cpml/cpml-arc.c

   1 /* CPML - Cairo Path Manipulation Library
   2  * Copyright (C) 2008, 2009  Nicola Fontana <ntd at entidi.it>
   3  *
   4  * This library is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU Lesser General Public
   6  * License as published by the Free Software Foundation; either
   7  * version 2 of the License, or (at your option) any later version.
   8  *
   9  * This library is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * Lesser General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU Lesser General Public
  15  * License along with this library; if not, write to the
  16  * Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  17  * Boston, MA  02110-1301, USA.
  18  */
  19
  20
  21 /**
  22  * SECTION:arc
  23  * @Section_Id:CpmlArc
  24  * @title: CpmlArc
  25  * @short_description: Manipulation of circular arcs
  26  *
  27  * The following functions manipulate #CAIRO_PATH_ARC_TO #CpmlPrimitive.
  28  * No validation is made on the input so use the following methods
  29  * only when you are sure the <varname>primitive</varname> argument
  30  * is effectively an arc-to.
  31  *
  32  * The arc primitive is defined by 3 points: the first one is the usual
  33  * implicit point got from the previous primitive, the second point is
  34  * an arbitrary intermediate point laying on the arc and the third point
  35  * is the end of the arc. These points identify univocally an arc:
  36  * furthermore, the intermediate point also gives the side of
  37  * the arc.
  38  *
  39  * As a special case, when the first point is coincident with the end
  40  * point the primitive is considered a circle with diameter defined by
  41  * the segment between the first and the intermediate point.
  42  *
  43  * <important>
  44  * <para>
  45  * An arc is not a native cairo primitive and should be treated specially.
  46  * </para>
  47  * </important>
  48  *
  49  * Using these CPML APIs you are free to use #CAIRO_PATH_ARC_TO whenever
  50  * you want but, if you are directly accessing the struct fields, you
  51  * are responsible of converting arcs to curves before passing them
  52  * to cairo. In other words, do not directly feed #CpmlPath struct to
  53  * cairo (throught cairo_append_path() for example) or at least do not
  54  * expect it will work.
  55  *
  56  * The conversion is provided by two APIs: cpml_arc_to_cairo() and
  57  * cpml_arc_to_curves(). The former directly renders to a cairo context
  58  * and is internally used by all the ..._to_cairo() functions when an
  59  * arc is met. The latter provided a more powerful (and more complex)
  60  * approach as it allows to specify the number of curves to use and do
  61  * not need a cairo context.
  62  **/
  63
  64
  65 #include "cpml-arc.h"
  66 #include "cpml-pair.h"
  67
  68 #include <stdlib.h>
  69 #include <math.h>
  70
  71
  72 /* Hardcoded max angle of the arc to be approximated by a Bézier curve:
  73  * this influence the arc quality (the default value is got from cairo) */
  74 #define ARC_MAX_ANGLE   M_PI_2
  75
  76
  77 static cairo_bool_t     get_center      (const CpmlPair *p,
  78                                          CpmlPair       *dest);
  79 static void             get_angles      (const CpmlPair *p,
  80                                          const CpmlPair *center,
  81                                          double         *start,
  82                                          double         *end);
  83 static void             arc_to_curve    (CpmlPrimitive  *curve,
  84                                          const CpmlPair *center,
  85                                          double          r,
  86                                          double          start,
  87                                          double          end);
  88
  89
  90 /**
  91  * cpml_arc_type_get_npoints:
  92  *
  93  * Returns the number of point needed to properly specify an arc primitive.
  94  *
  95  * Return value: 3
  96  **/
  97 int
  98 cpml_arc_type_get_npoints(void)
  99 {
 100     return 3;
 101 }
 102
 103 /**
 104  * cpml_arc_info:
 105  * @arc:    the #CpmlPrimitive arc data
 106  * @center: where to store the center coordinates (can be %NULL)
 107  * @r:      where to store the radius (can be %NULL)
 108  * @start:  where to store the starting angle (can be %NULL)
 109  * @end:    where to store the ending angle (can be %NULL)
 110  *
 111  * Given an @arc, this function calculates and returns its basic data.
 112  * Any pointer can be %NULL, in which case the requested info is not
 113  * returned. This function can fail (when the three points lay on a
 114  * straight line, for example) in which case 0 is returned and no
 115  * data can be considered valid.
 116  *
 117  * The radius @r can be 0 when the three points are coincidents: a
 118  * circle with radius 0 is considered a valid path.
 119  *
 120  * When the start and end angle are returned, together with their
 121  * values these angles implicitely gives another important information:
 122  * the arc direction.
 123  *
 124  * If @start < @end the arc must be rendered with increasing angle
 125  * value (clockwise direction using the ordinary cairo coordinate
 126  * system) while if @start > @end the arc must be rendered in reverse
 127  * order (that is counterclockwise in the cairo world). This is the
 128  * reason the angle values are returned in the range
 129  * { -M_PI < value < 3*M_PI } inclusive instead of the usual
 130  * { -M_PI < value < M_PI } range.
 131  *
 132  * Return value: 1 if the function worked succesfully, 0 on errors
 133  **/
 134 cairo_bool_t
 135 cpml_arc_info(const CpmlPrimitive *arc, CpmlPair *center,
 136               double *r, double *start, double *end)
 137 {
 138     CpmlPair p[3], l_center;
 139
 140     cpml_pair_from_cairo(&p[0], arc->org);
 141     cpml_pair_from_cairo(&p[1], &arc->data[1]);
 142     cpml_pair_from_cairo(&p[2], &arc->data[2]);
 143
 144     if (!get_center(p, &l_center))
 145         return 0;
 146
 147     if (center)
 148         *center = l_center;
 149
 150     if (r != NULL)
 151         *r = cpml_pair_distance(&p[0], &l_center);
 152
 153     if (start != NULL || end != NULL) {
 154         double l_start, l_end;
 155
 156         get_angles(p, &l_center, &l_start, &l_end);
 157
 158         if (start != NULL)
 159             *start = l_start;
 160         if (end != NULL)
 161             *end = l_end;
 162     }
 163
 164     return 1;
 165 }
 166
 167 /**
 168  * cpml_arc_length:
 169  * @arc:  the #CpmlPrimitive arc data
 170  *
 171  * Given the @arc primitive, returns its length.
 172  *
 173  * Return value: the requested length or 0 on errors
 174  **/
 175 double
 176 cpml_arc_length(const CpmlPrimitive *arc)
 177 {
 178     double r, start, end, delta;
 179
 180     if (!cpml_arc_info(arc, NULL, &r, &start, &end) || start == end)
 181         return 0.;
 182
 183     delta = end - start;
 184     if (delta < 0)
 185         delta += M_PI*2;
 186
 187     return r*delta;
 188 }
 189
 190 /**
 191  * cpml_arc_pair_at:
 192  * @arc:  the #CpmlPrimitive arc data
 193  * @pair: the destination #CpmlPair
 194  * @pos:  the position value
 195  *
 196  * Given an @arc, finds the coordinates at position @pos (where 0 is
 197  * the start and 1 is the end) and stores the result in @pair.
 198  *
 199  * @pos can also be outside the 0..1 limit, as interpolating on an
 200  * arc is quite trivial.
 201  **/
 202 void
 203 cpml_arc_pair_at(const CpmlPrimitive *arc, CpmlPair *pair, double pos)
 204 {
 205     if (pos == 0.) {
 206         cpml_pair_from_cairo(pair, arc->org);
 207     } else if (pos == 1.) {
 208         cpml_pair_from_cairo(pair, &arc->data[2]);
 209     } else {
 210         CpmlPair center;
 211         double r, start, end, angle;
 212
 213         if (!cpml_arc_info(arc, &center, &r, &start, &end))
 214             return;
 215
 216         angle = (end-start)*pos + start;
 217         cpml_vector_from_angle(pair, angle, r);
 218         cpml_pair_add(pair, &center);
 219     }
 220 }
 221
 222 /**
 223  * cpml_arc_vector_at:
 224  * @arc:    the #CpmlPrimitive arc data
 225  * @vector: the destination vector
 226  * @pos:    the position value
 227  *
 228  * Given an @arc, finds the slope at position @pos (where 0 is
 229  * the start and 1 is the end) and stores the result in @vector.
 230  *
 231  * @pos can also be outside the 0..1 limit, as interpolating on an
 232  * arc is quite trivial.
 233  **/
 234 void
 235 cpml_arc_vector_at(const CpmlPrimitive *arc, CpmlVector *vector, double pos)
 236 {
 237     double start, end, angle;
 238
 239     if (!cpml_arc_info(arc, NULL, NULL, &start, &end))
 240         return;
 241
 242     angle = (end-start)*pos + start;
 243     cpml_vector_from_angle(vector, angle, 1.);
 244     cpml_vector_normal(vector);
 245 }
 246
 247 /**
 248  * cpml_arc_near_pos:
 249  * @arc: the #CpmlPrimitive arc data
 250  * @pair: the coordinates of the subject point
 251  *
 252  * Returns the pos value of the point on @arc nearest to @pair.
 253  * The returned value is always between 0 and 1.
 254  *
 255  * <important>
 256  * <title>TODO</title>
 257  * <itemizedlist>
 258  * <listitem>To be implemented...</listitem>
 259  * </itemizedlist>
 260  * </important>
 261  *
 262  * Return value: the pos value, always between 0 and 1
 263  **/
 264 double
 265 cpml_arc_near_pos(const CpmlPrimitive *arc, const CpmlPair *pair)
 266 {
 267     /* TODO */
 268
 269     return 0;
 270 }
 271
 272 /**
 273  * cpml_arc_intersection:
 274  * @arc:  the first arc
 275  * @arc2: the second arc
 276  * @dest: a vector of #CpmlPair
 277  * @max:  maximum number of intersections to return
 278  *        (that is, the size of @dest)
 279  *
 280  * Given two arcs (@arc and @arc2), gets their intersection points
 281  * and store the result in @dest. Keep in mind two arcs can have
 282  * up to 2 intersections.
 283  *
 284  * If @max is 0, the function returns 0 immediately without any
 285  * further processing. If @arc and @arc2 are cohincident (same
 286  * center and same radius), their intersections are not considered.
 287  *
 288  * <important>
 289  * <title>TODO</title>
 290  * <itemizedlist>
 291  * <listitem>To be implemented...</listitem>
 292  * </itemizedlist>
 293  * </important>
 294  *
 295  * Return value: the number of intersections found (max 2)
 296  *               or 0 if the primitives do not intersect
 297  **/
 298 int
 299 cpml_arc_intersection(const CpmlPrimitive *arc, const CpmlPrimitive *arc2,
 300                       CpmlPair *dest, int max)
 301 {
 302     return 0;
 303 }
 304
 305 /**
 306  * cpml_arc_intersection_with_line:
 307  * @arc:  an arc
 308  * @line: a line
 309  * @dest: a vector of #CpmlPair
 310  * @max:  maximum number of intersections to return
 311  *        (that is, the size of @dest)
 312  *
 313  * Given an @arc and a @line, gets their intersection points
 314  * and store the result in @dest. Keep in mind an arc and a
 315  * line can have up to 2 intersections.
 316  *
 317  * If @max is 0, the function returns 0 immediately without any
 318  * further processing.
 319  *
 320  * <important>
 321  * <title>TODO</title>
 322  * <itemizedlist>
 323  * <listitem>To be implemented...</listitem>
 324  * </itemizedlist>
 325  * </important>
 326  *
 327  * Return value: the number of intersections found (max 2)
 328  *               or 0 if the primitives do not intersect
 329  **/
 330 int
 331 cpml_arc_intersection_with_line(const CpmlPrimitive *arc,
 332                                 const CpmlPrimitive *line,
 333                                 CpmlPair *dest, int max)
 334 {
 335     return 0;
 336 }
 337
 338 /**
 339  * cpml_arc_offset:
 340  * @arc:    the #CpmlPrimitive arc data
 341  * @offset: distance for the computed parallel arc
 342  *
 343  * Given an @arc, this function computes the parallel arc at
 344  * distance @offset. The three points needed to build the
 345  * new arc are returned in the @arc data (substituting the
 346  * previous ones.
 347  **/
 348 void
 349 cpml_arc_offset(CpmlPrimitive *arc, double offset)
 350 {
 351     CpmlPair p[3], center;
 352     double r;
 353
 354     cpml_pair_from_cairo(&p[0], arc->org);
 355     cpml_pair_from_cairo(&p[1], &arc->data[1]);
 356     cpml_pair_from_cairo(&p[2], &arc->data[2]);
 357
 358     if (!get_center(p, &center))
 359         return;
 360
 361     r = cpml_pair_distance(&p[0], &center) + offset;
 362
 363     /* Offset the three points by calculating their vector from the center,
 364      * setting the new radius as length and readding the center */
 365     cpml_pair_sub(&p[0], &center);
 366     cpml_pair_sub(&p[1], &center);
 367     cpml_pair_sub(&p[2], &center);
 368
 369     cpml_vector_set_length(&p[0], r);
 370     cpml_vector_set_length(&p[1], r);
 371     cpml_vector_set_length(&p[2], r);
 372
 373     cpml_pair_add(&p[0], &center);
 374     cpml_pair_add(&p[1], &center);
 375     cpml_pair_add(&p[2], &center);
 376
 377     cpml_pair_to_cairo(&p[0], arc->org);
 378     cpml_pair_to_cairo(&p[1], &arc->data[1]);
 379     cpml_pair_to_cairo(&p[2], &arc->data[2]);
 380 }
 381
 382 /**
 383  * cpml_arc_to_cairo:
 384  * @arc: the #CpmlPrimitive arc data
 385  * @cr:  the destination cairo context
 386  *
 387  * Renders @arc to the @cr cairo context. As cairo does not support
 388  * arcs natively, it is approximated using one or more Bézier curves.
 389  *
 390  * The number of curves used is dependent from the angle of the arc.
 391  * Anyway, this function uses internally the hardcoded %M_PI_2 value
 392  * as threshold value. This means the maximum arc approximated by a
 393  * single curve will be a quarter of a circle and, consequently, a
 394  * whole circle will be approximated by 4 Bézier curves.
 395  **/
 396 void
 397 cpml_arc_to_cairo(const CpmlPrimitive *arc, cairo_t *cr)
 398 {
 399     CpmlPair center;
 400     double r, start, end;
 401     int n_curves;
 402     double step, angle;
 403     CpmlPrimitive curve;
 404     cairo_path_data_t data[4];
 405
 406     if (!cpml_arc_info(arc, &center, &r, &start, &end))
 407         return;
 408
 409     n_curves = ceil(fabs(end-start) / ARC_MAX_ANGLE);
 410     step = (end-start) / (double) n_curves;
 411     curve.data = data;
 412
 413     for (angle = start; n_curves--; angle += step) {
 414         arc_to_curve(&curve, &center, r, angle, angle+step);
 415         cairo_curve_to(cr,
 416                        curve.data[1].point.x, curve.data[1].point.y,
 417                        curve.data[2].point.x, curve.data[2].point.y,
 418                        curve.data[3].point.x, curve.data[3].point.y);
 419     }
 420 }
 421
 422 /**
 423  * cpml_arc_to_curves:
 424  * @arc:      the #CpmlPrimitive arc data
 425  * @segment:  the destination #CpmlSegment
 426  * @n_curves: number of Bézier to use
 427  *
 428  * Converts @arc to a serie of @n_curves Bézier curves and puts them
 429  * inside @segment. Obviously, @segment must have enough space to
 430  * contain at least @n_curves curves.
 431  *
 432  * This function works in a similar way as cpml_arc_to_cairo() but
 433  * has two important differences: it does not need a cairo context
 434  * and the number of curves to be generated is explicitely defined.
 435  * The latter difference allows a more specific error control from
 436  * the application: in the file src/cairo-arc.c, found in the cairo
 437  * tarball (at least in cairo-1.9.1), there is a table showing the
 438  * magnitude of error of this curve approximation algorithm.
 439  **/
 440 void
 441 cpml_arc_to_curves(const CpmlPrimitive *arc, CpmlSegment *segment,
 442                    int n_curves)
 443 {
 444     CpmlPair center;
 445     double r, start, end;
 446     double step, angle;
 447     CpmlPrimitive curve;
 448
 449     if (!cpml_arc_info(arc, &center, &r, &start, &end))
 450         return;
 451
 452     step = (end-start) / (double) n_curves;
 453     segment->num_data = n_curves*4;
 454     curve.segment = segment;
 455     curve.data = segment->data;
 456
 457     for (angle = start; n_curves--; angle += step) {
 458         arc_to_curve(&curve, &center, r, angle, angle+step);
 459         curve.data += 4;
 460     }
 461 }
 462
 463
 464 static cairo_bool_t
 465 get_center(const CpmlPair *p, CpmlPair *dest)
 466 {
 467     CpmlPair b, c;
 468     double d, b2, c2;
 469
 470     /* When p[0] == p[2], p[0]..p[1] is considered the diameter of a circle */
 471     if (p[0].x == p[2].x && p[0].y == p[2].y) {
 472         dest->x = (p[0].x + p[1].x) / 2;
 473         dest->y = (p[0].y + p[1].y) / 2;
 474         return 1;
 475     }
 476
 477     /* Translate the 3 points of -p0, to simplify the formula */
 478     cpml_pair_sub(cpml_pair_copy(&b, &p[1]), &p[0]);
 479     cpml_pair_sub(cpml_pair_copy(&c, &p[2]), &p[0]);
 480
 481     /* Check for division by 0, that is the case where the 3 given points
 482      * are laying on a straight line and there is no fitting circle */
 483     d = (b.x*c.y - b.y*c.x) * 2;
 484     if (d == 0.)
 485         return 0;
 486
 487     b2 = b.x*b.x + b.y*b.y;
 488     c2 = c.x*c.x + c.y*c.y;
 489
 490     dest->x = (c.y*b2 - b.y*c2) / d + p[0].x;
 491     dest->y = (b.x*c2 - c.x*b2) / d + p[0].y;
 492
 493     return 1;
 494 }
 495
 496 static void
 497 get_angles(const CpmlPair *p, const CpmlPair *center,
 498            double *start, double *end)
 499 {
 500     CpmlVector vector;
 501     double mid;
 502
 503     /* Calculate the starting angle */
 504     cpml_pair_sub(cpml_pair_copy(&vector, &p[0]), center);
 505     *start = cpml_vector_angle(&vector);
 506
 507     if (p[0].x == p[2].x && p[0].y == p[2].y) {
 508         /* When p[0] and p[2] are cohincidents, p[0]..p[1] is the diameter
 509          * of a circle: return by convention start=start end=start+2PI */
 510         *end = *start + M_PI*2;
 511     } else {
 512         /* Calculate the mid and end angle */
 513         cpml_pair_sub(cpml_pair_copy(&vector, &p[1]), center);
 514         mid = cpml_vector_angle(&vector);
 515         cpml_pair_sub(cpml_pair_copy(&vector, &p[2]), center);
 516         *end = cpml_vector_angle(&vector);
 517
 518         if (*end > *start) {
 519             if (mid > *end || mid < *start)
 520                 *start += M_PI*2;
 521         } else {
 522             if (mid < *end || mid > *start)
 523                 *end += M_PI*2;
 524         }
 525     }
 526 }
 527
 528 static void
 529 arc_to_curve(CpmlPrimitive *curve, const CpmlPair *center,
 530              double r, double start, double end)
 531 {
 532     double r_sin1, r_cos1;
 533     double r_sin2, r_cos2;
 534     double h;
 535
 536     r_sin1 = r*sin(start);
 537     r_cos1 = r*cos(start);
 538     r_sin2 = r*sin(end);
 539     r_cos2 = r*cos(end);
 540
 541     h = 4./3. * tan((end-start) / 4.);
 542
 543     curve->data[0].header.type = CAIRO_PATH_CURVE_TO;
 544     curve->data[0].header.length = 4;
 545     curve->data[1].point.x = center->x + r_cos1 - h*r_sin1;
 546     curve->data[1].point.y = center->y + r_sin1 + h*r_cos1;
 547     curve->data[2].point.x = center->x + r_cos2 + h*r_sin2;
 548     curve->data[2].point.y = center->y + r_sin2 - h*r_cos2;
 549     curve->data[3].point.x = center->x + r_cos2;
 550     curve->data[3].point.y = center->y + r_sin2;
 551 }