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comparison mupdf-source/thirdparty/tesseract/src/textord/textlineprojection.h @ 2:b50eed0cc0ef upstream

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ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4. The directory name has changed: no version number in the expanded directory now.
author Franz Glasner <fzglas.hg@dom66.de>
date Mon, 15 Sep 2025 11:43:07 +0200
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1 // Copyright 2011 Google Inc. All Rights Reserved.
2 // Author: rays@google.com (Ray Smith)
3 //
4 // Licensed under the Apache License, Version 2.0 (the "License");
5 // you may not use this file except in compliance with the License.
6 // You may obtain a copy of the License at
7 // http://www.apache.org/licenses/LICENSE-2.0
8 // Unless required by applicable law or agreed to in writing, software
9 // distributed under the License is distributed on an "AS IS" BASIS,
10 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
11 // See the License for the specific language governing permissions and
12 // limitations under the License.
13
14 #ifndef TESSERACT_TEXTORD_TEXTLINEPROJECTION_H_
15 #define TESSERACT_TEXTORD_TEXTLINEPROJECTION_H_
16
17 #include "blobgrid.h" // For BlobGrid
18
19 struct Pix;
20
21 namespace tesseract {
22
23 class DENORM;
24 struct TPOINT;
25 class ColPartition;
26
27 // Simple class to encapsulate the computation of an image representing
28 // local textline density, and function(s) to make use of it.
29 // The underlying principle is that if you smear connected components
30 // horizontally (vertically for components on a vertically written textline)
31 // and count the number of smeared components in an image, then the resulting
32 // image shows the density of the textlines at each image position.
33 class TESS_API TextlineProjection {
34 public:
35 // The down-scaling factor is computed to obtain a projection resolution
36 // of about 100 dpi, whatever the input.
37 explicit TextlineProjection(int resolution);
38 ~TextlineProjection();
39
40 // Build the projection profile given the input_block containing lists of
41 // blobs, a rotation to convert to image coords,
42 // and a full-resolution nontext_map, marking out areas to avoid.
43 // During construction, we have the following assumptions:
44 // The rotation is a multiple of 90 degrees, ie no deskew yet.
45 // The blobs have had their left and right rules set to also limit
46 // the range of projection.
47 void ConstructProjection(TO_BLOCK *input_block, const FCOORD &rotation, Image nontext_map);
48
49 // Display the blobs in the window colored according to textline quality.
50 void PlotGradedBlobs(BLOBNBOX_LIST *blobs, ScrollView *win);
51
52 // Moves blobs that look like they don't sit well on a textline from the
53 // input blobs list to the output small_blobs list.
54 // This gets them away from initial textline finding to stop diacritics
55 // from forming incorrect textlines. (Introduced mainly to fix Thai.)
56 void MoveNonTextlineBlobs(BLOBNBOX_LIST *blobs, BLOBNBOX_LIST *small_blobs) const;
57
58 // Create a window and display the projection in it.
59 void DisplayProjection() const;
60
61 // Compute the distance of the box from the partition using curved projection
62 // space. As DistanceOfBoxFromBox, except that the direction is taken from
63 // the ColPartition and the median bounds of the ColPartition are used as
64 // the to_box.
65 int DistanceOfBoxFromPartition(const TBOX &box, const ColPartition &part, const DENORM *denorm,
66 bool debug) const;
67
68 // Compute the distance from the from_box to the to_box using curved
69 // projection space. Separation that involves a decrease in projection
70 // density (moving from the from_box to the to_box) is weighted more heavily
71 // than constant density, and an increase is weighted less.
72 // If horizontal_textline is true, then curved space is used vertically,
73 // as for a diacritic on the edge of a textline.
74 // The projection uses original image coords, so denorm is used to get
75 // back to the image coords from box/part space.
76 int DistanceOfBoxFromBox(const TBOX &from_box, const TBOX &to_box, bool horizontal_textline,
77 const DENORM *denorm, bool debug) const;
78
79 // Compute the distance between (x, y1) and (x, y2) using the rule that
80 // a decrease in textline density is weighted more heavily than an increase.
81 // The coordinates are in source image space, ie processed by any denorm
82 // already, but not yet scaled by scale_factor_.
83 // Going from the outside of a textline to the inside should measure much
84 // less distance than going from the inside of a textline to the outside.
85 int VerticalDistance(bool debug, int x, int y1, int y2) const;
86
87 // Compute the distance between (x1, y) and (x2, y) using the rule that
88 // a decrease in textline density is weighted more heavily than an increase.
89 int HorizontalDistance(bool debug, int x1, int x2, int y) const;
90
91 // Returns true if the blob appears to be outside of a horizontal textline.
92 // Such blobs are potentially diacritics (even if large in Thai) and should
93 // be kept away from initial textline finding.
94 bool BoxOutOfHTextline(const TBOX &box, const DENORM *denorm, bool debug) const;
95
96 // Evaluates the textlineiness of a ColPartition. Uses EvaluateBox below,
97 // but uses the median top/bottom for horizontal and median left/right for
98 // vertical instead of the bounding box edges.
99 // Evaluates for both horizontal and vertical and returns the best result,
100 // with a positive value for horizontal and a negative value for vertical.
101 int EvaluateColPartition(const ColPartition &part, const DENORM *denorm, bool debug) const;
102
103 // Computes the mean projection gradients over the horizontal and vertical
104 // edges of the box:
105 // -h-h-h-h-h-h
106 // |------------| mean=htop -v|+v--------+v|-v
107 // |+h+h+h+h+h+h| -v|+v +v|-v
108 // | | -v|+v +v|-v
109 // | box | -v|+v box +v|-v
110 // | | -v|+v +v|-v
111 // |+h+h+h+h+h+h| -v|+v +v|-v
112 // |------------| mean=hbot -v|+v--------+v|-v
113 // -h-h-h-h-h-h
114 // mean=vleft mean=vright
115 //
116 // Returns MAX(htop,hbot) - MAX(vleft,vright), which is a positive number
117 // for a horizontal textline, a negative number for a vertical textline,
118 // and near zero for undecided. Undecided is most likely non-text.
119 int EvaluateBox(const TBOX &box, const DENORM *denorm, bool debug) const;
120
121 private:
122 // Internal version of EvaluateBox returns the unclipped gradients as well
123 // as the result of EvaluateBox.
124 // hgrad1 and hgrad2 are the gradients for the horizontal textline.
125 int EvaluateBoxInternal(const TBOX &box, const DENORM *denorm, bool debug, int *hgrad1,
126 int *hgrad2, int *vgrad1, int *vgrad2) const;
127
128 // Helper returns the mean gradient value for the horizontal row at the given
129 // y, (in the external coordinates) by subtracting the mean of the transformed
130 // row 2 pixels above from the mean of the transformed row 2 pixels below.
131 // This gives a positive value for a good top edge and negative for bottom.
132 // Returns the best result out of +2/-2, +3/-1, +1/-3 pixels from the edge.
133 int BestMeanGradientInRow(const DENORM *denorm, int16_t min_x, int16_t max_x, int16_t y,
134 bool best_is_max) const;
135
136 // Helper returns the mean gradient value for the vertical column at the
137 // given x, (in the external coordinates) by subtracting the mean of the
138 // transformed column 2 pixels left from the mean of the transformed column
139 // 2 pixels to the right.
140 // This gives a positive value for a good left edge and negative for right.
141 // Returns the best result out of +2/-2, +3/-1, +1/-3 pixels from the edge.
142 int BestMeanGradientInColumn(const DENORM *denorm, int16_t x, int16_t min_y, int16_t max_y,
143 bool best_is_max) const;
144
145 // Helper returns the mean pixel value over the line between the start_pt and
146 // end_pt (inclusive), but shifted perpendicular to the line in the projection
147 // image by offset pixels. For simplicity, it is assumed that the vector is
148 // either nearly horizontal or nearly vertical. It works on skewed textlines!
149 // The end points are in external coordinates, and will be denormalized with
150 // the denorm if not nullptr before further conversion to pix coordinates.
151 // After all the conversions, the offset is added to the direction
152 // perpendicular to the line direction. The offset is thus in projection image
153 // coordinates, which allows the caller to get a guaranteed displacement
154 // between pixels used to calculate gradients.
155 int MeanPixelsInLineSegment(const DENORM *denorm, int offset, TPOINT start_pt,
156 TPOINT end_pt) const;
157
158 // Helper function to add 1 to a rectangle in source image coords to the
159 // internal projection pix_.
160 void IncrementRectangle8Bit(const TBOX &box);
161 // Inserts a list of blobs into the projection.
162 // Rotation is a multiple of 90 degrees to get from blob coords to
163 // nontext_map coords, image_box is the bounds of the nontext_map.
164 // Blobs are spread horizontally or vertically according to their internal
165 // flags, but the spreading is truncated by set pixels in the nontext_map
166 // and also by the horizontal rule line limits on the blobs.
167 void ProjectBlobs(BLOBNBOX_LIST *blobs, const FCOORD &rotation, const TBOX &image_box,
168 Image nontext_map);
169 // Pads the bounding box of the given blob according to whether it is on
170 // a horizontal or vertical text line, taking into account tab-stops near
171 // the blob. Returns true if padding was in the horizontal direction.
172 bool PadBlobBox(BLOBNBOX *blob, TBOX *bbox);
173
174 // Helper denormalizes the TPOINT with the denorm if not nullptr, then
175 // converts to pix_ coordinates.
176 void TransformToPixCoords(const DENORM *denorm, TPOINT *pt) const;
177
178 // Helper truncates the TPOINT to be within the pix_.
179 void TruncateToImageBounds(TPOINT *pt) const;
180
181 // Transform tesseract coordinates to coordinates used in the pix.
182 int ImageXToProjectionX(int x) const;
183 int ImageYToProjectionY(int y) const;
184
185 // The down-sampling scale factor used in building the image.
186 int scale_factor_;
187 // The blob coordinates of the top-left (origin of the pix_) in tesseract
188 // coordinates. Used to transform the bottom-up tesseract coordinates to
189 // the top-down coordinates of the pix.
190 int x_origin_;
191 int y_origin_;
192 // The image of horizontally smeared blob boxes summed to provide a
193 // textline density map. As with a horizontal projection, the map has
194 // dips in the gaps between textlines.
195 Image pix_;
196 };
197
198 } // namespace tesseract.
199
200 #endif // TESSERACT_TEXTORD_TEXTLINEPROJECTION_H_