tesseract  5.0.0
tablerecog.h
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1 // File: tablerecog.h
3 // Description: Functions to detect structure of tables.
4 // Author: Nicholas Beato
5 //
6 // (C) Copyright 2010, Google Inc.
7 // Licensed under the Apache License, Version 2.0 (the "License");
8 // you may not use this file except in compliance with the License.
9 // You may obtain a copy of the License at
10 // http://www.apache.org/licenses/LICENSE-2.0
11 // Unless required by applicable law or agreed to in writing, software
12 // distributed under the License is distributed on an "AS IS" BASIS,
13 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 // See the License for the specific language governing permissions and
15 // limitations under the License.
16 //
18 
19 #ifndef TABLERECOG_H_
20 #define TABLERECOG_H_
21 
22 #include "colpartitiongrid.h"
23 
24 namespace tesseract {
25 
26 // There are 2 classes in this file. They have 2 different purposes.
27 // - StructuredTable contains the methods to find the structure given
28 // a specific bounding box and grow that structure.
29 // - TableRecognizer contains the methods to adjust the possible positions
30 // of a table without worrying about structure.
31 //
32 // To use these classes, the assumption is that the TableFinder will
33 // have a guess of the location of a table (or possibly over/undersegmented
34 // tables). The TableRecognizer is responsible for finding the table boundaries
35 // at a high level. The StructuredTable class is responsible for determining
36 // the structure of the table and trying to maximize its bounds while retaining
37 // the structure.
38 // (The latter part is not implemented yet, but that was the goal).
39 //
40 // While on the boundary discussion, keep in mind that this is a first pass.
41 // There should eventually be some things like internal structure checks,
42 // and, more importantly, surrounding text flow checks.
43 //
44 
45 // Usage:
46 // The StructuredTable class contains methods to query a potential table.
47 // It has functions to find structure, count rows, find ColPartitions that
48 // intersect gridlines, etc. It is not meant to blindly find a table. It
49 // is meant to start with a known table location and enhance it.
50 // Usage:
51 // ColPartitionGrid text_grid, line_grid; // init
52 // TBOX table_box; // known location of table location
53 //
54 // StructuredTable table;
55 // table.Init(); // construction code
56 // table.set_text_grid(/* text */); // These 2 grids can be the same!
57 // table.set_line_grid(/* lines */);
58 // table.set_min_text_height(10); // Filter vertical and tall text.
59 // // IMPORTANT! The table needs to be told where it is!
60 // table.set_bounding_box(table_box); // Set initial table location.
61 // if (table.FindWhitespacedStructure()) {
62 // // process table
63 // table.column_count(); // number of columns
64 // table.row_count(); // number of rows
65 // table.cells_count(); // number of cells
66 // table.bounding_box(); // updated bounding box
67 // // etc.
68 // }
69 //
71 public:
73  ~StructuredTable() = default;
74 
75  // Initialization code. Must be called after the constructor.
76  void Init();
77 
78  // Sets the grids used by the table. These can be changed between
79  // calls to Recognize. They are treated as read-only data.
80  void set_text_grid(ColPartitionGrid *text);
81  void set_line_grid(ColPartitionGrid *lines);
82  // Filters text partitions that are ridiculously tall to prevent
83  // merging rows.
84  void set_max_text_height(int height);
85 
86  // Basic accessors. Some are treated as attributes despite having indirect
87  // representation.
88  bool is_lined() const;
89  unsigned row_count() const;
90  unsigned column_count() const;
91  unsigned cell_count() const;
92  void set_bounding_box(const TBOX &box);
93  const TBOX &bounding_box() const;
94  int median_cell_height();
95  int median_cell_width();
96  int row_height(unsigned row) const;
97  int column_width(unsigned column) const;
98  int space_above() const;
99  int space_below() const;
100 
101  // Given enough horizontal and vertical lines in a region, create this table
102  // based on the structure given by the lines. Return true if it worked out.
103  // Code assumes the lines exist. It is the caller's responsibility to check
104  // for lines and find an appropriate bounding box.
105  bool FindLinedStructure();
106 
107  // The main subroutine for finding generic table structure. The function
108  // finds the grid structure in the given box. Returns true if a good grid
109  // exists, implying that "this" table is valid.
110  bool FindWhitespacedStructure();
111 
115 
116  // Returns true if inserting part into the table does not cause any
117  // cell merges.
118  bool DoesPartitionFit(const ColPartition &part) const;
119  // Checks if a sub-table has multiple data cells filled.
120  int CountFilledCells();
121  int CountFilledCellsInRow(int row);
122  int CountFilledCellsInColumn(int column);
123  int CountFilledCells(unsigned row_start, unsigned row_end, unsigned column_start, unsigned column_end);
124 
125  // Makes sure that at least one cell in a row has substantial area filled.
126  // This can filter out large whitespace caused by growing tables too far
127  // and page numbers.
128  // (currently bugged for some reason).
129  bool VerifyRowFilled(int row);
130  // Finds the filled area in a cell.
131  double CalculateCellFilledPercentage(unsigned row, unsigned column);
132 
133  // Debug display, draws the table in the given color. If the table is not
134  // valid, the table and "best" grid lines are still drawn in the given color.
135  void Display(ScrollView *window, ScrollView::Color color);
136 
137 protected:
138  // Clear the structure information.
139  void ClearStructure();
140 
144 
145  // Verifies the lines do not intersect partitions. This happens when
146  // the lines are in column boundaries and extend the full page. As a result,
147  // the grid lines go through column text. The condition is detectable.
148  bool VerifyLinedTableCells();
149 
153 
154  // This is the function to change if you want to filter resulting tables
155  // better. Right now it just checks for a minimum cell count and such.
156  // You could add things like maximum number of ColPartitions per cell or
157  // similar.
158  bool VerifyWhitespacedTable();
159  // Find the columns of a table using whitespace.
160  void FindWhitespacedColumns();
161  // Find the rows of a table using whitespace.
162  void FindWhitespacedRows();
163 
167 
168  // Calculates the whitespace around the table using the table boundary and
169  // the supplied grids (set_text_grid and set_line_grid).
170  void CalculateMargins();
171  // Update the table margins with the supplied grid. This is
172  // only called by calculate margins to use multiple grid sources.
173  void UpdateMargins(ColPartitionGrid *grid);
174  int FindVerticalMargin(ColPartitionGrid *grid, int start_x, bool decrease) const;
175  int FindHorizontalMargin(ColPartitionGrid *grid, int start_y, bool decrease) const;
176  // Calculates stats on the table, namely the median cell height and width.
177  void CalculateStats();
178 
182 
183  // Given a whitespaced table, this looks for bordering lines that might
184  // be page layout boxes around the table. It is necessary to get the margins
185  // correct on the table. If the lines are not joined, the margins will be
186  // the distance to the line, which is not right.
187  void AbsorbNearbyLines();
188 
189  // Nice utility function for finding partition gaps. You feed it a sorted
190  // list of all of the mins/maxes of the partitions in the table, and it gives
191  // you the gaps (middle). This works for both vertical and horizontal
192  // gaps.
193  //
194  // If you want to allow slight overlap in the division and the partitions,
195  // just scale down the partitions before inserting them in the list.
196  // Likewise, you can force at least some space between partitions.
197  // This trick is how the horizontal partitions are done (since the page
198  // skew could make it hard to find splits in the text).
199  //
200  // As a result, "0 distance" between closest partitions causes a gap.
201  // This is not a programmatic assumption. It is intentional and simplifies
202  // things.
203  //
204  // "max_merged" indicates both the minimum number of stacked partitions
205  // to cause a cell (add 1 to it), and the maximum number of partitions that
206  // a grid line can intersect. For example, if max_merged is 0, then lines
207  // are inserted wherever space exists between partitions. If it is 2,
208  // lines may intersect 2 partitions at most, but you also need at least
209  // 2 partitions to generate a line.
210  static void FindCellSplitLocations(const std::vector<int> &min_list,
211  const std::vector<int> &max_list, int max_merged,
212  std::vector<int> *locations);
213 
217 
218  // Counts the number of ColPartitions that intersect vertical cell
219  // division at this x value. Used by VerifyLinedTable.
220  int CountVerticalIntersections(int x);
221  int CountHorizontalIntersections(int y);
222 
223  // Counts how many text partitions are in this box.
224  int CountPartitions(const TBOX &box);
225 
229 
230  // Input data, used as read only data to make decisions.
231  ColPartitionGrid *text_grid_; // Text ColPartitions
232  ColPartitionGrid *line_grid_; // Line ColPartitions
233  // Table structure.
234  // bounding box is a convenient external representation.
235  // cell_x_ and cell_y_ indicate the grid lines.
236  TBOX bounding_box_; // Bounding box
237  std::vector<int> cell_x_; // Locations of vertical divisions (sorted)
238  std::vector<int> cell_y_; // Locations of horizontal divisions (sorted)
239  bool is_lined_; // Is the table backed up by a line structure
240  // Table margins, set via CalculateMargins
247  // Filters, used to prevent awkward partitions from destroying structure.
249 };
250 
252 public:
253  TableRecognizer() = default;
254  ~TableRecognizer() = default;
255 
256  // Initialization code. Must be called after the constructor.
257  void Init();
258 
262 
263  // Sets the grids used by the table. These can be changed between
264  // calls to Recognize. They are treated as read-only data.
265  void set_text_grid(ColPartitionGrid *text);
266  void set_line_grid(ColPartitionGrid *lines);
267  // Sets some additional constraints on the table.
268  void set_min_height(int height);
269  void set_min_width(int width);
270  // Filters text partitions that are ridiculously tall to prevent
271  // merging rows. Note that "filters" refers to allowing horizontal
272  // cells to slice through them on the premise that they were
273  // merged text rows during previous layout.
274  void set_max_text_height(int height);
275 
276  // Given a guess location, the RecognizeTable function will try to find a
277  // structured grid in the area. On success, it will return a new
278  // StructuredTable (and assumes you will delete it). Otherwise,
279  // nullptr is returned.
280  //
281  // Keep in mind, this may "overgrow" or "undergrow" the size of guess.
282  // Ideally, there is a either a one-to-one correspondence between
283  // the guess and table or no table at all. This is not the best of
284  // assumptions right now, but was made to try to keep things simple in
285  // the first pass.
286  //
287  // If a line structure is available on the page in the given region,
288  // the table will use the linear structure as it is.
289  // Otherwise, it will try to maximize the whitespace around it while keeping
290  // a grid structure. This is somewhat working.
291  //
292  // Since the combination of adjustments can get high, effort was
293  // originally made to keep the number of adjustments linear in the number
294  // of partitions. The underlying structure finding code used to be
295  // much more complex. I don't know how necessary this constraint is anymore.
296  // The evaluation of a possible table is kept within O(nlogn) in the size of
297  // the table (where size is the number of partitions in the table).
298  // As a result, the algorithm is capable of O(n^2 log n). Depending
299  // on the grid search size, it may be higher.
300  //
301  // Last note: it is possible to just try all partition boundaries at a high
302  // level O(n^4) and do a verification scheme (at least O(nlogn)). If there
303  // area 200 partitions on a page, this could be too costly. Effort could go
304  // into pruning the search, but I opted for something quicker. I'm confident
305  // that the independent adjustments can get similar results and keep the
306  // complextiy down. However, the other approach could work without using
307  // TableFinder at all if it is fast enough. It comes down to properly
308  // deciding what is a table. The code currently relies on TableFinder's
309  // guess to the location of a table for that.
310  StructuredTable *RecognizeTable(const TBOX &guess_box);
311 
312 protected:
316 
317  // Returns true if the given box has a lined table within it. The
318  // table argument will be updated with the table if the table exists.
319  bool RecognizeLinedTable(const TBOX &guess_box, StructuredTable *table);
320  // Returns true if the given box has a large number of horizontal and
321  // vertical lines present. If so, we assume the extent of these lines
322  // uniquely defines a table and find that table via SolveLinedTable.
323  bool HasSignificantLines(const TBOX &guess);
324 
325  // Given enough horizontal and vertical lines in a region, find a bounding
326  // box that encloses all of them (as well as newly introduced lines).
327  // The bounding box is the smallest box that encloses the lines in guess
328  // without having any lines sticking out of it.
329  // bounding_box is an in/out parameter.
330  // On input, it in the extents of the box to search.
331  // On output, it is the resulting bounding box.
332  bool FindLinesBoundingBox(TBOX *bounding_box);
333  // Iteration in above search.
334  // bounding_box is an in/out parameter.
335  // On input, it in the extents of the box to search.
336  // On output, it is the resulting bounding box.
337  bool FindLinesBoundingBoxIteration(TBOX *bounding_box);
338 
342 
343  // Returns true if the given box has a whitespaced table within it. The
344  // table argument will be updated if the table exists. Also note
345  // that this method will fail if the guess_box center is not
346  // mostly within the table.
347  bool RecognizeWhitespacedTable(const TBOX &guess_box, StructuredTable *table);
348 
349  // Finds the location of a horizontal split relative to y.
350  // This function is mostly unused now. If the SolveWhitespacedTable
351  // changes much, it can be removed. Note, it isn't really as reliable
352  // as I thought. I went with alternatives for most of the other uses.
353  int NextHorizontalSplit(int left, int right, int y, bool top_to_bottom);
354 
355  // Indicates that a table row is weak. This means that it has
356  // many missing data cells or very large cell heights compared.
357  // to the rest of the table.
358  static bool IsWeakTableRow(StructuredTable *table, int row);
359 
360  // Input data, used as read only data to make decisions.
361  ColPartitionGrid *text_grid_ = nullptr; // Text ColPartitions
362  ColPartitionGrid *line_grid_ = nullptr; // Line ColPartitions
363  // Table constraints, a "good" table must satisfy these.
364  int min_height_ = 0;
365  int min_width_ = 0;
366  // Filters, used to prevent awkward partitions from destroying structure.
367  int max_text_height_ = INT32_MAX; // Horizontal lines may intersect taller text.
368 };
369 
370 } // namespace tesseract
371 
372 #endif /* TABLERECOG_H_ */
std::vector< int > cell_y_
Definition: tablerecog.h:238
ColPartitionGrid * text_grid_
Definition: tablerecog.h:231
std::vector< int > cell_x_
Definition: tablerecog.h:237
ColPartitionGrid * line_grid_
Definition: tablerecog.h:232
#define TESS_API
Definition: export.h:34