/* Warning: This function reorders the elements of the array provided. */
cairo_status_t
_cairo_traps_tessellate_rectangle (cairo_traps_t *traps, cairo_point_t q[4])
{
cairo_status_t status;
qsort (q, 4, sizeof (cairo_point_t), _compare_point_fixed_by_y);
if (q[1].x > q[2].x) {
status = _cairo_traps_add_trap_from_points (traps,
q[0].y, q[1].y, q[0], q[2], q[0], q[1]);
if (status)
return status;
status = _cairo_traps_add_trap_from_points (traps,
q[1].y, q[2].y, q[0], q[2], q[1], q[3]);
if (status)
return status;
status = _cairo_traps_add_trap_from_points (traps,
q[2].y, q[3].y, q[2], q[3], q[1], q[3]);
if (status)
return status;
} else {
status = _cairo_traps_add_trap_from_points (traps,
q[0].y, q[1].y, q[0], q[1], q[0], q[2]);
if (status)
return status;
status = _cairo_traps_add_trap_from_points (traps,
q[1].y, q[2].y, q[1], q[3], q[0], q[2]);
if (status)
return status;
status = _cairo_traps_add_trap_from_points (traps,
q[2].y, q[3].y, q[1], q[3], q[2], q[3]);
if (status)
return status;
}
return CAIRO_STATUS_SUCCESS;
}
cairo_status_t
_cairo_traps_tessellate_convex_quad (cairo_traps_t *traps, cairo_point_t q[4])
{
int a, b, c, d;
int i;
cairo_slope_t ab, ad;
cairo_bool_t b_left_of_d;
/* Choose a as a point with minimal y */
a = 0;
for (i = 1; i < 4; i++)
if (_compare_point_fixed_by_y (&q[i], &q[a]) < 0)
a = i;
/* b and d are adjacent to a, while c is opposite */
b = (a + 1) % 4;
c = (a + 2) % 4;
d = (a + 3) % 4;
/* Choose between b and d so that b.y is less than d.y */
if (_compare_point_fixed_by_y (&q[d], &q[b]) < 0) {
b = (a + 3) % 4;
d = (a + 1) % 4;
}
/* Without freedom left to choose anything else, we have four
* cases to tessellate.
*
* First, we have to determine the Y-axis sort of the four
* vertices, (either abcd or abdc). After that we need to detemine
* which edges will be "left" and which will be "right" in the
* resulting trapezoids. This can be determined by computing a
* slope comparison of ab and ad to determine if b is left of d or
* not.
*
* Note that "left of" here is in the sense of which edges should
* be the left vs. right edges of the trapezoid. In particular, b
* left of d does *not* mean that b.x is less than d.x.
*
* This should hopefully be made clear in the lame ASCII art
* below. Since the same slope comparison is used in all cases, we
* compute it before testing for the Y-value sort. */
/* Note: If a == b then the ab slope doesn't give us any
* information. In that case, we can replace it with the ac (or
* equivalenly the bc) slope which gives us exactly the same
* information we need. At worst the names of the identifiers ab
* and b_left_of_d are inaccurate in this case, (would be ac, and
* c_left_of_d). */
if (q[a].x == q[b].x && q[a].y == q[b].y)
_cairo_slope_init (&ab, &q[a], &q[c]);
else
_cairo_slope_init (&ab, &q[a], &q[b]);
_cairo_slope_init (&ad, &q[a], &q[d]);
b_left_of_d = (_cairo_slope_compare (&ab, &ad) > 0);
if (q[c].y <= q[d].y) {
if (b_left_of_d) {
/* Y-sort is abcd and b is left of d, (slope(ab) > slope (ad))
*
* top bot left right
* _a a a
* / / /| |\ a.y b.y ab ad
* b / b | b \
* / / | | \ \ b.y c.y bc ad
* c / c | c \
* | / \| \ \ c.y d.y cd ad
* d d d
*/
_cairo_traps_add_trap_from_points (traps,
q[a].y, q[b].y,
q[a], q[b], q[a], q[d]);
_cairo_traps_add_trap_from_points (traps,
q[b].y, q[c].y,
q[b], q[c], q[a], q[d]);
_cairo_traps_add_trap_from_points (traps,
q[c].y, q[d].y,
q[c], q[d], q[a], q[d]);
} else {
/* Y-sort is abcd and b is right of d, (slope(ab) <= slope (ad))
*
* a a a_
* /| |\ \ \ a.y b.y ad ab
* / b | b \ b
* / / | | \ \ b.y c.y ad bc
* / c | c \ c
* / / |/ \ | c.y d.y ad cd
* d d d
*/
_cairo_traps_add_trap_from_points (traps,
q[a].y, q[b].y,
q[a], q[d], q[a], q[b]);
_cairo_traps_add_trap_from_points (traps,
q[b].y, q[c].y,
q[a], q[d], q[b], q[c]);
_cairo_traps_add_trap_from_points (traps,
q[c].y, q[d].y,
q[a], q[d], q[c], q[d]);
}
} else {
if (b_left_of_d) {
/* Y-sort is abdc and b is left of d, (slope (ab) > slope (ad))
*
* a a a
* // / \ |\ a.y b.y ab ad
* /b/ b \ b \
* / / \ \ \ \ b.y d.y bc ad
* /d/ \ d \ d
* // \ / \| d.y c.y bc dc
* c c c
*/
_cairo_traps_add_trap_from_points (traps,
q[a].y, q[b].y,
q[a], q[b], q[a], q[d]);
_cairo_traps_add_trap_from_points (traps,
q[b].y, q[d].y,
q[b], q[c], q[a], q[d]);
_cairo_traps_add_trap_from_points (traps,
q[d].y, q[c].y,
q[b], q[c], q[d], q[c]);
} else {
/* Y-sort is abdc and b is right of d, (slope (ab) <= slope (ad))
*
* a a a
* /| / \ \\ a.y b.y ad ab
* / b / b \b\
* / / / / \ \ b.y d.y ad bc
* d / d / \d\
* |/ \ / \\ d.y c.y dc bc
* c c c
*/
_cairo_traps_add_trap_from_points (traps,
q[a].y, q[b].y,
q[a], q[d], q[a], q[b]);
_cairo_traps_add_trap_from_points (traps,
q[b].y, q[d].y,
q[a], q[d], q[b], q[c]);
_cairo_traps_add_trap_from_points (traps,
q[d].y, q[c].y,
q[d], q[c], q[b], q[c]);
}
}
return traps->status;
}
cairo_status_t
_cairo_traps_tessellate_triangle (cairo_traps_t *traps, cairo_point_t t[3])
{
cairo_status_t status;
cairo_line_t line;
cairo_fixed_16_16_t intersect;
cairo_point_t tsort[3];
memcpy (tsort, t, 3 * sizeof (cairo_point_t));
qsort (tsort, 3, sizeof (cairo_point_t), _compare_point_fixed_by_y);
/* horizontal top edge requires special handling */
if (tsort[0].y == tsort[1].y) {
if (tsort[0].x < tsort[1].x)
status = _cairo_traps_add_trap_from_points (traps,
tsort[1].y, tsort[2].y,
tsort[0], tsort[2],
tsort[1], tsort[2]);
else
status = _cairo_traps_add_trap_from_points (traps,
tsort[1].y, tsort[2].y,
tsort[1], tsort[2],
tsort[0], tsort[2]);
return status;
}
line.p1 = tsort[0];
line.p2 = tsort[1];
intersect = _compute_x (&line, tsort[2].y);
if (intersect < tsort[2].x) {
status = _cairo_traps_add_trap_from_points (traps,
tsort[0].y, tsort[1].y,
tsort[0], tsort[1],
tsort[0], tsort[2]);
if (status)
return status;
status = _cairo_traps_add_trap_from_points (traps,
tsort[1].y, tsort[2].y,
tsort[1], tsort[2],
tsort[0], tsort[2]);
if (status)
return status;
} else {
status = _cairo_traps_add_trap_from_points (traps,
tsort[0].y, tsort[1].y,
tsort[0], tsort[2],
tsort[0], tsort[1]);
if (status)
return status;
status = _cairo_traps_add_trap_from_points (traps,
tsort[1].y, tsort[2].y,
tsort[0], tsort[2],
tsort[1], tsort[2]);
if (status)
return status;
}
return CAIRO_STATUS_SUCCESS;
}
