
+                    filtered.Select(w => funcPointsDist(w.BoundingBox)).ToList(),
+                    Distances.Euclidean, out double distWL);
+
+                if (index >= 0)
+                {
+                    var matchWL = filtered[index];
+                    return new double[]
+                    {
+                        (double)pivot.Letters.Select(l => l.FontSize).Mode(),
+                        finalDistMEasure(pointR, funcPointsDist(matchWL.BoundingBox))
+                    };
+                }
+            }
+            return null;
+        }
+
+        /// <summary>
+        /// Build lines via transitive closure.
+        /// </summary>
+        /// <param name="words"></param>
+        /// <param name="maxDist"></param>
+        /// <returns></returns>
+        private IEnumerable<TextLine> GetLines(Word[] words, double maxDist)
+        {
+            TextDirection textDirection = words[0].TextDirection;
+            var groupedIndexes = ClusteringAlgorithms.SimpleTransitiveClosure(words, Distances.Euclidean,
+                (w1, w2) => maxDist,
+                w => w.BoundingBox.BottomRight, w => w.BoundingBox.BottomLeft,
+                w => true,
+                (w1, w2) =>
+                {
+                    var angleWL = Distances.Angle(w1.BoundingBox.BottomRight, w2.BoundingBox.BottomLeft); // compare bottom right with bottom left for angle
+                    return (angleWL >= -30 && angleWL <= 30);
+                }).ToList();
+
+            Func<IEnumerable<Word>, IReadOnlyList<Word>> orderFunc = l => l.OrderBy(x => x.BoundingBox.Left).ToList();
+            if (textDirection == TextDirection.Rotate180)
+            {
+                orderFunc = l => l.OrderByDescending(x => x.BoundingBox.Right).ToList();
+            }
+            else if (textDirection == TextDirection.Rotate90)
+            {
+                orderFunc = l => l.OrderByDescending(x => x.BoundingBox.Top).ToList();
+            }
+            else if (textDirection == TextDirection.Rotate270)
+            {
+                orderFunc = l => l.OrderBy(x => x.BoundingBox.Bottom).ToList();
+            }
+
+            for (int a = 0; a < groupedIndexes.Count(); a++)
+            {
+                yield return new TextLine(orderFunc(groupedIndexes[a].Select(i => words[i])));
+            }
+        }
+
+        /// <summary>
+        /// Build blocks via transitive closure.
+        /// </summary>
+        /// <param name="lines"></param>
+        /// <param name="maxDist"></param>
+        /// <returns></returns>
+        private IEnumerable<TextBlock> GetLinesGroups(TextLine[] lines, double maxDist)
+        {
+            var groupedIndexes = ClusteringAlgorithms.SimpleTransitiveClosure(lines, Distances.Euclidean,
+                (l1, l2) => maxDist,
+                l => l.BoundingBox.TopLeft, l => l.BoundingBox.BottomLeft,
+                l => true, (l1, l2) => true).ToList();
+
+            for (int a = 0; a < groupedIndexes.Count(); a++)
+            {
+                yield return new TextBlock(groupedIndexes[a].Select(i => lines[i]).ToList());
+            }
+        }
+
+        /// <summary>
+        /// Get the average distance value of the peak bucket of the histogram.
+        /// </summary>
+        /// <param name="values">array[0]=font size, array[1]=distance</param>
+        /// <returns></returns>
+        private double GetPeakAverageDistance(IEnumerable<double[]> values)
+        {
+            int max = (int)values.Max(x => x[1]) + 1;
+            int[] distrib = new int[max];
+
+            // Create histogram with buckets of size 1.
+            for (int i = 0; i < max; i++)
+            {
+                distrib[i] = values.Where(x => x[1] > i && x[1] <= i + 1).Count();
+            }
+
+            var peakIndex = Array.IndexOf(distrib, distrib.Max());
+
+            return values.Where(v => v[1] > peakIndex && v[1] <= peakIndex + 1).Average(x => x[1]);
+        }
+    }
+}

src/UglyToad.PdfPig/DocumentLayoutAnalysis/IPageSegmenter.cs

@@ -0,0 +1,19 @@
+using System.Collections.Generic;
+using UglyToad.PdfPig.Content;
+
+namespace UglyToad.PdfPig.DocumentLayoutAnalysis
+{
+    /// <summary>
+    /// Page segmentation divides a page into areas, each consisting of a layout structure (blocks, lines, etc.).
+    /// <para> See 'Performance Comparison of Six Algorithms for Page Segmentation' by Faisal Shafait, Daniel Keysers, and Thomas M. Breuel.</para>
+    /// </summary>
+    public interface IPageSegmenter
+    {
+        /// <summary>
+        /// Get the text blocks.
+        /// </summary>
+        /// <param name="pageWords">The words to generate text blocks for.</param>
+        /// <returns>A list of text blocks from this approach.</returns>
+        IReadOnlyList<TextBlock> GetBlocks(IEnumerable<Word> pageWords);
+    }
+}

src/UglyToad.PdfPig/DocumentLayoutAnalysis/NearestNeighbourWordExtractor .cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
-using System.Threading.Tasks;
 using UglyToad.PdfPig.Content;
 using UglyToad.PdfPig.Geometry;
 using UglyToad.PdfPig.Util;
@@ -71,7 +70,7 @@ namespace UglyToad.PdfPig.DocumentLayoutAnalysis
         /// between 2 letters, e.g. GlyphRectangle.Width or GlyphRectangle.Height.</param>
         /// <param name="distMeasure">The distance measure between two start and end base line points,
         /// e.g. the Manhattan distance.</param>
-        private static List<Word> GetWords(IEnumerable<Letter> pageLetters,
+        private List<Word> GetWords(IEnumerable<Letter> pageLetters,
             Func<Letter, decimal> metric, Func<PdfPoint, PdfPoint, double> distMeasure)
         {
             if (pageLetters == null || pageLetters.Count() == 0) return new List<Word>();
@@ -97,116 +96,18 @@ namespace UglyToad.PdfPig.DocumentLayoutAnalysis
             }
 
             Letter[] letters = pageLetters.ToArray();
-            int lettersCount = letters.Length;
-            List<PdfPoint> startBaseLines = letters.Select(x => x.StartBaseLine).ToList();
 
-            int[] indexes = Enumerable.Repeat((int)-1, lettersCount).ToArray();
-
-            // Find nearest neighbours indexes
-            Parallel.For(0, lettersCount, c =>
-            {
-                var currentLetter = letters[c];
-                // only check neighbours if not a white space
-                if (!string.IsNullOrWhiteSpace(currentLetter.Value))
-                {
-                    int index = currentLetter.EndBaseLine.FindIndexNearest(startBaseLines, distMeasure, out double dist);
-                    var pairedLetter = letters[index];
-
-                    if (!string.IsNullOrWhiteSpace(pairedLetter.Value) &&
-                        string.Equals(currentLetter.FontName, pairedLetter.FontName, StringComparison.OrdinalIgnoreCase))
-                    {
-                        decimal minDist = Math.Max(Math.Abs(metric(currentLetter)), Math.Abs(metric(pairedLetter))) * 0.60m;
-                        if ((decimal)dist < minDist)
-                        {
-                            indexes[c] = index;
-                        }
-                    }
-                }
-            });
-
-            // Group indexes
-            List<List<int>> groupedIndexes = new List<List<int>>();
-            List<int> indexDone = new List<int>();
-            for (int c = 0; c < lettersCount; c++)
-            {
-                int i = indexes[c];
-                if (i == -1) continue;
-
-                bool isDoneC = indexDone.Contains(c);
-                bool isDoneI = indexDone.Contains(i);
-                if (isDoneC || isDoneI)
-                {
-                    if (isDoneC && !isDoneI)
-                    {
-                        foreach (var pair in groupedIndexes.Where(x => x.Contains(c)))
-                        {
-                            pair.Add(i);
-                        }
-                        indexDone.Add(i);
-                    }
-                    else if (!isDoneC && isDoneI)
-                    {
-                        foreach (var pair in groupedIndexes.Where(x => x.Contains(i)))
-                        {
-                            pair.Add(c);
-                        }
-                        indexDone.Add(c);
-                    }
-                    else
-                    {
-                        foreach (var pair in groupedIndexes.Where(x => x.Contains(i)))
-                        {
-                            if (!pair.Contains(c)) pair.Add(c);
-                        }
-
-                        foreach (var pair in groupedIndexes.Where(x => x.Contains(c)))
-                        {
-                            if (!pair.Contains(i)) pair.Add(i);
-                        }
-                    }
-                }
-                else
-                {
-                    List<int> pair = new List<int>() { c, i };
-                    groupedIndexes.Add(pair);
-                    indexDone.AddRange(pair);
-                }
-            }
-
-            // Merge lists with common index 
-            for (int c = 0; c < lettersCount; c++)
-            {
-                List<List<int>> candidates = groupedIndexes.Where(x => x.Any(t => t == c)).ToList();
-                if (candidates.Count < 2) continue; // only one group with this index
-
-                List<int> merged = candidates.First();
-                groupedIndexes.Remove(merged);
-                for (int i = 1; i < candidates.Count; i++)
-                {
-                    var current = candidates[i];
-                    merged = merged.Union(current).ToList();
-                    groupedIndexes.Remove(current);
-                }
-                groupedIndexes.Add(merged);
-            }
+            var groupedIndexes = ClusteringAlgorithms.SimpleTransitiveClosure(letters,
+                distMeasure,
+                (l1, l2) => Math.Max((double)metric(l1), (double)metric(l2)) * 0.60,
+                l => l.EndBaseLine, l => l.StartBaseLine,
+                l => !string.IsNullOrWhiteSpace(l.Value),
+                (l1, l2) => string.Equals(l1.FontName, l2.FontName, StringComparison.OrdinalIgnoreCase) && !string.IsNullOrWhiteSpace(l2.Value)).ToList();
 
             List<Word> words = new List<Word>();
             for (int a = 0; a < groupedIndexes.Count(); a++)
             {
-                List<Letter> groupedLetters = new List<Letter>();
-                foreach (int s in groupedIndexes[a])
-                {
-                    groupedLetters.Add(letters[s]);
-                }
-
-                words.Add(new Word(orderFunc(groupedLetters)));
-            }
-
-            List<int> indexesNotDone = Enumerable.Range(0, lettersCount).Except(groupedIndexes.SelectMany(x => x)).ToList();
-            for (int n = 0; n < indexesNotDone.Count(); n++)
-            {
-                Letter letter = letters[indexesNotDone[n]];
-                words.Add(new Word(new Letter[] { letter }));
+                words.Add(new Word(orderFunc(groupedIndexes[a].Select(i => letters[i]))));
             }
 
             return words;

src/UglyToad.PdfPig/DocumentLayoutAnalysis/RecursiveXYCut.cs

@@ -11,14 +11,31 @@ namespace UglyToad.PdfPig.DocumentLayoutAnalysis
     /// https://en.wikipedia.org/wiki/Recursive_X-Y_cut
     /// <para>See 'Recursive X-Y Cut using Bounding Boxes of Connected Components' by Jaekyu Ha, Robert M.Haralick and Ihsin T. Phillips</para>
     /// </summary>
-    public static class RecursiveXYCut
+    public class RecursiveXYCut : IPageSegmenter
     {
+        /// <summary>
+        /// Create an instance of Recursive X-Y Cut page segmenter, <see cref="RecursiveXYCut"/>.
+        /// </summary>
+        public static RecursiveXYCut Instance { get; } = new RecursiveXYCut();
+
         /// <summary>
         /// Get the blocks.
+        /// <para>Uses 'minimumWidth' = 0, 'dominantFontWidthFunc' = Mode(Width), 'dominantFontHeightFunc' = 1.5 x Mode(Height)</para>
+        /// </summary>
+        /// <param name="pageWords">The words in the page.</param>
+        /// <returns></returns>
+        public IReadOnlyList<TextBlock> GetBlocks(IEnumerable<Word> pageWords)
+        {
+            return GetBlocks(pageWords, 0);
+        }
+
+        /// <summary>
+        /// Get the blocks.
+        /// <para>Uses 'dominantFontWidthFunc' = Mode(Width), 'dominantFontHeightFunc' = 1.5 x Mode(Height)</para>
         /// </summary>
         /// <param name="pageWords">The words in the page.</param>
         /// <param name="minimumWidth">The minimum width for a block.</param>
-        public static XYNode GetBlocks(IEnumerable<Word> pageWords, decimal minimumWidth = 0)
+        public IReadOnlyList<TextBlock> GetBlocks(IEnumerable<Word> pageWords, decimal minimumWidth)
         {
             return GetBlocks(pageWords, minimumWidth, k => Math.Round(k.Mode(), 3), k => Math.Round(k.Mode() * 1.5m, 3));
         }
@@ -30,7 +47,7 @@ namespace UglyToad.PdfPig.DocumentLayoutAnalysis
         /// <param name="minimumWidth">The minimum width for a block.</param>
         /// <param name="dominantFontWidth">The dominant font width.</param>
         /// <param name="dominantFontHeight">The dominant font height.</param>
-        public static XYNode GetBlocks(IEnumerable<Word> pageWords, decimal minimumWidth,
+        public IReadOnlyList<TextBlock> GetBlocks(IEnumerable<Word> pageWords, decimal minimumWidth,
             decimal dominantFontWidth, decimal dominantFontHeight)
         {
             return GetBlocks(pageWords, minimumWidth, k => dominantFontWidth, k => dominantFontHeight);
@@ -43,15 +60,24 @@ namespace UglyToad.PdfPig.DocumentLayoutAnalysis
         /// <param name="minimumWidth">The minimum width for a block.</param>
         /// <param name="dominantFontWidthFunc">The function that determines the dominant font width.</param>
         /// <param name="dominantFontHeightFunc">The function that determines the dominant font height.</param>
-        public static XYNode GetBlocks(IEnumerable<Word> pageWords, decimal minimumWidth,
+        public IReadOnlyList<TextBlock> GetBlocks(IEnumerable<Word> pageWords, decimal minimumWidth,
             Func<IEnumerable<decimal>, decimal> dominantFontWidthFunc,
             Func<IEnumerable<decimal>, decimal> dominantFontHeightFunc)
         {
-            var root = new XYLeaf(pageWords); // Create a root node.
-            return VerticalCut(root, minimumWidth, dominantFontWidthFunc, dominantFontHeightFunc);
+            XYLeaf root = new XYLeaf(pageWords); // Create a root node.
+            XYNode node = VerticalCut(root, minimumWidth, dominantFontWidthFunc, dominantFontHeightFunc);
+
+            var leafs = node.GetLeafs();
+
+            if (leafs.Count > 0)
+            {
+                return leafs.Select(l => new TextBlock(l.GetLines())).ToList();
             }
 
-        private static XYNode VerticalCut(XYLeaf leaf, decimal minimumWidth,
+            return new List<TextBlock>();
+        }
+
+        private XYNode VerticalCut(XYLeaf leaf, decimal minimumWidth,
             Func<IEnumerable<decimal>, decimal> dominantFontWidthFunc,
             Func<IEnumerable<decimal>, decimal> dominantFontHeightFunc, int level = 0)
         {
@@ -144,7 +170,7 @@ namespace UglyToad.PdfPig.DocumentLayoutAnalysis
             return new XYNode(newNodes);
         }
 
-        private static XYNode HorizontalCut(XYLeaf leaf, decimal minimumWidth,
+        private XYNode HorizontalCut(XYLeaf leaf, decimal minimumWidth,
             Func<IEnumerable<decimal>, decimal> dominantFontWidthFunc,
             Func<IEnumerable<decimal>, decimal> dominantFontHeightFunc, int level = 0)
         {

src/UglyToad.PdfPig/DocumentLayoutAnalysis/XYLeaf.cs

@@ -9,7 +9,7 @@ namespace UglyToad.PdfPig.DocumentLayoutAnalysis
     /// <summary>
     /// A Leaf node used in the <see cref="RecursiveXYCut"/> algorithm, i.e. a block.
     /// </summary>
-    public class XYLeaf : XYNode
+    internal class XYLeaf : XYNode
     {
         /// <summary>
         /// Returns true if this node is a leaf, false otherwise.

src/UglyToad.PdfPig/DocumentLayoutAnalysis/XYNode.cs

@@ -8,7 +8,7 @@ namespace UglyToad.PdfPig.DocumentLayoutAnalysis
     /// <summary>
     /// A Node used in the <see cref="RecursiveXYCut"/> algorithm.
     /// </summary>
-    public class XYNode
+    internal class XYNode
     {
         /// <summary>
         /// Returns true if this node is a leaf, false otherwise.