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Treat points created by clipping and tiny polygon placeholders as not shared

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Points that clipping creates along a feature's edges, other than at the
existing vertices, and the vertices of tiny polygon placeholders, are
not vertices of the original geometry, so they are now marked as not
being shared nodes instead of being looked up in the global list of
shared nodes when they are simplified. This could only change the output
where a vertex of some other feature happens to fall exactly on one of
these new points, and in practice none of them ever turned out to be
shared nodes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P2nBqZisxNQfmEmon3vE9v
This commit is contained in:
Claude
2026-09-23 18:28:04 +00:00
parent 1dee1890e7
commit f03cc8ce0e
2 changed files with 29 additions and 12 deletions
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clip.cpp
+20 -9
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@@ -17,7 +17,6 @@
#include "read_json.hpp"
// A vertex being clipped, which remembers whether it is a shared node.
// Vertices created at the intersections with the clipping edges don't know.
struct clip_point_d {
double first;
double second;
@@ -146,9 +145,10 @@ drawvec clip_lines(drawvec &geom, long long minx, long long miny, long long maxx
int c = clip(&x1, &y1, &x2, &y2, minx, miny, maxx, maxy);
if (c > 1) { // clipped
// endpoints that weren't moved by clipping keep their node state
out.push_back(draw(VT_MOVETO, x1, y1, (x1 == geom[i - 1].x && y1 == geom[i - 1].y) ? geom[i - 1].node : NODE_UNKNOWN));
out.push_back(draw(VT_LINETO, x2, y2, (x2 == geom[i].x && y2 == geom[i].y) ? geom[i].node : NODE_UNKNOWN));
// endpoints that weren't moved by clipping keep their node state;
// new endpoints along the segment aren't original vertices, so aren't shared nodes
out.push_back(draw(VT_MOVETO, x1, y1, (x1 == geom[i - 1].x && y1 == geom[i - 1].y) ? geom[i - 1].node : NODE_NOT_SHARED));
out.push_back(draw(VT_LINETO, x2, y2, (x2 == geom[i].x && y2 == geom[i].y) ? geom[i].node : NODE_NOT_SHARED));
out.push_back(draw(VT_MOVETO, geom[i].x, geom[i].y, geom[i].node));
} else if (c == 1) { // unchanged
out.push_back(geom[i]);
@@ -800,6 +800,12 @@ static clip_point_d intersect1(clip_point_d const &a, clip_point_d const &b, int
// If the intersection is at one of the endpoints of the segment,
// it is the same vertex, and has the same node state.
//
// Otherwise it is a new point along the segment, which is not a vertex
// of the original geometry, so it is treated as not being a shared node.
// (It could only be one if some other feature had a vertex exactly where
// this feature's edge crosses the clipping line, without this feature
// having a vertex there too.)
static clip_point_d intersect(clip_point_d const &a, clip_point_d const &b, int edge, long long minx, long long miny, long long maxx, long long maxy) {
clip_point_d p = intersect1(a, b, edge, minx, miny, maxx, maxy);
double x = std::round(p.first), y = std::round(p.second);
@@ -808,6 +814,8 @@ static clip_point_d intersect(clip_point_d const &a, clip_point_d const &b, int
p.node = a.node;
} else if (x == b.first && y == b.second) {
p.node = b.node;
} else {
p.node = NODE_NOT_SHARED;
}
return p;
@@ -1123,11 +1131,14 @@ drawvec reduce_tiny_poly(drawvec const &geom, int z, int detail, bool *still_nee
if (area > 0 && *accum_area > pixel * pixel) {
// XXX use centroid;
out.emplace_back(VT_MOVETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2);
out.emplace_back(VT_LINETO, geom[i].x - pixel / 2 + pixel, geom[i].y - pixel / 2);
out.emplace_back(VT_LINETO, geom[i].x - pixel / 2 + pixel, geom[i].y - pixel / 2 + pixel);
out.emplace_back(VT_LINETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2 + pixel);
out.emplace_back(VT_LINETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2);
// This placeholder is synthetic, so none of its vertices
// can be shared nodes from the original geometry.
out.emplace_back(VT_MOVETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2, NODE_NOT_SHARED);
out.emplace_back(VT_LINETO, geom[i].x - pixel / 2 + pixel, geom[i].y - pixel / 2, NODE_NOT_SHARED);
out.emplace_back(VT_LINETO, geom[i].x - pixel / 2 + pixel, geom[i].y - pixel / 2 + pixel, NODE_NOT_SHARED);
out.emplace_back(VT_LINETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2 + pixel, NODE_NOT_SHARED);
out.emplace_back(VT_LINETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2, NODE_NOT_SHARED);
*accum_area -= pixel * pixel;
}