cairo-path-stroke.c

按照官方的说法：Cairo is a vector graphics library with cross-device output support. 翻译过来

    double line_dx, line_dy;
    double face_dx, face_dy;
    cairo_point_double_t usr_vector;
    cairo_point_t offset_ccw, offset_cw;

    line_dx = _cairo_fixed_to_double (slope->dx);
    line_dy = _cairo_fixed_to_double (slope->dy);

    /* faces are normal in user space, not device space */
    cairo_matrix_transform_distance (stroker->ctm_inverse, &line_dx, &line_dy);

    mag = sqrt (line_dx * line_dx + line_dy * line_dy);
    if (mag == 0) {
	/* XXX: Can't compute other face points. Do we want a tag in the face for this case? */
	return;
    }

    /* normalize to unit length */
    line_dx /= mag;
    line_dy /= mag;

    usr_vector.x = line_dx;
    usr_vector.y = line_dy;

    /*
     * rotate to get a line_width/2 vector along the face, note that
     * the vector must be rotated the right direction in device space,
     * but by 90° in user space. So, the rotation depends on
     * whether the ctm reflects or not, and that can be determined
     * by looking at the determinant of the matrix.
     */
    _cairo_matrix_compute_determinant (stroker->ctm, &det);
    if (det >= 0)
    {
	face_dx = - line_dy * (stroker->style->line_width / 2.0);
	face_dy = line_dx * (stroker->style->line_width / 2.0);
    }
    else
    {
	face_dx = line_dy * (stroker->style->line_width / 2.0);
	face_dy = - line_dx * (stroker->style->line_width / 2.0);
    }

    /* back to device space */
    cairo_matrix_transform_distance (stroker->ctm, &face_dx, &face_dy);

    offset_ccw.x = _cairo_fixed_from_double (face_dx);
    offset_ccw.y = _cairo_fixed_from_double (face_dy);
    offset_cw.x = -offset_ccw.x;
    offset_cw.y = -offset_ccw.y;

    face->ccw = *point;
    _translate_point (&face->ccw, &offset_ccw);

    face->point = *point;

    face->cw = *point;
    _translate_point (&face->cw, &offset_cw);

    face->usr_vector.x = usr_vector.x;
    face->usr_vector.y = usr_vector.y;

    face->dev_vector = *slope;
}

static cairo_status_t
_cairo_stroker_add_sub_edge (cairo_stroker_t *stroker, cairo_point_t *p1, cairo_point_t *p2,
			     cairo_slope_t *slope, cairo_stroke_face_t *start,
			     cairo_stroke_face_t *end)
{
    cairo_status_t status;
    cairo_polygon_t polygon;

    _compute_face (p1, slope, stroker, start);

    /* XXX: This could be optimized slightly by not calling
       _compute_face again but rather  translating the relevant
       fields from start. */
    _compute_face (p2, slope, stroker, end);

    if (p1->x == p2->x && p1->y == p2->y)
	return CAIRO_STATUS_SUCCESS;

    /* XXX: I should really check the return value of the
       move_to/line_to functions here to catch out of memory
       conditions. But since that would be ugly, I'd prefer to add a
       status flag to the polygon object that I could check only once
       at then end of this sequence, (like we do with cairo_t
       already). */
    _cairo_polygon_init (&polygon);
    _cairo_polygon_move_to (&polygon, &start->cw);
    _cairo_polygon_line_to (&polygon, &start->ccw);
    _cairo_polygon_line_to (&polygon, &end->ccw);
    _cairo_polygon_line_to (&polygon, &end->cw);
    _cairo_polygon_close (&polygon);

    /* XXX: We can't use tessellate_rectangle as the matrix may have
       skewed this into a non-rectangular shape. Perhaps it would be
       worth checking the matrix for skew so that the common case
       could use the faster tessellate_rectangle rather than
       tessellate_polygon? */
    status = _cairo_traps_tessellate_polygon (stroker->traps,
					      &polygon, CAIRO_FILL_RULE_WINDING);

    _cairo_polygon_fini (&polygon);

    return status;
}

static cairo_status_t
_cairo_stroker_move_to (void *closure, cairo_point_t *point)
{
    cairo_status_t status;
    cairo_stroker_t *stroker = closure;

    status = _cairo_stroker_add_caps (stroker);
    if (status)
	return status;

    stroker->first_point = *point;
    stroker->current_point = *point;

    stroker->has_first_face = FALSE;
    stroker->has_current_face = FALSE;
    stroker->has_sub_path = FALSE;

    return CAIRO_STATUS_SUCCESS;
}

static cairo_status_t
_cairo_stroker_move_to_dashed (void *closure, cairo_point_t *point)
{
    /* reset the dash pattern for new sub paths */
    cairo_stroker_t *stroker = closure;
    _cairo_stroker_start_dash (stroker);

    return _cairo_stroker_move_to (closure, point);
}

static cairo_status_t
_cairo_stroker_line_to (void *closure, cairo_point_t *point)
{
    cairo_status_t status;
    cairo_stroker_t *stroker = closure;
    cairo_stroke_face_t start, end;
    cairo_point_t *p1 = &stroker->current_point;
    cairo_point_t *p2 = point;
    cairo_slope_t slope;

    stroker->has_sub_path = TRUE;

    if (p1->x == p2->x && p1->y == p2->y)
	return CAIRO_STATUS_SUCCESS;

    _cairo_slope_init (&slope, p1, p2);

    status = _cairo_stroker_add_sub_edge (stroker, p1, p2, &slope, &start, &end);
    if (status)
	return status;

    if (stroker->has_current_face) {
	status = _cairo_stroker_join (stroker, &stroker->current_face, &start);
	if (status)
	    return status;
    } else {
	if (!stroker->has_first_face) {
	    stroker->first_face = start;
	    stroker->has_first_face = TRUE;
	}
    }
    stroker->current_face = end;
    stroker->has_current_face = TRUE;

    stroker->current_point = *point;

    return CAIRO_STATUS_SUCCESS;
}

/*
 * Dashed lines.  Cap each dash end, join around turns when on
 */
static cairo_status_t
_cairo_stroker_line_to_dashed (void *closure, cairo_point_t *point)
{
    cairo_status_t status = CAIRO_STATUS_SUCCESS;
    cairo_stroker_t *stroker = closure;
    double mag, remain, tmp;
    double dx, dy;
    double dx2, dy2;
    cairo_point_t fd1, fd2;
    cairo_bool_t first = TRUE;
    cairo_stroke_face_t sub_start, sub_end;
    cairo_point_t *p1 = &stroker->current_point;
    cairo_point_t *p2 = point;
    cairo_slope_t slope;

    if (p1->x == p2->x && p1->y == p2->y)
	return CAIRO_STATUS_SUCCESS;

    _cairo_slope_init (&slope, p1, p2);

    dx = _cairo_fixed_to_double (p2->x - p1->x);
    dy = _cairo_fixed_to_double (p2->y - p1->y);

    cairo_matrix_transform_distance (stroker->ctm_inverse, &dx, &dy);

    mag = sqrt (dx *dx + dy * dy);
    remain = mag;
    fd1 = *p1;
    while (remain) {
	tmp = stroker->dash_remain;
	if (tmp > remain)
	    tmp = remain;
	remain -= tmp;
        dx2 = dx * (mag - remain)/mag;
	dy2 = dy * (mag - remain)/mag;
	cairo_matrix_transform_distance (stroker->ctm, &dx2, &dy2);
	fd2.x = _cairo_fixed_from_double (dx2);
	fd2.y = _cairo_fixed_from_double (dy2);
	fd2.x += p1->x;
	fd2.y += p1->y;
	/*
	 * XXX simplify this case analysis
	 */
	if (stroker->dash_on) {
	    status = _cairo_stroker_add_sub_edge (stroker, &fd1, &fd2, &slope, &sub_start, &sub_end);
	    if (status)
		return status;
	    if (!first) {
		/*
		 * Not first dash in this segment, cap start
		 */
		status = _cairo_stroker_add_leading_cap (stroker, &sub_start);
		if (status)
		    return status;
	    } else {
		/*
		 * First in this segment, join to any current_face, else
		 * if at start of sub-path, mark position, else
		 * cap
		 */
		if (stroker->has_current_face) {
		    status = _cairo_stroker_join (stroker, &stroker->current_face, &sub_start);
		    if (status)
			return status;
		} else {
		    if (!stroker->has_first_face) {
			stroker->first_face = sub_start;
			stroker->has_first_face = TRUE;
		    } else {
			status = _cairo_stroker_add_leading_cap (stroker, &sub_start);
			if (status)
			    return status;
		    }
		}
	    }
	    if (remain) {
		/*
		 * Cap if not at end of segment
		 */
		status = _cairo_stroker_add_trailing_cap (stroker, &sub_end);
		if (status)
		    return status;
	    } else {
		/*
		 * Mark previous line face and fix up next time
		 * through
		 */
		stroker->current_face = sub_end;
		stroker->has_current_face = TRUE;
	    }
	} else {
	    /*
	     * If starting with off dash, check previous face
	     * and cap if necessary
	     */
	    if (first) {
		if (stroker->has_current_face) {
		    status = _cairo_stroker_add_trailing_cap (stroker, &stroker->current_face);
		    if (status)
			return status;
		}
	    }
	    if (!remain)
		stroker->has_current_face = FALSE;
	}
	_cairo_stroker_step_dash (stroker, tmp);
	fd1 = fd2;
	first = FALSE;
    }

    stroker->current_point = *point;

    return status;
}

static cairo_status_t
_cairo_stroker_curve_to (void *closure,
			 cairo_point_t *b,
			 cairo_point_t *c,
			 cairo_point_t *d)
{
    cairo_status_t status = CAIRO_STATUS_SUCCESS;
    cairo_stroker_t *stroker = closure;
    cairo_spline_t spline;
    cairo_pen_t pen;
    cairo_stroke_face_t start, end;
    cairo_point_t extra_points[4];
    cairo_point_t *a = &stroker->current_point;

    status = _cairo_spline_init (&spline, a, b, c, d);
    if (status == CAIRO_INT_STATUS_DEGENERATE)
	return CAIRO_STATUS_SUCCESS;

    status = _cairo_pen_init_copy (&pen, &stroker->pen);
    if (status)
	goto CLEANUP_SPLINE;

    _compute_face (a, &spline.initial_slope, stroker, &start);
    _compute_face (d, &spline.final_slope, stroker, &end);

    if (stroker->has_current_face) {
	status = _cairo_stroker_join (stroker, &stroker->current_face, &start);
	if (status)
	    return status;
    } else {
	if (!stroker->has_first_face) {
	    stroker->first_face = start;
	    stroker->has_first_face = TRUE;
	}
    }
    stroker->current_face = end;
    stroker->has_current_face = TRUE;

    extra_points[0] = start.cw;
    extra_points[0].x -= start.point.x;
    extra_points[0].y -= start.point.y;
    extra_points[1] = start.ccw;
    extra_points[1].x -= start.point.x;
    extra_points[1].y -= start.point.y;
    extra_points[2] = end.cw;
    extra_points[2].x -= end.point.x;
    extra_points[2].y -= end.point.y;
    extra_points[3] = end.ccw;
    extra_points[3].x -= end.point.x;
    extra_points[3].y -= end.point.y;

    status = _cairo_pen_add_points (&pen, extra_points, 4);
    if (status)
	goto CLEANUP_PEN;

    status = _cairo_pen_stroke_spline (&pen, &spline, stroker->tolerance, stroker->traps);
    if (status)
	goto CLEANUP_PEN;

  CLEANUP_PEN:
    _cairo_pen_fini (&pen);
  CLEANUP_SPLINE:
    _cairo_spline_fini (&spline);

    stroker->current_point = *d;

    return status;
}

/* We're using two different algorithms here for dashed and un-dashed
 * splines. The dashed alogorithm uses the existing line dashing
 * code. It's linear in path length, but gets subtly wrong results for
 * self-intersecting paths (an outstanding but for self-intersecting
 * non-curved paths as well). The non-dashed algorithm tessellates a
 * single polygon for the whole curve. It handles the
 * self-intersecting problem, but it's (unsurprisingly) not O(n) and
 * more significantly, it doesn't yet handle dashes.
 *
 * The only reason we're doing split algortihms here is to
 * minimize the impact of fixing the splines-aren't-dashed bug for
 * 1.0.2. Long-term the right answer is to rewrite the whole pile
 * of stroking code so that the entire result is computed as a
 * single polygon that is tessellated, (that is, stroking can be
 * built on top of filling). That will solve the self-intersecting
 * problem. It will also increase the importance of implementing
 * an efficient and more robust tessellator.
 */
static cairo_status_t
_cairo_stroker_curve_to_dashed (void *closure,
				cairo_point_t *b,
				cairo_point_t *c,
				cairo_point_t *d)
{
    cairo_status_t status = CAIRO_STATUS_SUCCESS;
    cairo_stroker_t *stroker = closure;
    cairo_spline_t spline;
    cairo_point_t *a = &stroker->current_point;
    cairo_line_join_t line_join_save;
    int i;

    status = _cairo_spline_init (&spline, a, b, c, d);
    if (status == CAIRO_INT_STATUS_DEGENERATE)
	return CAIRO_STATUS_SUCCESS;

    /* If the line width is so small that the pen is reduced to a
       single point, then we have nothing to do. */
    if (stroker->pen.num_vertices <= 1)
	goto CLEANUP_SPLINE;

    /* Temporarily modify the stroker to use round joins to guarantee
     * smooth stroked curves. */
    line_join_save = stroker->style->line_join;
    stroker->style->line_join = CAIRO_LINE_JOIN_ROUND;

    status = _cairo_spline_decompose (&spline, stroker->tolerance);
    if (status)
	goto CLEANUP_GSTATE;

    for (i = 1; i < spline.num_points; i++) {
	if (stroker->dashed)
	    status = _cairo_stroker_line_to_dashed (stroker, &spline.points[i]);
	else
	    status = _cairo_stroker_line_to (stroker, &spline.points[i]);
	if (status)
	    break;
    }

  CLEANUP_GSTATE:
    stroker->style->line_join = line_join_save;

  CLEANUP_SPLINE:
    _cairo_spline_fini (&spline);

    return status;
}

static cairo_status_t
_cairo_stroker_close_path (void *closure)
{
    cairo_status_t status;
    cairo_stroker_t *stroker = closure;

    if (stroker->dashed)
	status = _cairo_stroker_line_to_dashed (stroker, &stroker->first_point);
    else
	status = _cairo_stroker_line_to (stroker, &stroker->first_point);
    if (status)
	return status;

    if (stroker->has_first_face && stroker->has_current_face) {
	status = _cairo_stroker_join (stroker, &stroker->current_face, &stroker->first_face);
	if (status)
	    return status;
    } else {
	status = _cairo_stroker_add_caps (stroker);
	if (status)
	    return status;
    }

    stroker->has_sub_path = FALSE;
    stroker->has_first_face = FALSE;
    stroker->has_current_face = FALSE;

    return CAIRO_STATUS_SUCCESS;
}

cairo_status_t
_cairo_path_fixed_stroke_to_traps (cairo_path_fixed_t	*path,
				   cairo_stroke_style_t	*stroke_style,
				   cairo_matrix_t	*ctm,
				   cairo_matrix_t	*ctm_inverse,
				   double		 tolerance,
				   cairo_traps_t	*traps)
{
    cairo_status_t status = CAIRO_STATUS_SUCCESS;
    cairo_stroker_t stroker;

    _cairo_stroker_init (&stroker, stroke_style,
			 ctm, ctm_inverse, tolerance,
			 traps);

    if (stroker.style->dash)
	status = _cairo_path_fixed_interpret (path,
					      CAIRO_DIRECTION_FORWARD,
					      _cairo_stroker_move_to_dashed,
					      _cairo_stroker_line_to_dashed,
					      _cairo_stroker_curve_to_dashed,
					      _cairo_stroker_close_path,
					      &stroker);
    else
	status = _cairo_path_fixed_interpret (path,
					      CAIRO_DIRECTION_FORWARD,
					      _cairo_stroker_move_to,
					      _cairo_stroker_line_to,
					      _cairo_stroker_curve_to,
					      _cairo_stroker_close_path,
					      &stroker);
    if (status)
	goto BAIL;

    status = _cairo_stroker_add_caps (&stroker);

BAIL:
    _cairo_stroker_fini (&stroker);

    return status;
}