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tablerecog.h
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1
2 // File: tablerecog.h
3 // Description: Functions to detect structure of tables.
4 // Author: Nicholas Beato
5 // Created: Aug 17, 2010
6 //
7 // (C) Copyright 2010, Google Inc.
8 // Licensed under the Apache License, Version 2.0 (the "License");
9 // you may not use this file except in compliance with the License.
10 // You may obtain a copy of the License at
11 // http://www.apache.org/licenses/LICENSE-2.0
12 // Unless required by applicable law or agreed to in writing, software
13 // distributed under the License is distributed on an "AS IS" BASIS,
14 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15 // See the License for the specific language governing permissions and
16 // limitations under the License.
17 //
19
20 #ifndef TABLERECOG_H_
21 #define TABLERECOG_H_
22
23 #include "colpartitiongrid.h"
24 #include "genericvector.h"
25
26 namespace tesseract {
27
28 // There are 2 classes in this file. They have 2 different purposes.
29 // - StructuredTable contains the methods to find the structure given
30 // a specific bounding box and grow that structure.
31 // - TableRecognizer contains the methods to adjust the possible positions
32 // of a table without worrying about structure.
33 //
34 // To use these classes, the assumption is that the TableFinder will
35 // have a guess of the location of a table (or possibly over/undersegmented
36 // tables). The TableRecognizer is responsible for finding the table boundaries
37 // at a high level. The StructuredTable class is responsible for determining
38 // the structure of the table and trying to maximize its bounds while retaining
39 // the structure.
40 // (The latter part is not implemented yet, but that was the goal).
41 //
42 // While on the boundary discussion, keep in mind that this is a first pass.
43 // There should eventually be some things like internal structure checks,
44 // and, more importantly, surrounding text flow checks.
45 //
46
47 // Usage:
48 // The StructuredTable class contains methods to query a potential table.
49 // It has functions to find structure, count rows, find ColPartitions that
50 // intersect gridlines, etc. It is not meant to blindly find a table. It
51 // is meant to start with a known table location and enhance it.
52 // Usage:
53 // ColPartitionGrid text_grid, line_grid; // init
54 // TBOX table_box; // known location of table location
55 //
56 // StructuredTable table;
57 // table.Init(); // construction code
58 // table.set_text_grid(/* text */); // These 2 grids can be the same!
59 // table.set_line_grid(/* lines */);
60 // table.set_min_text_height(10); // Filter vertical and tall text.
61 // // IMPORTANT! The table needs to be told where it is!
62 // table.set_bounding_box(table_box); // Set initial table location.
63 // if (table.FindWhitespacedStructure()) {
64 // // process table
65 // table.column_count(); // number of columns
66 // table.row_count(); // number of rows
67 // table.cells_count(); // number of cells
68 // table.bounding_box(); // updated bounding box
69 // // etc.
70 // }
71 //
72 class StructuredTable {
73 public:
74 StructuredTable();
75 ~StructuredTable();
76
77 // Initialization code. Must be called after the constructor.
78 void Init();
79
80 // Sets the grids used by the table. These can be changed between
81 // calls to Recognize. They are treated as read-only data.
82 void set_text_grid(ColPartitionGrid* text);
83 void set_line_grid(ColPartitionGrid* lines);
84 // Filters text partitions that are ridiculously tall to prevent
85 // merging rows.
86 void set_max_text_height(int height);
87
88 // Basic accessors. Some are treated as attributes despite having indirect
89 // representation.
90 bool is_lined() const;
91 int row_count() const;
92 int column_count() const;
93 int cell_count() const;
94 void set_bounding_box(const TBOX& box);
95 const TBOX& bounding_box() const;
96 int median_cell_height();
97 int median_cell_width();
98 int row_height(int row) const;
99 int column_width(int column) const;
100 int space_above() const;
101 int space_below() const;
102
103 // Given enough horizontal and vertical lines in a region, create this table
104 // based on the structure given by the lines. Return true if it worked out.
105 // Code assumes the lines exist. It is the caller's responsibility to check
106 // for lines and find an appropriate bounding box.
107 bool FindLinedStructure();
108
109 // The main subroutine for finding generic table structure. The function
110 // finds the grid structure in the given box. Returns true if a good grid
111 // exists, implying that "this" table is valid.
112 bool FindWhitespacedStructure();
113
117
118 // Returns true if inserting part into the table does not cause any
119 // cell merges.
120 bool DoesPartitionFit(const ColPartition& part) const;
121 // Checks if a sub-table has multiple data cells filled.
122 int CountFilledCells();
123 int CountFilledCellsInRow(int row);
124 int CountFilledCellsInColumn(int column);
125 int CountFilledCells(int row_start, int row_end,
126 int column_start, int column_end);
127
128 // Makes sure that at least one cell in a row has substantial area filled.
129 // This can filter out large whitespace caused by growing tables too far
130 // and page numbers.
131 // (currently bugged for some reason).
132 bool VerifyRowFilled(int row);
133 // Finds the filled area in a cell.
134 double CalculateCellFilledPercentage(int row, int column);
135
136 // Debug display, draws the table in the given color. If the table is not
137 // valid, the table and "best" grid lines are still drawn in the given color.
138 void Display(ScrollView* window, ScrollView::Color color);
139
140 protected:
141 // Clear the structure information.
142 void ClearStructure();
143
147
148 // Verifies the lines do not intersect partitions. This happens when
149 // the lines are in column boundaries and extend the full page. As a result,
150 // the grid lines go through column text. The condition is detectable.
151 bool VerifyLinedTableCells();
152
156
157 // This is the function to change if you want to filter resulting tables
158 // better. Right now it just checks for a minimum cell count and such.
159 // You could add things like maximum number of ColPartitions per cell or
160 // similar.
161 bool VerifyWhitespacedTable();
162 // Find the columns of a table using whitespace.
163 void FindWhitespacedColumns();
164 // Find the rows of a table using whitespace.
165 void FindWhitespacedRows();
166
170
171 // Calculates the whitespace around the table using the table boundary and
172 // the supplied grids (set_text_grid and set_line_grid).
173 void CalculateMargins();
174 // Update the table margins with the supplied grid. This is
175 // only called by calculate margins to use multiple grid sources.
176 void UpdateMargins(ColPartitionGrid* grid);
177 int FindVerticalMargin(ColPartitionGrid* grid, int start_x,
178 bool decrease) const;
179 int FindHorizontalMargin(ColPartitionGrid* grid, int start_y,
180 bool decrease) const;
181 // Calculates stats on the table, namely the median cell height and width.
182 void CalculateStats();
183
187
188 // Given a whitespaced table, this looks for bordering lines that might
189 // be page layout boxes around the table. It is necessary to get the margins
190 // correct on the table. If the lines are not joined, the margins will be
191 // the distance to the line, which is not right.
192 void AbsorbNearbyLines();
193
194 // Nice utility function for finding partition gaps. You feed it a sorted
195 // list of all of the mins/maxes of the partitions in the table, and it gives
196 // you the gaps (middle). This works for both vertical and horizontal
197 // gaps.
198 //
199 // If you want to allow slight overlap in the division and the partitions,
200 // just scale down the partitions before inserting them in the list.
201 // Likewise, you can force at least some space between partitions.
202 // This trick is how the horizontal partitions are done (since the page
203 // skew could make it hard to find splits in the text).
204 //
205 // As a result, "0 distance" between closest partitions causes a gap.
206 // This is not a programmatic assumption. It is intentional and simplifies
207 // things.
208 //
209 // "max_merged" indicates both the minimum number of stacked partitions
210 // to cause a cell (add 1 to it), and the maximum number of partitions that
211 // a grid line can intersect. For example, if max_merged is 0, then lines
212 // are inserted wherever space exists between partitions. If it is 2,
213 // lines may intersect 2 partitions at most, but you also need at least
214 // 2 partitions to generate a line.
215 static void FindCellSplitLocations(const GenericVector<int>& min_list,
216 const GenericVector<int>& max_list,
217 int max_merged,
218 GenericVector<int>* locations);
219
223
224 // Counts the number of ColPartitions that intersect vertical cell
225 // division at this x value. Used by VerifyLinedTable.
226 int CountVerticalIntersections(int x);
227 int CountHorizontalIntersections(int y);
228
229 // Counts how many text partitions are in this box.
230 int CountPartitions(const TBOX& box);
231
235
236 // Input data, used as read only data to make decisions.
237 ColPartitionGrid* text_grid_; // Text ColPartitions
238 ColPartitionGrid* line_grid_; // Line ColPartitions
239 // Table structure.
240 // bounding box is a convenient external representation.
241 // cell_x_ and cell_y_ indicate the grid lines.
242 TBOX bounding_box_; // Bounding box
243 GenericVectorEqEq<int> cell_x_; // Locations of vertical divisions (sorted)
244 GenericVectorEqEq<int> cell_y_; // Locations of horizontal divisions (sorted)
245 bool is_lined_; // Is the table backed up by a line structure
246 // Table margins, set via CalculateMargins
247 int space_above_;
248 int space_below_;
249 int space_left_;
250 int space_right_;
251 int median_cell_height_;
252 int median_cell_width_;
253 // Filters, used to prevent awkward partitions from destroying structure.
254 int max_text_height_;
255 };
256
257 class TableRecognizer {
258 public:
259 TableRecognizer();
260 ~TableRecognizer();
261
262 // Initialization code. Must be called after the constructor.
263 void Init();
264
268
269 // Sets the grids used by the table. These can be changed between
270 // calls to Recognize. They are treated as read-only data.
271 void set_text_grid(ColPartitionGrid* text);
272 void set_line_grid(ColPartitionGrid* lines);
273 // Sets some additional constraints on the table.
274 void set_min_height(int height);
275 void set_min_width(int width);
276 // Filters text partitions that are ridiculously tall to prevent
277 // merging rows. Note that "filters" refers to allowing horizontal
278 // cells to slice through them on the premise that they were
279 // merged text rows during previous layout.
280 void set_max_text_height(int height);
281
282 // Given a guess location, the RecognizeTable function will try to find a
283 // structured grid in the area. On success, it will return a new
284 // StructuredTable (and assumes you will delete it). Otherwise,
285 // NULL is returned.
286 //
287 // Keep in mind, this may "overgrow" or "undergrow" the size of guess.
288 // Ideally, there is a either a one-to-one correspondence between
289 // the guess and table or no table at all. This is not the best of
290 // assumptions right now, but was made to try to keep things simple in
291 // the first pass.
292 //
293 // If a line structure is available on the page in the given region,
294 // the table will use the linear structure as it is.
295 // Otherwise, it will try to maximize the whitespace around it while keeping
296 // a grid structure. This is somewhat working.
297 //
298 // Since the combination of adjustments can get high, effort was
299 // originally made to keep the number of adjustments linear in the number
300 // of partitions. The underlying structure finding code used to be
301 // much more complex. I don't know how necessary this constraint is anymore.
302 // The evaluation of a possible table is kept within O(nlogn) in the size of
303 // the table (where size is the number of partitions in the table).
304 // As a result, the algorithm is capable of O(n^2 log n). Depending
305 // on the grid search size, it may be higher.
306 //
307 // Last note: it is possible to just try all partition boundaries at a high
308 // level O(n^4) and do a verification scheme (at least O(nlogn)). If there
309 // area 200 partitions on a page, this could be too costly. Effort could go
310 // into pruning the search, but I opted for something quicker. I'm confident
311 // that the independent adjustments can get similar results and keep the
312 // complextiy down. However, the other approach could work without using
313 // TableFinder at all if it is fast enough. It comes down to properly
314 // deciding what is a table. The code currently relies on TableFinder's
315 // guess to the location of a table for that.
316 StructuredTable* RecognizeTable(const TBOX& guess_box);
317
318 protected:
322
323 // Returns true if the given box has a lined table within it. The
324 // table argument will be updated with the table if the table exists.
325 bool RecognizeLinedTable(const TBOX& guess_box, StructuredTable* table);
326 // Returns true if the given box has a large number of horizontal and
327 // vertical lines present. If so, we assume the extent of these lines
328 // uniquely defines a table and find that table via SolveLinedTable.
329 bool HasSignificantLines(const TBOX& guess);
330
331 // Given enough horizontal and vertical lines in a region, find a bounding
332 // box that encloses all of them (as well as newly introduced lines).
333 // The bounding box is the smallest box that encloses the lines in guess
334 // without having any lines sticking out of it.
335 // bounding_box is an in/out parameter.
336 // On input, it in the extents of the box to search.
337 // On output, it is the resulting bounding box.
338 bool FindLinesBoundingBox(TBOX* bounding_box);
339 // Iteration in above search.
340 // bounding_box is an in/out parameter.
341 // On input, it in the extents of the box to search.
342 // On output, it is the resulting bounding box.
343 bool FindLinesBoundingBoxIteration(TBOX* bounding_box);
344
348
349 // Returns true if the given box has a whitespaced table within it. The
350 // table argument will be updated if the table exists. Also note
351 // that this method will fail if the guess_box center is not
352 // mostly within the table.
353 bool RecognizeWhitespacedTable(const TBOX& guess_box, StructuredTable* table);
354
355 // Finds the location of a horizontal split relative to y.
356 // This function is mostly unused now. If the SolveWhitespacedTable
357 // changes much, it can be removed. Note, it isn't really as reliable
358 // as I thought. I went with alternatives for most of the other uses.
359 int NextHorizontalSplit(int left, int right, int y, bool top_to_bottom);
360
361 // Indicates that a table row is weak. This means that it has
362 // many missing data cells or very large cell heights compared.
363 // to the rest of the table.
364 static bool IsWeakTableRow(StructuredTable* table, int row);
365
366 // Input data, used as read only data to make decisions.
367 ColPartitionGrid* text_grid_; // Text ColPartitions
368 ColPartitionGrid* line_grid_; // Line ColPartitions
369 // Table constraints, a "good" table must satisfy these.
370 int min_height_;
371 int min_width_;
372 // Filters, used to prevent awkward partitions from destroying structure.
373 int max_text_height_; // Horizontal lines may intersect taller text.
374 };
375
376 } // namespace tesseract
377
378 #endif /* TABLERECOG_H_ */