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215 lines
10 KiB
C++
215 lines
10 KiB
C++
/**********************************************************************
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* File: fixxht.cpp (Formerly fixxht.c)
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* Description: Improve x_ht and look out for case inconsistencies
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* Author: Phil Cheatle
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* Created: Thu Aug 5 14:11:08 BST 1993
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*
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* (C) Copyright 1992, Hewlett-Packard Ltd.
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** Licensed under the Apache License, Version 2.0 (the "License");
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** you may not use this file except in compliance with the License.
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** You may obtain a copy of the License at
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** http://www.apache.org/licenses/LICENSE-2.0
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** Unless required by applicable law or agreed to in writing, software
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** distributed under the License is distributed on an "AS IS" BASIS,
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** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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** See the License for the specific language governing permissions and
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** limitations under the License.
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*
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**********************************************************************/
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#include <string.h>
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#include <ctype.h>
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#include "params.h"
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#include "float2int.h"
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#include "tesseractclass.h"
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namespace tesseract {
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// Fixxht overview.
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// Premise: Initial estimate of x-height is adequate most of the time, but
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// occasionally it is incorrect. Most notable causes of failure are:
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// 1. Small caps, where the top of the caps is the same as the body text
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// xheight. For small caps words the xheight needs to be reduced to correctly
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// recognize the caps in the small caps word.
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// 2. All xheight lines, such as summer. Here the initial estimate will have
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// guessed that the blob tops are caps and will have placed the xheight too low.
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// 3. Noise/logos beside words, or changes in font size on a line. Such
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// things can blow the statistics and cause an incorrect estimate.
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// 4. Incorrect baseline. Can happen when 2 columns are incorrectly merged.
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// In this case the x-height is often still correct.
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//
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// Algorithm.
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// Compare the vertical position (top only) of alphnumerics in a word with
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// the range of positions in training data (in the unicharset).
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// See CountMisfitTops. If any characters disagree sufficiently with the
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// initial xheight estimate, then recalculate the xheight, re-run OCR on
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// the word, and if the number of vertical misfits goes down, along with
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// either the word rating or certainty, then keep the new xheight.
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// The new xheight is calculated as follows:ComputeCompatibleXHeight
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// For each alphanumeric character that has a vertically misplaced top
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// (a misfit), yet its bottom is within the acceptable range (ie it is not
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// likely a sub-or super-script) calculate the range of acceptable xheight
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// positions from its range of tops, and give each value in the range a
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// number of votes equal to the distance of its top from its acceptance range.
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// The x-height position with the median of the votes becomes the new
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// x-height. This assumes that most characters will be correctly recognized
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// even if the x-height is incorrect. This is not a terrible assumption, but
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// it is not great. An improvement would be to use a classifier that does
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// not care about vertical position or scaling at all.
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// Separately collect stats on shifted baselines and apply the same logic to
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// computing a best-fit shift to fix the error. If the baseline needs to be
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// shifted, but the x-height is OK, returns the original x-height along with
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// the baseline shift to indicate that recognition needs to re-run.
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// If the max-min top of a unicharset char is bigger than kMaxCharTopRange
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// then the char top cannot be used to judge misfits or suggest a new top.
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const int kMaxCharTopRange = 48;
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// Returns the number of misfit blob tops in this word.
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int Tesseract::CountMisfitTops(WERD_RES *word_res) {
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int bad_blobs = 0;
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int num_blobs = word_res->rebuild_word->NumBlobs();
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for (int blob_id = 0; blob_id < num_blobs; ++blob_id) {
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TBLOB* blob = word_res->rebuild_word->blobs[blob_id];
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UNICHAR_ID class_id = word_res->best_choice->unichar_id(blob_id);
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if (unicharset.get_isalpha(class_id) || unicharset.get_isdigit(class_id)) {
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int top = blob->bounding_box().top();
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if (top >= INT_FEAT_RANGE)
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top = INT_FEAT_RANGE - 1;
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int min_bottom, max_bottom, min_top, max_top;
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unicharset.get_top_bottom(class_id, &min_bottom, &max_bottom,
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&min_top, &max_top);
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if (max_top - min_top > kMaxCharTopRange)
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continue;
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bool bad = top < min_top - x_ht_acceptance_tolerance ||
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top > max_top + x_ht_acceptance_tolerance;
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if (bad)
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++bad_blobs;
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if (debug_x_ht_level >= 1) {
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tprintf("Class %s is %s with top %d vs limits of %d->%d, +/-%d\n",
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unicharset.id_to_unichar(class_id),
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bad ? "Misfit" : "OK", top, min_top, max_top,
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static_cast<int>(x_ht_acceptance_tolerance));
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}
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}
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}
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return bad_blobs;
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}
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// Returns a new x-height maximally compatible with the result in word_res.
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// See comment above for overall algorithm.
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float Tesseract::ComputeCompatibleXheight(WERD_RES *word_res,
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float* baseline_shift) {
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STATS top_stats(0, MAX_UINT8);
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STATS shift_stats(-MAX_UINT8, MAX_UINT8);
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int bottom_shift = 0;
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int num_blobs = word_res->rebuild_word->NumBlobs();
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do {
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top_stats.clear();
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shift_stats.clear();
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for (int blob_id = 0; blob_id < num_blobs; ++blob_id) {
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TBLOB* blob = word_res->rebuild_word->blobs[blob_id];
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UNICHAR_ID class_id = word_res->best_choice->unichar_id(blob_id);
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if (unicharset.get_isalpha(class_id) ||
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unicharset.get_isdigit(class_id)) {
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int top = blob->bounding_box().top() + bottom_shift;
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// Clip the top to the limit of normalized feature space.
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if (top >= INT_FEAT_RANGE)
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top = INT_FEAT_RANGE - 1;
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int bottom = blob->bounding_box().bottom() + bottom_shift;
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int min_bottom, max_bottom, min_top, max_top;
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unicharset.get_top_bottom(class_id, &min_bottom, &max_bottom,
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&min_top, &max_top);
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// Chars with a wild top range would mess up the result so ignore them.
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if (max_top - min_top > kMaxCharTopRange)
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continue;
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int misfit_dist = MAX((min_top - x_ht_acceptance_tolerance) - top,
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top - (max_top + x_ht_acceptance_tolerance));
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int height = top - kBlnBaselineOffset;
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if (debug_x_ht_level >= 2) {
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tprintf("Class %s: height=%d, bottom=%d,%d top=%d,%d, actual=%d,%d: ",
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unicharset.id_to_unichar(class_id),
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height, min_bottom, max_bottom, min_top, max_top,
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bottom, top);
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}
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// Use only chars that fit in the expected bottom range, and where
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// the range of tops is sensibly near the xheight.
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if (min_bottom <= bottom + x_ht_acceptance_tolerance &&
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bottom - x_ht_acceptance_tolerance <= max_bottom &&
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min_top > kBlnBaselineOffset &&
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max_top - kBlnBaselineOffset >= kBlnXHeight &&
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misfit_dist > 0) {
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// Compute the x-height position using proportionality between the
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// actual height and expected height.
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int min_xht = DivRounded(height * kBlnXHeight,
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max_top - kBlnBaselineOffset);
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int max_xht = DivRounded(height * kBlnXHeight,
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min_top - kBlnBaselineOffset);
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if (debug_x_ht_level >= 2) {
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tprintf(" xht range min=%d, max=%d\n", min_xht, max_xht);
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}
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// The range of expected heights gets a vote equal to the distance
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// of the actual top from the expected top.
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for (int y = min_xht; y <= max_xht; ++y)
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top_stats.add(y, misfit_dist);
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} else if ((min_bottom > bottom + x_ht_acceptance_tolerance ||
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bottom - x_ht_acceptance_tolerance > max_bottom) &&
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bottom_shift == 0) {
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// Get the range of required bottom shift.
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int min_shift = min_bottom - bottom;
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int max_shift = max_bottom - bottom;
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if (debug_x_ht_level >= 2) {
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tprintf(" bottom shift min=%d, max=%d\n", min_shift, max_shift);
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}
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// The range of expected shifts gets a vote equal to the min distance
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// of the actual bottom from the expected bottom, spread over the
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// range of its acceptance.
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int misfit_weight = abs(min_shift);
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if (max_shift > min_shift)
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misfit_weight /= max_shift - min_shift;
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for (int y = min_shift; y <= max_shift; ++y)
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shift_stats.add(y, misfit_weight);
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} else {
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if (bottom_shift == 0) {
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// Things with bottoms that are already ok need to say so, on the
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// 1st iteration only.
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shift_stats.add(0, kBlnBaselineOffset);
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}
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if (debug_x_ht_level >= 2) {
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tprintf(" already OK\n");
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}
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}
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}
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}
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if (shift_stats.get_total() > top_stats.get_total()) {
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bottom_shift = IntCastRounded(shift_stats.median());
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if (debug_x_ht_level >= 2) {
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tprintf("Applying bottom shift=%d\n", bottom_shift);
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}
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}
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} while (bottom_shift != 0 &&
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top_stats.get_total() < shift_stats.get_total());
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// Baseline shift is opposite sign to the bottom shift.
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*baseline_shift = -bottom_shift / word_res->denorm.y_scale();
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if (debug_x_ht_level >= 2) {
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tprintf("baseline shift=%g\n", *baseline_shift);
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}
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if (top_stats.get_total() == 0)
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return bottom_shift != 0 ? word_res->x_height : 0.0f;
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// The new xheight is just the median vote, which is then scaled out
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// of BLN space back to pixel space to get the x-height in pixel space.
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float new_xht = top_stats.median();
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if (debug_x_ht_level >= 2) {
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tprintf("Median xht=%f\n", new_xht);
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tprintf("Mode20:A: New x-height = %f (norm), %f (orig)\n",
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new_xht, new_xht / word_res->denorm.y_scale());
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}
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// The xheight must change by at least x_ht_min_change to be used.
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if (fabs(new_xht - kBlnXHeight) >= x_ht_min_change)
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return new_xht / word_res->denorm.y_scale();
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else
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return bottom_shift != 0 ? word_res->x_height : 0.0f;
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}
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} // namespace tesseract
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