Subversion Repositories Kolibri OS

/* cairo - a vector graphics library with display and print output

 *

 * Copyright © 2002 University of Southern California

 *

 * This library is free software; you can redistribute it and/or

 * modify it either under the terms of the GNU Lesser General Public

 * License version 2.1 as published by the Free Software Foundation

 * (the "LGPL") or, at your option, under the terms of the Mozilla

 * Public License Version 1.1 (the "MPL"). If you do not alter this

 * notice, a recipient may use your version of this file under either

 * the MPL or the LGPL.

 *

 * You should have received a copy of the LGPL along with this library

 * in the file COPYING-LGPL-2.1; if not, write to the Free Software

 * Foundation, Inc., 51 Franklin Street, Suite 500, Boston, MA 02110-1335, USA

 * You should have received a copy of the MPL along with this library

 * in the file COPYING-MPL-1.1

 *

 * The contents of this file are subject to the Mozilla Public License

 * Version 1.1 (the "License"); you may not use this file except in

 * compliance with the License. You may obtain a copy of the License at

 * http://www.mozilla.org/MPL/

 *

 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY

 * OF ANY KIND, either express or implied. See the LGPL or the MPL for

 * the specific language governing rights and limitations.

 *

 * The Original Code is the cairo graphics library.

 *

 * The Initial Developer of the Original Code is University of Southern

 * California.

 *

 * Contributor(s):

 *	Carl D. Worth 

 */

#include "cairoint.h"

#include "cairo-arc-private.h"

#define MAX_FULL_CIRCLES 65536

/* Spline deviation from the circle in radius would be given by:

	error = sqrt (x**2 + y**2) - 1

   A simpler error function to work with is:

	e = x**2 + y**2 - 1

   From "Good approximation of circles by curvature-continuous Bezier

   curves", Tor Dokken and Morten Daehlen, Computer Aided Geometric

   Design 8 (1990) 22-41, we learn:

	abs (max(e)) = 4/27 * sin**6(angle/4) / cos**2(angle/4)

   and

	abs (error) =~ 1/2 * e

   Of course, this error value applies only for the particular spline

   approximation that is used in _cairo_gstate_arc_segment.

*/

static double

_arc_error_normalized (double angle)

{

    return 2.0/27.0 * pow (sin (angle / 4), 6) / pow (cos (angle / 4), 2);

}

static double

_arc_max_angle_for_tolerance_normalized (double tolerance)

{

    double angle, error;

    int i;

    /* Use table lookup to reduce search time in most cases. */

    struct {

	double angle;

	double error;

    } table[] = {

	{ M_PI / 1.0,   0.0185185185185185036127 },

	{ M_PI / 2.0,   0.000272567143730179811158 },

	{ M_PI / 3.0,   2.38647043651461047433e-05 },

	{ M_PI / 4.0,   4.2455377443222443279e-06 },

	{ M_PI / 5.0,   1.11281001494389081528e-06 },

	{ M_PI / 6.0,   3.72662000942734705475e-07 },

	{ M_PI / 7.0,   1.47783685574284411325e-07 },

	{ M_PI / 8.0,   6.63240432022601149057e-08 },

	{ M_PI / 9.0,   3.2715520137536980553e-08 },

	{ M_PI / 10.0,  1.73863223499021216974e-08 },

	{ M_PI / 11.0,  9.81410988043554039085e-09 },

    };

    int table_size = ARRAY_LENGTH (table);

    for (i = 0; i < table_size; i++)

	if (table[i].error < tolerance)

	    return table[i].angle;

    ++i;

    do {

	angle = M_PI / i++;

	error = _arc_error_normalized (angle);

    } while (error > tolerance);

    return angle;

}

static int

_arc_segments_needed (double	      angle,

		      double	      radius,

		      cairo_matrix_t *ctm,

		      double	      tolerance)

{

    double major_axis, max_angle;

    /* the error is amplified by at most the length of the

     * major axis of the circle; see cairo-pen.c for a more detailed analysis

     * of this. */

    major_axis = _cairo_matrix_transformed_circle_major_axis (ctm, radius);

    max_angle = _arc_max_angle_for_tolerance_normalized (tolerance / major_axis);

    return ceil (fabs (angle) / max_angle);

}

/* We want to draw a single spline approximating a circular arc radius

   R from angle A to angle B. Since we want a symmetric spline that

   matches the endpoints of the arc in position and slope, we know

   that the spline control points must be:

	(R * cos(A), R * sin(A))

	(R * cos(A) - h * sin(A), R * sin(A) + h * cos (A))

	(R * cos(B) + h * sin(B), R * sin(B) - h * cos (B))

	(R * cos(B), R * sin(B))

   for some value of h.

   "Approximation of circular arcs by cubic polynomials", Michael

   Goldapp, Computer Aided Geometric Design 8 (1991) 227-238, provides

   various values of h along with error analysis for each.

   From that paper, a very practical value of h is:

	h = 4/3 * R * tan(angle/4)

   This value does not give the spline with minimal error, but it does

   provide a very good approximation, (6th-order convergence), and the

   error expression is quite simple, (see the comment for

   _arc_error_normalized).

*/

static void

_cairo_arc_segment (cairo_t *cr,

		    double   xc,

		    double   yc,

		    double   radius,

		    double   angle_A,

		    double   angle_B)

{

    double r_sin_A, r_cos_A;

    double r_sin_B, r_cos_B;

    double h;

    r_sin_A = radius * sin (angle_A);

    r_cos_A = radius * cos (angle_A);

    r_sin_B = radius * sin (angle_B);

    r_cos_B = radius * cos (angle_B);

    h = 4.0/3.0 * tan ((angle_B - angle_A) / 4.0);

    cairo_curve_to (cr,

		    xc + r_cos_A - h * r_sin_A,

		    yc + r_sin_A + h * r_cos_A,

		    xc + r_cos_B + h * r_sin_B,

		    yc + r_sin_B - h * r_cos_B,

		    xc + r_cos_B,

		    yc + r_sin_B);

}

static void

_cairo_arc_in_direction (cairo_t	  *cr,

			 double		   xc,

			 double		   yc,

			 double		   radius,

			 double		   angle_min,

			 double		   angle_max,

			 cairo_direction_t dir)

{

    if (cairo_status (cr))

        return;

    assert (angle_max >= angle_min);

    if (angle_max - angle_min > 2 * M_PI * MAX_FULL_CIRCLES) {

	angle_max = fmod (angle_max - angle_min, 2 * M_PI);

	angle_min = fmod (angle_min, 2 * M_PI);

	angle_max += angle_min + 2 * M_PI * MAX_FULL_CIRCLES;

    }

    /* Recurse if drawing arc larger than pi */

    if (angle_max - angle_min > M_PI) {

	double angle_mid = angle_min + (angle_max - angle_min) / 2.0;

	if (dir == CAIRO_DIRECTION_FORWARD) {

	    _cairo_arc_in_direction (cr, xc, yc, radius,

				     angle_min, angle_mid,

				     dir);

	    _cairo_arc_in_direction (cr, xc, yc, radius,

				     angle_mid, angle_max,

				     dir);

	} else {

	    _cairo_arc_in_direction (cr, xc, yc, radius,

				     angle_mid, angle_max,

				     dir);

	    _cairo_arc_in_direction (cr, xc, yc, radius,

				     angle_min, angle_mid,

				     dir);

	}

    } else if (angle_max != angle_min) {

	cairo_matrix_t ctm;

	int i, segments;

	double step;

	cairo_get_matrix (cr, &ctm);

	segments = _arc_segments_needed (angle_max - angle_min,

					 radius, &ctm,

					 cairo_get_tolerance (cr));

	step = (angle_max - angle_min) / segments;

	segments -= 1;

	if (dir == CAIRO_DIRECTION_REVERSE) {

	    double t;

	    t = angle_min;

	    angle_min = angle_max;

	    angle_max = t;

	    step = -step;

	}

	for (i = 0; i < segments; i++, angle_min += step) {

	    _cairo_arc_segment (cr, xc, yc, radius,

				angle_min, angle_min + step);

	}

	_cairo_arc_segment (cr, xc, yc, radius,

			    angle_min, angle_max);

    } else {

	cairo_line_to (cr,

		       xc + radius * cos (angle_min),

		       yc + radius * sin (angle_min));

    }

}

/**

 * _cairo_arc_path:

 * @cr: a cairo context

 * @xc: X position of the center of the arc

 * @yc: Y position of the center of the arc

 * @radius: the radius of the arc

 * @angle1: the start angle, in radians

 * @angle2: the end angle, in radians

 *

 * Compute a path for the given arc and append it onto the current

 * path within @cr. The arc will be accurate within the current

 * tolerance and given the current transformation.

 **/

void

_cairo_arc_path (cairo_t *cr,

		 double	  xc,

		 double	  yc,

		 double	  radius,

		 double	  angle1,

		 double	  angle2)

{

    _cairo_arc_in_direction (cr, xc, yc,

			     radius,

			     angle1, angle2,

			     CAIRO_DIRECTION_FORWARD);

}

/**

 * _cairo_arc_path_negative:

 * @xc: X position of the center of the arc

 * @yc: Y position of the center of the arc

 * @radius: the radius of the arc

 * @angle1: the start angle, in radians

 * @angle2: the end angle, in radians

 * @ctm: the current transformation matrix

 * @tolerance: the current tolerance value

 * @path: the path onto which the arc will be appended

 *

 * Compute a path for the given arc (defined in the negative

 * direction) and append it onto the current path within @cr. The arc

 * will be accurate within the current tolerance and given the current

 * transformation.

 **/

void

_cairo_arc_path_negative (cairo_t *cr,

			  double   xc,

			  double   yc,

			  double   radius,

			  double   angle1,

			  double   angle2)

{

    _cairo_arc_in_direction (cr, xc, yc,

			     radius,

			     angle2, angle1,

			     CAIRO_DIRECTION_REVERSE);

}