cairotwisted.c

Pango is a library for layout and rendering of text, with an emphasis on internationalization. Pang

/* Example code to show how to use pangocairo to render text
 * projected on a path.
 *
 *
 * Written by Behdad Esfahbod, 2006..2007
 *
 * Permission to use, copy, modify, distribute, and sell this example
 * for any purpose is hereby granted without fee.
 * It is provided "as is" without express or implied warranty.
 */

#include <math.h>
#include <stdlib.h>
#include <pango/pangocairo.h>

void fancy_cairo_stroke (cairo_t *cr);
void fancy_cairo_stroke_preserve (cairo_t *cr);

/* A fancy cairo_stroke[_preserve]() that draws points and control
 * points, and connects them together.
 */
static void
_fancy_cairo_stroke (cairo_t *cr, cairo_bool_t preserve)
{
  int i;
  double line_width;
  cairo_path_t *path;
  cairo_path_data_t *data;
  const double dash[] = {10, 10};

  cairo_save (cr);
  cairo_set_source_rgb (cr, 1.0, 0.0, 0.0);

  line_width = cairo_get_line_width (cr);
  path = cairo_copy_path (cr);
  cairo_new_path (cr);

  cairo_save (cr);
  cairo_set_line_width (cr, line_width / 3);
  cairo_set_dash (cr, dash, G_N_ELEMENTS (dash), 0);
  for (i=0; i < path->num_data; i += path->data[i].header.length) {
    data = &path->data[i];
    switch (data->header.type) {
    case CAIRO_PATH_MOVE_TO:
    case CAIRO_PATH_LINE_TO:
	cairo_move_to (cr, data[1].point.x, data[1].point.y);
	break;
    case CAIRO_PATH_CURVE_TO:
	cairo_line_to (cr, data[1].point.x, data[1].point.y);
	cairo_move_to (cr, data[2].point.x, data[2].point.y);
	cairo_line_to (cr, data[3].point.x, data[3].point.y);
	break;
    case CAIRO_PATH_CLOSE_PATH:
	break;
    default:
	g_assert_not_reached ();
    }
  }
  cairo_stroke (cr);
  cairo_restore (cr);

  cairo_save (cr);
  cairo_set_line_width (cr, line_width * 4);
  cairo_set_line_cap (cr, CAIRO_LINE_CAP_ROUND);
  for (i=0; i < path->num_data; i += path->data[i].header.length) {
    data = &path->data[i];
    switch (data->header.type) {
    case CAIRO_PATH_MOVE_TO:
	cairo_move_to (cr, data[1].point.x, data[1].point.y);
	break;
    case CAIRO_PATH_LINE_TO:
	cairo_rel_line_to (cr, 0, 0);
	cairo_move_to (cr, data[1].point.x, data[1].point.y);
	break;
    case CAIRO_PATH_CURVE_TO:
	cairo_rel_line_to (cr, 0, 0);
	cairo_move_to (cr, data[1].point.x, data[1].point.y);
	cairo_rel_line_to (cr, 0, 0);
	cairo_move_to (cr, data[2].point.x, data[2].point.y);
	cairo_rel_line_to (cr, 0, 0);
	cairo_move_to (cr, data[3].point.x, data[3].point.y);
	break;
    case CAIRO_PATH_CLOSE_PATH:
	cairo_rel_line_to (cr, 0, 0);
	break;
    default:
	g_assert_not_reached ();
    }
  }
  cairo_rel_line_to (cr, 0, 0);
  cairo_stroke (cr);
  cairo_restore (cr);

  for (i=0; i < path->num_data; i += path->data[i].header.length) {
    data = &path->data[i];
    switch (data->header.type) {
    case CAIRO_PATH_MOVE_TO:
	cairo_move_to (cr, data[1].point.x, data[1].point.y);
	break;
    case CAIRO_PATH_LINE_TO:
	cairo_line_to (cr, data[1].point.x, data[1].point.y);
	break;
    case CAIRO_PATH_CURVE_TO:
	cairo_curve_to (cr, data[1].point.x, data[1].point.y,
			    data[2].point.x, data[2].point.y,
			    data[3].point.x, data[3].point.y);
	break;
    case CAIRO_PATH_CLOSE_PATH:
	cairo_close_path (cr);
	break;
    default:
	g_assert_not_reached ();
    }
  }
  cairo_stroke (cr);

  if (preserve)
    cairo_append_path (cr, path);

  cairo_path_destroy (path);

  cairo_restore (cr);
}

/* A fancy cairo_stroke() that draws points and control points, and
 * connects them together.
 */
void
fancy_cairo_stroke (cairo_t *cr)
{
  _fancy_cairo_stroke (cr, FALSE);
}

/* A fancy cairo_stroke_preserve() that draws points and control
 * points, and connects them together.
 */
void
fancy_cairo_stroke_preserve (cairo_t *cr)
{
  _fancy_cairo_stroke (cr, TRUE);
}


/* Returns Euclidean distance between two points */
static double
two_points_distance (cairo_path_data_t *a, cairo_path_data_t *b)
{
  double dx, dy;

  dx = b->point.x - a->point.x;
  dy = b->point.y - a->point.y;

  return sqrt (dx * dx + dy * dy);
}

/* Returns length of a Bezier curve.
 * Seems like computing that analytically is not easy.  The
 * code just flattens the curve using cairo and adds the length
 * of segments.
 */
static double
curve_length (double x0, double y0,
	      double x1, double y1,
	      double x2, double y2,
	      double x3, double y3)
{
  cairo_surface_t *surface;
  cairo_t *cr;
  cairo_path_t *path;
  cairo_path_data_t *data, current_point;
  int i;
  double length;

  surface = cairo_image_surface_create (CAIRO_FORMAT_A8, 0, 0);
  cr = cairo_create (surface);
  cairo_surface_destroy (surface);

  cairo_move_to (cr, x0, y0);
  cairo_curve_to (cr, x1, y1, x2, y2, x3, y3);

  length = 0;
  path = cairo_copy_path_flat (cr);
  for (i=0; i < path->num_data; i += path->data[i].header.length) {
    data = &path->data[i];
    switch (data->header.type) {

    case CAIRO_PATH_MOVE_TO:
	current_point = data[1];
	break;

    case CAIRO_PATH_LINE_TO:
	length += two_points_distance (&current_point, &data[1]);
	current_point = data[1];
	break;

    default:
    case CAIRO_PATH_CURVE_TO:
    case CAIRO_PATH_CLOSE_PATH:
	g_assert_not_reached ();
    }
  }
  cairo_path_destroy (path);

  cairo_destroy (cr);

  return length;
}


typedef double parametrization_t;

/* Compute parametrization info.  That is, for each part of the 
 * cairo path, tags it with its length.
 */
static parametrization_t *
parametrize_path (cairo_path_t *path)
{
  int i;
  cairo_path_data_t *data, current_point;
  parametrization_t *parametrization;

  parametrization = malloc (path->num_data * sizeof (parametrization[0]));

  for (i=0; i < path->num_data; i += path->data[i].header.length) {
    data = &path->data[i];
    parametrization[i] = 0.0;
    switch (data->header.type) {
    case CAIRO_PATH_MOVE_TO:
	current_point = data[1];
	break;
    case CAIRO_PATH_LINE_TO:
	parametrization[i] = two_points_distance (&current_point, &data[1]);
	current_point = data[1];
	break;
    case CAIRO_PATH_CURVE_TO:
	/* naive curve-length, treating bezier as three line segments:
	parametrization[i] = two_points_distance (&current_point, &data[1])
			   + two_points_distance (&data[1], &data[2])
			   + two_points_distance (&data[2], &data[3]);
	*/
	parametrization[i] = curve_length (current_point.point.x, current_point.point.x,
					   data[1].point.x, data[1].point.y,
					   data[2].point.x, data[2].point.y,
					   data[3].point.x, data[3].point.y);

	current_point = data[3];
	break;
    case CAIRO_PATH_CLOSE_PATH:
	break;
    default:
	g_assert_not_reached ();
    }
  }

  return parametrization;
}


typedef void (*transform_point_func_t) (void *closure, double *x, double *y);

/* Project a path using a function.  Each point of the path (including
 * Bezier control points) is passed to the function for transformation.
 */
static void
transform_path (cairo_path_t *path, transform_point_func_t f, void *closure)
{
  int i;
  cairo_path_data_t *data;

  for (i=0; i < path->num_data; i += path->data[i].header.length) {
    data = &path->data[i];
    switch (data->header.type) {
    case CAIRO_PATH_CURVE_TO:
      f (closure, &data[3].point.x, &data[3].point.y);
      f (closure, &data[2].point.x, &data[2].point.y);
    case CAIRO_PATH_MOVE_TO:
    case CAIRO_PATH_LINE_TO:
      f (closure, &data[1].point.x, &data[1].point.y);
      break;
    case CAIRO_PATH_CLOSE_PATH:
      break;
    default:
	g_assert_not_reached ();
    }
  }
}


/* Simple struct to hold a path and its parametrization */
typedef struct {
  cairo_path_t *path;
  parametrization_t *parametrization;
} parametrized_path_t;


/* Project a point X,Y onto a parameterized path.  The final point is
 * where you get if you walk on the path forward from the beginning for X
 * units, then stop there and walk another Y units perpendicular to the
 * path at that point.  In more detail:
 *
 * There's three pieces of math involved:
 *
 *   - The parametric form of the Line equation
 *     http://en.wikipedia.org/wiki/Line
 *
 *   - The parametric form of the Cubic Bézier curve equation