detlinefit.h
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1 // File: detlinefit.h
3 // Description: Deterministic least upper-quartile squares line fitting.
4 // Author: Ray Smith
5 // Created: Thu Feb 28 14:35:01 PDT 2008
6 //
9 // you may not use this file except in compliance with the License.
10 // You may obtain a copy of the License at
12 // Unless required by applicable law or agreed to in writing, software
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 TESSERACT_CCSTRUCT_DETLINEFIT_H_
21 #define TESSERACT_CCSTRUCT_DETLINEFIT_H_
22
23 #include "genericvector.h"
24 #include "kdpair.h"
25 #include "points.h"
26
27 namespace tesseract {
28
29 // This class fits a line to a set of ICOORD points.
30 // There is no restriction on the direction of the line, as it
31 // uses a vector method, ie no concern over infinite gradients.
32 // The fitted line has the least upper quartile of squares of perpendicular
33 // distances of all source points from the line, subject to the constraint
34 // that the line is made from one of the pairs of [{p1,p2,p3},{pn-2, pn-1, pn}]
35 // i.e. the 9 combinations of one of the first 3 and last 3 points.
36 // A fundamental assumption of this algorithm is that one of the first 3 and
37 // one of the last 3 points are near the best line fit.
38 // The points must be Added in line order for the algorithm to work properly.
39 // No floating point calculations are needed* to make an accurate fit,
40 // and no random numbers are needed** so the algorithm is deterministic,
41 // architecture-stable, and compiler-stable as well as stable to minor
42 // changes in the input.
43 // *A single floating point division is used to compute each line's distance.
44 // This is unlikely to result in choice of a different line, but if it does,
45 // it would be easy to replace with a 64 bit integer calculation.
46 // **Random numbers are used in the nth_item function, but the worst
47 // non-determinism that can result is picking a different result among equals,
48 // and that wouldn't make any difference to the end-result distance, so the
49 // randomness does not affect the determinism of the algorithm. The random
50 // numbers are only there to guarantee average linear time.
51 // Fitting time is linear, but with a high constant, as it tries 9 different
52 // lines and computes the distance of all points each time.
53 // This class is aimed at replacing the LLSQ (linear least squares) and
54 // LMS (least median of squares) classes that are currently used for most
55 // of the line fitting in Tesseract.
56 class DetLineFit {
57  public:
58  DetLineFit();
59  ~DetLineFit();
60
61  // Delete all Added points.
62  void Clear();
63
64  // Adds a new point. Takes a copy - the pt doesn't need to stay in scope.
65  // Add must be called on points in sequence along the line.
67  // Associates a half-width with the given point if a point overlaps the
68  // previous point by more than half the width, and its distance is further
69  // than the previous point, then the more distant point is ignored in the
70  // distance calculation. Useful for ignoring i dots and other diacritics.
71  void Add(const ICOORD& pt, int halfwidth);
72
73  // Fits a line to the points, returning the fitted line as a pair of
74  // points, and the upper quartile error.
75  double Fit(ICOORD* pt1, ICOORD* pt2) {
76  return Fit(0, 0, pt1, pt2);
77  }
78  // Fits a line to the points, ignoring the skip_first initial points and the
79  // skip_last final points, returning the fitted line as a pair of points,
80  // and the upper quartile error.
81  double Fit(int skip_first, int skip_last, ICOORD* pt1, ICOORD* pt2);
82
83  // Constrained fit with a supplied direction vector. Finds the best line_pt,
84  // that is one of the supplied points having the median cross product with
85  // direction, ignoring points that have a cross product outside of the range
86  // [min_dist, max_dist]. Returns the resulting error metric using the same
87  // reduced set of points.
88  // *Makes use of floating point arithmetic*
89  double ConstrainedFit(const FCOORD& direction,
90  double min_dist, double max_dist,
91  bool debug, ICOORD* line_pt);
92
93  // Returns true if there were enough points at the last call to Fit or
94  // ConstrainedFit for the fitted points to be used on a badly fitted line.
96
97  // Backwards compatible fit returning a gradient and constant.
98  // Deprecated. Prefer Fit(ICOORD*, ICOORD*) where possible, but use this
99  // function in preference to the LMS class.
100  double Fit(float* m, float* c);
101
102  // Backwards compatible constrained fit with a supplied gradient.
103  // Deprecated. Use ConstrainedFit(const FCOORD& direction) where possible
104  // to avoid potential difficulties with infinite gradients.
105  double ConstrainedFit(double m, float* c);
106
107  private:
108  // Simple struct to hold an ICOORD point and a halfwidth representing half
109  // the "width" (supposedly approximately parallel to the direction of the
110  // line) of each point, such that distant points can be discarded when they
111  // overlap nearer points. (Think i dot and other diacritics or noise.)
112  struct PointWidth {
113  PointWidth() : pt(ICOORD(0, 0)), halfwidth(0) {}
114  PointWidth(const ICOORD& pt0, int halfwidth0)
115  : pt(pt0), halfwidth(halfwidth0) {}
116
117  ICOORD pt;
118  int halfwidth;
119  };
120  // Type holds the distance of each point from the fitted line and the point
121  // itself. Use of double allows integer distances from ICOORDs to be stored
122  // exactly, and also the floating point results from ConstrainedFit.
123  typedef KDPairInc<double, ICOORD> DistPointPair;
124
125  // Computes and returns the squared evaluation metric for a line fit.
126  double EvaluateLineFit();
127
128  // Computes the absolute values of the precomputed distances_,
129  // and returns the squared upper-quartile error distance.
130  double ComputeUpperQuartileError();
131
132  // Returns the number of sample points that have an error more than threshold.
133  int NumberOfMisfittedPoints(double threshold) const;
134
135  // Computes all the cross product distances of the points from the line,
136  // storing the actual (signed) cross products in distances_.
137  // Ignores distances of points that are further away than the previous point,
138  // and overlaps the previous point by at least half.
139  void ComputeDistances(const ICOORD& start, const ICOORD& end);
140
141  // Computes all the cross product distances of the points perpendicular to
142  // the given direction, ignoring distances outside of the give distance range,
143  // storing the actual (signed) cross products in distances_.
144  void ComputeConstrainedDistances(const FCOORD& direction,
145  double min_dist, double max_dist);
146
147  // Stores all the source points in the order they were given and their
148  // halfwidths, if any.
150  // Stores the computed perpendicular distances of (some of) the pts_ from a
151  // given vector (assuming it goes through the origin, making it a line).
152  // Since the distances may be a subset of the input points, and get
153  // re-ordered by the nth_item function, the original point is stored
154  // along side the distance.
155  GenericVector<DistPointPair> distances_; // Distances of points.
156  // The squared length of the vector used to compute distances_.
157  double square_length_;
158 };
159
160 } // namespace tesseract.
161
162 #endif // TESSERACT_CCSTRUCT_DETLINEFIT_H_
163
164
int direction(EDGEPT *point)
Definition: vecfuncs.cpp:43
bool SufficientPointsForIndependentFit() const
Definition: detlinefit.cpp:163
integer coordinate
Definition: points.h:30