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polygon: Fix generic case of edge clipping

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The edge clipper is more complicated than it should be and contains a
subtle bug: when an edge is almost horizontal, it is always considered
as having a positive slope.

Explain what should be done and do it in a simpler way.

Fixes horizontal-clip.
This commit is contained in:
Andrea Canciani 2011-07-27 18:58:03 +02:00
parent 6aa6b7daa8
commit d7abdab931
1 changed files with 49 additions and 74 deletions
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123
src/cairo-polygon.c
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@ -333,97 +333,72 @@ _add_clipped_edge (cairo_polygon_t *polygon,
} else {
/* The edge and the box intersect in a generic way */
cairo_fixed_t left_y, right_y;
int p1_y, p2_y;
cairo_point_t p[2];
cairo_bool_t top_left_to_bottom_right;
left_y = _cairo_edge_compute_intersection_y_for_x (p1, p2,
limits->p1.x);
right_y = _cairo_edge_compute_intersection_y_for_x (p1, p2,
limits->p2.x);
p1_y = top;
p2_y = bottom;
/*
* The edge intersects the lines corresponding to the left
* and right sides of the limit box at left_y and right_y,
* but we need to add edges for the range from top_y to
* bot_y.
*
* For both intersections, there are three cases:
*
* 1) It is outside the vertical range of the limit
* box. In this case we can simply further clip the
* edge we will be emitting (i.e. restrict its
* top/bottom limits to those of the limit box).
*
* 2) It is inside the vertical range of the limit
* box. In this case, we need to add the vertical edge
* connecting the correct vertex to the intersection,
* in order to preserve the winding count.
*
* 3) It is exactly on the box. In this case, do nothing.
*
* These operations restrict the active range (stored in
* top_y/bot_y) so that the p1-p2 edge is completely
* inside the box if it is clipped to this vertical range.
*/
if (left_y < right_y) {
if (p1->x < limits->p1.x && left_y > top) {
p[0].x = limits->p1.x;
p[0].y = limits->p1.y;
top_y = p1_y;
if (top_y < p[0].y)
top_y = p[0].y;
top_left_to_bottom_right = (p1->x < p2->x) == (p1->y < p2->y);
p[1].x = limits->p1.x;
p[1].y = limits->p2.y;
bot_y = left_y;
if (bot_y > p[1].y)
bot_y = p[1].y;
if (bot_y > top_y)
_add_edge (polygon, &p[0], &p[1], top_y, bot_y, dir);
p1_y = bot_y;
if (top_left_to_bottom_right) {
left_y = MIN (left_y, bot_y);
if (top_y < left_y) {
_add_edge (polygon, &limits->p1, &bot_left,
top_y, left_y, dir);
top_y = left_y;
}
if (p2->x > limits->p2.x && right_y < bottom) {
p[0].x = limits->p2.x;
p[0].y = limits->p1.y;
top_y = right_y;
if (top_y < p[0].y)
top_y = p[0].y;
p[1].x = limits->p2.x;
p[1].y = limits->p2.y;
bot_y = p2_y;
if (bot_y > p[1].y)
bot_y = p[1].y;
if (bot_y > top_y)
_add_edge (polygon, &p[0], &p[1], top_y, bot_y, dir);
p2_y = top_y;
right_y = MAX (right_y, top_y);
if (bot_y > right_y) {
_add_edge (polygon, &top_right, &limits->p2,
right_y, bot_y, dir);
bot_y = right_y;
}
} else {
if (p1->x > limits->p2.x && right_y > top) {
p[0].x = limits->p2.x;
p[0].y = limits->p1.y;
top_y = p1_y;
if (top_y < p[0].y)
top_y = p[0].y;
p[1].x = limits->p2.x;
p[1].y = limits->p2.y;
bot_y = right_y;
if (bot_y > p[1].y)
bot_y = p[1].y;
if (bot_y > top_y)
_add_edge (polygon, &p[0], &p[1], top_y, bot_y, dir);
p1_y = bot_y;
right_y = MIN (right_y, bot_y);
if (top_y < right_y) {
_add_edge (polygon, &top_right, &limits->p2,
top_y, right_y, dir);
top_y = right_y;
}
if (p2->x < limits->p1.x && left_y < bottom) {
p[0].x = limits->p1.x;
p[0].y = limits->p1.y;
top_y = left_y;
if (top_y < p[0].y)
top_y = p[0].y;
p[1].x = limits->p1.x;
p[1].y = limits->p2.y;
bot_y = p2_y;
if (bot_y > p[1].y)
bot_y = p[1].y;
if (bot_y > top_y)
_add_edge (polygon, &p[0], &p[1], top_y, bot_y, dir);
p2_y = top_y;
left_y = MAX (left_y, top_y);
if (bot_y > left_y) {
_add_edge (polygon, &limits->p1, &bot_left,
left_y, bot_y, dir);
bot_y = left_y;
}
}
if (p1_y < limits->p1.y)
p1_y = limits->p1.y;
if (p2_y > limits->p2.y)
p2_y = limits->p2.y;
if (p2_y > p1_y)
_add_edge (polygon, p1, p2, p1_y, p2_y, dir);
if (top_y != bot_y)
_add_edge (polygon, p1, p2, top_y, bot_y, dir);
}
}
}