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565 lines
21 KiB
C
565 lines
21 KiB
C
/*====================================================================*
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- Copyright (C) 2001 Leptonica. All rights reserved.
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- This software is distributed in the hope that it will be
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- useful, but with NO WARRANTY OF ANY KIND.
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- No author or distributor accepts responsibility to anyone for the
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- consequences of using this software, or for whether it serves any
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- particular purpose or works at all, unless he or she says so in
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- writing. Everyone is granted permission to copy, modify and
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- redistribute this source code, for commercial or non-commercial
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- purposes, with the following restrictions: (1) the origin of this
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- source code must not be misrepresented; (2) modified versions must
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- be plainly marked as such; and (3) this notice may not be removed
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- or altered from any source or modified source distribution.
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*====================================================================*/
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/*
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* baseline.c
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*
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* Locate text baselines in an image
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* NUMA *pixFindBaselines()
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*
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* Projective transform to remove local skew
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* PIX *pixDeskewLocal()
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*
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* Determine local skew
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* l_int32 pixGetLocalSkewTransform()
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* NUMA *pixGetLocalSkewAngles()
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*
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* We have two apparently different functions here:
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* - finding baselines
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* - finding a projective transform to remove keystone warping
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* The function pixGetLocalSkewAngles() returns an array of angles,
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* one for each raster line, and the baselines of the text lines
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* should intersect the left edge of the image with that angle.
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <math.h>
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#include "allheaders.h"
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#ifndef NO_CONSOLE_IO
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#define DEBUG_PLOT 0
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#endif /* NO_CONSOLE_IO */
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/* Min to travel after finding max before abandoning peak */
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static const l_int32 MIN_DIST_IN_PEAK = 35;
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/* Thresholds for peaks and zeros, relative to the max peak */
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static const l_int32 PEAK_THRESHOLD_RATIO = 20;
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static const l_int32 ZERO_THRESHOLD_RATIO = 100;
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/* Default values for determining local skew */
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static const l_int32 DEFAULT_SLICES = 10;
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static const l_int32 DEFAULT_SWEEP_REDUCTION = 2;
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static const l_int32 DEFAULT_BS_REDUCTION = 1;
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static const l_float32 DEFAULT_SWEEP_RANGE = 5.; /* degrees */
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static const l_float32 DEFAULT_SWEEP_DELTA = 1.; /* degrees */
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static const l_float32 DEFAULT_MINBS_DELTA = 0.01; /* degrees */
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/* Overlap slice fraction added to top and bottom of each slice */
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static const l_float32 OVERLAP_FRACTION = 0.5;
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/* Minimum allowed confidence (ratio) for accepting a value */
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static const l_float32 MIN_ALLOWED_CONFIDENCE = 3.0;
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/*---------------------------------------------------------------------*
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* Locate text baselines in an image *
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*---------------------------------------------------------------------*/
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/*!
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* pixFindBaselines()
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*
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* Input: pixs (1 bpp)
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* &pta (<optional return> pairs of pts corresponding to
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* approx. ends of each text line)
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* debug (usually 0; set to 1 for debugging output)
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* Return: na (of baseline y values), or null on error
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*
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* Notes:
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* (1) Input binary image must have text lines already aligned
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* horizontally. This can be done by either rotating the
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* image with pixDeskew(), or, if a projective transform
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* is required, by doing pixDeskewLocal() first.
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* (2) Input null for &pta if you don't want this returned.
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* The pta will come in pairs of points (left and right end
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* of each baseline).
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* (3) Caution: this will not work properly on text with multiple
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* columns, where the lines are not aligned between columns.
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* If there are multiple columns, they should be extracted
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* separately before finding the baselines.
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* (4) This function constructs different types of output
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* for baselines; namely, a set of raster line values and
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* a set of end points of each baseline.
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* (5) This function was designed to handle short and long text lines
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* without using dangerous thresholds on the peak heights. It does
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* this by combining the differential signal with a morphological
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* analysis of the locations of the text lines. One can also
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* combine this data to normalize the peak heights, by weighting
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* the differential signal in the region of each baseline
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* by the inverse of the width of the text line found there.
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* (6) There are various debug sections that can be turned on
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* with the debug flag.
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*/
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NUMA *
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pixFindBaselines(PIX *pixs,
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PTA **ppta,
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l_int32 debug)
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{
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l_int32 w, h, i, j, nbox, val1, val2, ndiff, bx, by, bw, bh;
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l_int32 imaxloc, peakthresh, zerothresh, inpeak;
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l_int32 mintosearch, max, maxloc, nloc, locval;
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l_int32 *array;
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l_float32 maxval;
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BOXA *boxa1, *boxa2, *boxa3;
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GPLOT *gplot;
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NUMA *nasum, *nadiff, *naloc, *naval;
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PIX *pixt1, *pixt2;
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PTA *pta;
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PROCNAME("pixFindBaselines");
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if (!pixs)
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return (NUMA *)ERROR_PTR("pixs not defined", procName, NULL);
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pta = NULL;
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if (ppta) {
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pta = ptaCreate(0);
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*ppta = pta;
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}
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/* Close up the text characters, removing noise */
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pixt1 = pixMorphSequence(pixs, "c25.1 + e3.1", 0);
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/* Save the difference of adjacent row sums.
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* The high positive-going peaks are the baselines */
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if ((nasum = pixCountPixelsByRow(pixt1, NULL)) == NULL)
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return (NUMA *)ERROR_PTR("nasum not made", procName, NULL);
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w = pixGetWidth(pixs);
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h = pixGetHeight(pixs);
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nadiff = numaCreate(h);
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numaGetIValue(nasum, 0, &val2);
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for (i = 0; i < h - 1; i++) {
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val1 = val2;
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numaGetIValue(nasum, i + 1, &val2);
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numaAddNumber(nadiff, val1 - val2);
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}
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if (debug) /* show the difference signal */
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gplotSimple1(nadiff, GPLOT_X11, "junkdiff", "difference");
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/* Use the zeroes of the profile to locate each baseline. */
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array = numaGetIArray(nadiff);
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ndiff = numaGetCount(nadiff);
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numaGetMax(nadiff, &maxval, &imaxloc);
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/* Use this to begin locating a new peak: */
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peakthresh = (l_int32)maxval / PEAK_THRESHOLD_RATIO;
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/* Use this to begin a region between peaks: */
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zerothresh = (l_int32)maxval / ZERO_THRESHOLD_RATIO;
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naloc = numaCreate(0);
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naval = numaCreate(0);
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inpeak = FALSE;
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for (i = 0; i < ndiff; i++) {
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if (inpeak == FALSE) {
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if (array[i] > peakthresh) { /* transition to in-peak */
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inpeak = TRUE;
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mintosearch = i + MIN_DIST_IN_PEAK; /* accept no zeros
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* between i and mintosearch */
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max = array[i];
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maxloc = i;
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}
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}
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else { /* inpeak == TRUE; look for max */
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if (array[i] > max) {
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max = array[i];
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maxloc = i;
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mintosearch = i + MIN_DIST_IN_PEAK;
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}
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else if (i > mintosearch && array[i] <= zerothresh) { /* leave */
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inpeak = FALSE;
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numaAddNumber(naval, max);
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numaAddNumber(naloc, maxloc);
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}
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}
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}
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/* If array[ndiff-1] is max, eg. no descenders, baseline at bottom */
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if (inpeak) {
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numaAddNumber(naval, max);
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numaAddNumber(naloc, maxloc);
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}
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FREE(array);
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if (debug) { /* show the raster locations for the peaks */
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gplot = gplotCreate("junkloc", GPLOT_X11, "Peak locations",
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"rasterline", "height");
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gplotAddPlot(gplot, naloc, naval, GPLOT_POINTS, "locs");
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gplotMakeOutput(gplot);
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gplotDestroy(&gplot);
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}
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/* Generate an approximate profile of text line width.
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* First, filter the boxes of text, where there may be
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* more than one box for a given textline. */
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pixt2 = pixMorphSequence(pixt1, "r11 + c25.1 + o7.1 +c1.3", 0);
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boxa1 = pixConnComp(pixt2, NULL, 4);
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boxa2 = boxaTransform(boxa1, 0, 0, 4., 4.);
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boxa3 = boxaSort(boxa2, L_SORT_BY_Y, L_SORT_INCREASING, NULL);
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/* Then find the baseline segments */
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if (pta) {
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nloc = numaGetCount(naloc);
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nbox = boxaGetCount(boxa3);
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for (i = 0; i < nbox; i++) {
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boxaGetBoxGeometry(boxa3, i, &bx, &by, &bw, &bh);
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for (j = 0; j < nloc; j++) {
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numaGetIValue(naloc, j, &locval);
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if (L_ABS(locval - (by + bh)) > 25)
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continue;
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ptaAddPt(pta, bx, locval);
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ptaAddPt(pta, bx + bw, locval);
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break;
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}
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}
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}
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if (debug) { /* display baselines */
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PIX *pixd;
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l_int32 npts, x1, y1, x2, y2;
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if (pta) {
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pixd = pixConvertTo32(pixs);
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npts = ptaGetCount(pta);
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for (i = 0; i < npts; i += 2) {
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ptaGetIPt(pta, i, &x1, &y1);
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ptaGetIPt(pta, i + 1, &x2, &y2);
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pixRenderLineArb(pixd, x1, y1, x2, y2, 1, 255, 0, 0);
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}
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pixDisplay(pixd, 200, 200);
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pixWrite("junkbaselines", pixd, IFF_PNG);
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pixDestroy(&pixd);
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}
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}
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boxaDestroy(&boxa1);
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boxaDestroy(&boxa2);
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boxaDestroy(&boxa3);
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pixDestroy(&pixt1);
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pixDestroy(&pixt2);
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numaDestroy(&nasum);
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numaDestroy(&nadiff);
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numaDestroy(&naval);
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return naloc;
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}
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/*---------------------------------------------------------------------*
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* Projective transform to remove local skew *
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*---------------------------------------------------------------------*/
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/*!
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* pixDeskewLocal()
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*
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* Input: pixs
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* nslices (the number of horizontal overlapping slices; must
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* be larger than 1 and not exceed 20; use 0 for default)
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* redsweep (sweep reduction factor: 1, 2, 4 or 8;
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* use 0 for default value)
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* redsearch (search reduction factor: 1, 2, 4 or 8, and
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* not larger than redsweep; use 0 for default value)
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* sweeprange (half the full range, assumed about 0; in degrees;
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* use 0.0 for default value)
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* sweepdelta (angle increment of sweep; in degrees;
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* use 0.0 for default value)
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* minbsdelta (min binary search increment angle; in degrees;
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* use 0.0 for default value)
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* Return: pixd, or null on error
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*
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* Notes:
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* (1) This function allows deskew of a page whose skew changes
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* approximately linearly with vertical position. It uses
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* a projective tranform that in effect does a differential
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* shear about the LHS of the page, and makes all text lines
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* horizontal.
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* (2) The origin of the keystoning can be either a cheap document
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* feeder that rotates the page as it is passed through, or a
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* camera image taken from either the left or right side
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* of the vertical.
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* (3) The image transformation is a projective warping,
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* not a rotation. Apart from this function, the text lines
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* must be properly aligned vertically with respect to each
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* other. This can be done by pre-processing the page; e.g.,
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* by rotating or horizontally shearing it.
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* Typically, this can be achieved by vertically aligning
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* the page edge.
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*/
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PIX *
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pixDeskewLocal(PIX *pixs,
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l_int32 nslices,
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l_int32 redsweep,
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l_int32 redsearch,
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l_float32 sweeprange,
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l_float32 sweepdelta,
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l_float32 minbsdelta)
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{
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l_int32 ret;
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PIX *pixd;
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PTA *ptas, *ptad;
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PROCNAME("pixDeskewLocal");
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if (!pixs)
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return (PIX *)ERROR_PTR("pixs not defined", procName, NULL);
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/* Skew array gives skew angle (deg) as fctn of raster line
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* where it intersects the LHS of the image */
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ret = pixGetLocalSkewTransform(pixs, nslices, redsweep, redsearch,
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sweeprange, sweepdelta, minbsdelta,
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&ptas, &ptad);
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if (ret != 0)
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return (PIX *)ERROR_PTR("transform pts not found", procName, NULL);
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/* Use a projective transform */
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pixd = pixProjectiveSampledPta(pixs, ptad, ptas, L_BRING_IN_WHITE);
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ptaDestroy(&ptas);
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ptaDestroy(&ptad);
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return pixd;
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}
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/*---------------------------------------------------------------------*
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* Determine the local skew *
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*---------------------------------------------------------------------*/
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/*!
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* pixGetLocalSkewTransform()
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*
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* Input: pixs
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* nslices (the number of horizontal overlapping slices; must
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* be larger than 1 and not exceed 20; use 0 for default)
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* redsweep (sweep reduction factor: 1, 2, 4 or 8;
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* use 0 for default value)
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* redsearch (search reduction factor: 1, 2, 4 or 8, and
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* not larger than redsweep; use 0 for default value)
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* sweeprange (half the full range, assumed about 0; in degrees;
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* use 0.0 for default value)
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* sweepdelta (angle increment of sweep; in degrees;
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* use 0.0 for default value)
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* minbsdelta (min binary search increment angle; in degrees;
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* use 0.0 for default value)
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* &ptas (<return> 4 points in the source)
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* &ptad (<return> the corresponding 4 pts in the dest)
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* Return: 0 if OK, 1 on error
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*
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* Notes:
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* (1) This generates two pairs of points in the src, each pair
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* corresponding to a pair of points that would lie along
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* the same raster line in a transformed (dewarped) image.
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* (2) The sets of 4 src and 4 dest points returned by this function
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* can then be used, in a projective or bilinear transform,
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* to remove keystoning in the src.
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*/
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l_int32
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pixGetLocalSkewTransform(PIX *pixs,
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l_int32 nslices,
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l_int32 redsweep,
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l_int32 redsearch,
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l_float32 sweeprange,
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l_float32 sweepdelta,
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l_float32 minbsdelta,
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PTA **pptas,
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PTA **pptad)
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{
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l_int32 w, h, i;
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l_float32 deg2rad, angr, angd, dely;
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NUMA *naskew;
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PTA *ptas, *ptad;
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PROCNAME("pixGetLocalSkewTransform");
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if (!pptas || !pptad)
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return ERROR_INT("&ptas and &ptad not defined", procName, 1);
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*pptas = *pptad = NULL;
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if (!pixs)
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return ERROR_INT("pixs not defined", procName, 1);
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if (nslices < 2 || nslices > 20)
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nslices = DEFAULT_SLICES;
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if (redsweep < 1 || redsweep > 8)
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redsweep = DEFAULT_SWEEP_REDUCTION;
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if (redsearch < 1 || redsearch > redsweep)
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redsearch = DEFAULT_BS_REDUCTION;
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if (sweeprange == 0.0)
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sweeprange = DEFAULT_SWEEP_RANGE;
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if (sweepdelta == 0.0)
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sweepdelta = DEFAULT_SWEEP_DELTA;
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if (minbsdelta == 0.0)
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minbsdelta = DEFAULT_MINBS_DELTA;
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naskew = pixGetLocalSkewAngles(pixs, nslices, redsweep, redsearch,
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sweeprange, sweepdelta, minbsdelta,
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NULL, NULL);
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if (!naskew)
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return ERROR_INT("naskew not made", procName, 1);
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deg2rad = 3.14159265 / 180.;
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w = pixGetWidth(pixs);
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h = pixGetHeight(pixs);
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ptas = ptaCreate(4);
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ptad = ptaCreate(4);
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*pptas = ptas;
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*pptad = ptad;
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/* Find i for skew line that intersects LHS at i and RHS at h / 20 */
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for (i = 0; i < h; i++) {
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numaGetFValue(naskew, i, &angd);
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angr = angd * deg2rad;
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dely = w * tan(angr);
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if (i - dely > 0.05 * h)
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break;
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}
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ptaAddPt(ptas, 0, i);
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ptaAddPt(ptas, w - 1, i - dely);
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ptaAddPt(ptad, 0, i);
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ptaAddPt(ptad, w - 1, i);
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/* Find i for skew line that intersects LHS at i and RHS at 19h / 20 */
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for (i = h - 1; i > 0; i--) {
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numaGetFValue(naskew, i, &angd);
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angr = angd * deg2rad;
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dely = w * tan(angr);
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if (i - dely < 0.95 * h)
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break;
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}
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ptaAddPt(ptas, 0, i);
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ptaAddPt(ptas, w - 1, i - dely);
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ptaAddPt(ptad, 0, i);
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ptaAddPt(ptad, w - 1, i);
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numaDestroy(&naskew);
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return 0;
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}
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/*!
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* pixGetLocalSkewAngles()
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*
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* Input: pixs
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* nslices (the number of horizontal overlapping slices; must
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* be larger than 1 and not exceed 20; use 0 for default)
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* redsweep (sweep reduction factor: 1, 2, 4 or 8;
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* use 0 for default value)
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* redsearch (search reduction factor: 1, 2, 4 or 8, and
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* not larger than redsweep; use 0 for default value)
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* sweeprange (half the full range, assumed about 0; in degrees;
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* use 0.0 for default value)
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* sweepdelta (angle increment of sweep; in degrees;
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* use 0.0 for default value)
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* minbsdelta (min binary search increment angle; in degrees;
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* use 0.0 for default value)
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* &a (<optional return> slope of skew as fctn of y)
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* &b (<optional return> intercept at y=0 of skew as fctn of y)
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* Return: naskew, or null on error
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*
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* Notes:
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* (1) The local skew is measured in a set of overlapping strips.
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* We then do a least square linear fit parameters to get
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* the slope and intercept parameters a and b in
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* skew-angle = a * y + b (degrees)
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* for the local skew as a function of raster line y.
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* This is then used to make naskew, which can be interpreted
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* as the computed skew angle (in degrees) at the left edge
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* of each raster line.
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* (2) naskew can then be used to find the baselines of text, because
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* each text line has a baseline that should intersect
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* the left edge of the image with the angle given by this
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* array, evaluated at the raster line of intersection.
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*/
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NUMA *
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pixGetLocalSkewAngles(PIX *pixs,
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l_int32 nslices,
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l_int32 redsweep,
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l_int32 redsearch,
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l_float32 sweeprange,
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l_float32 sweepdelta,
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l_float32 minbsdelta,
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l_float32 *pa,
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l_float32 *pb)
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{
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l_int32 w, h, hs, i, ystart, yend, ovlap, npts;
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l_float32 angle, conf, ycenter, a, b;
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BOX *box;
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NUMA *naskew;
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PIX *pix;
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PTA *pta;
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|
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PROCNAME("pixGetLocalSkewAngles");
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if (!pixs)
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return (NUMA *)ERROR_PTR("pixs not defined", procName, NULL);
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if (nslices < 2 || nslices > 20)
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nslices = DEFAULT_SLICES;
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if (redsweep < 1 || redsweep > 8)
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redsweep = DEFAULT_SWEEP_REDUCTION;
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if (redsearch < 1 || redsearch > redsweep)
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redsearch = DEFAULT_BS_REDUCTION;
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if (sweeprange == 0.0)
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sweeprange = DEFAULT_SWEEP_RANGE;
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if (sweepdelta == 0.0)
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sweepdelta = DEFAULT_SWEEP_DELTA;
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if (minbsdelta == 0.0)
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minbsdelta = DEFAULT_MINBS_DELTA;
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|
|
|
h = pixGetHeight(pixs);
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w = pixGetWidth(pixs);
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hs = h / nslices;
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ovlap = (l_int32)(OVERLAP_FRACTION * hs);
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pta = ptaCreate(nslices);
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|
for (i = 0; i < nslices; i++) {
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ystart = L_MAX(0, hs * i - ovlap);
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yend = L_MIN(h - 1, hs * (i + 1) + ovlap);
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|
ycenter = (ystart + yend) / 2;
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|
box = boxCreate(0, ystart, w, yend - ystart + 1);
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|
pix = pixClipRectangle(pixs, box, NULL);
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|
pixFindSkewSweepAndSearch(pix, &angle, &conf, redsweep, redsearch,
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|
sweeprange, sweepdelta, minbsdelta);
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|
if (conf > MIN_ALLOWED_CONFIDENCE)
|
|
ptaAddPt(pta, ycenter, angle);
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|
pixDestroy(&pix);
|
|
boxDestroy(&box);
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|
}
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|
/* ptaWriteStream(stderr, pta, 0); */
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|
|
|
/* Do linear least squares fit */
|
|
if ((npts = ptaGetCount(pta)) < 2) {
|
|
ptaDestroy(&pta);
|
|
return (NUMA *)ERROR_PTR("can't fit skew", procName, NULL);
|
|
}
|
|
ptaGetLinearLSF(pta, &a, &b, NULL);
|
|
if (pa) *pa = a;
|
|
if (pb) *pb = b;
|
|
|
|
/* Make skew angle array as function of raster line */
|
|
naskew = numaCreate(h);
|
|
for (i = 0; i < h; i++) {
|
|
angle = a * i + b;
|
|
numaAddNumber(naskew, angle);
|
|
}
|
|
|
|
#if DEBUG_PLOT
|
|
{ NUMA *nax, *nay;
|
|
GPLOT *gplot;
|
|
ptaGetArrays(pta, &nax, &nay);
|
|
gplot = gplotCreate("junkskew", GPLOT_X11, "skew as fctn of y",
|
|
"y (in raster lines from top)", "angle (in degrees)");
|
|
gplotAddPlot(gplot, NULL, naskew, GPLOT_POINTS, "linear lsf");
|
|
gplotAddPlot(gplot, nax, nay, GPLOT_POINTS, "actual data pts");
|
|
gplotMakeOutput(gplot);
|
|
gplotDestroy(&gplot);
|
|
numaDestroy(&nax);
|
|
numaDestroy(&nay);
|
|
}
|
|
#endif /* DEBUG_PLOT */
|
|
|
|
ptaDestroy(&pta);
|
|
return naskew;
|
|
}
|
|
|
|
|