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Avoid placing polygon labels in holes (felt#62)
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* Avoid placing polygon labels in holes

* Traverse vertices in hilbert order to find potential label points

* Update test to reflect new label placements

* Sorting by Y coordinate is better and easier than Hilbert order

* Give a bonus for being near the center of mass

* Make label points a little more border-shy

* Tall places, not just wide places; border lines, not just border points

* Clean up

* Try diagonals through the features too

* Limit the search for label points to prevent slowing down too much

* At this point the center of mass bonus is doing more harm than good

* Forgot to update test
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@e-n-f
e-n-f authored Jan 27, 2023
1 parent c58a8e3 commit e615668
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4 changes: 4 additions & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
@@ -1,3 +1,7 @@
## 2.21.0

* Improve label placement to avoid placing labels in polygon holes

## 2.20.0

* Round coordinates instead of truncating them, for better precision when overzooming
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233 changes: 198 additions & 35 deletions geometry.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,7 @@
#include "main.hpp"
#include "options.hpp"
#include "errors.hpp"
#include "projection.hpp"

static int clip(double *x0, double *y0, double *x1, double *y1, double xmin, double ymin, double xmax, double ymax);

Expand Down Expand Up @@ -1481,25 +1482,117 @@ draw centerOfMass(const drawvec &dv, size_t start, size_t end, draw centre) {
}
}

double label_goodness(const drawvec &dv, size_t start, size_t count, long long x, long long y) {
if (!pnpoly(dv, start, count, x, y)) {
double label_goodness(const drawvec &dv, long long x, long long y) {
int nesting = 0;

for (size_t i = 0; i < dv.size(); i++) {
if (dv[i].op == VT_MOVETO) {
size_t j;
for (j = i + 1; j < dv.size(); j++) {
if (dv[j].op != VT_LINETO) {
break;
}
}

// if it's inside the ring, and it's an outer ring,
// we are nested more; if it's an inner ring, we are
// nested less.
if (pnpoly(dv, i, j - i, x, y)) {
if (get_area(dv, i, j) >= 0) {
nesting++;
} else {
nesting--;
}
}

i = j - 1;
}
}

if (nesting < 1) {
return 0; // outside the polygon is as bad as it gets
}

double closest = INFINITY; // square of closest distance to the border

for (size_t i = start; i < start + count; i++) {
for (size_t i = 0; i < dv.size(); i++) {
double dx = dv[i].x - x;
double dy = dv[i].y - y;
double squared = dx * dx + dy * dy;
if (squared < closest) {
closest = squared;
}

if (i > 0 && dv[i].op == VT_LINETO) {
squared = square_distance_from_line(x, y, dv[i - 1].x, dv[i - 1].y, dv[i].x, dv[i].y);
if (squared < closest) {
closest = squared;
}
}
}

return sqrt(closest);
}

struct sorty {
long long x;
long long y;
};

struct sorty_sorter {
int kind;
sorty_sorter(int k) : kind(k) {};

bool operator()(const sorty &a, const sorty &b) const {
long long xa, ya, xb, yb;

if (kind == 0) { // Y first
xa = a.x;
ya = a.y;

xb = b.x;
yb = b.y;
} else if (kind == 1) { // X first
xa = a.y;
ya = a.x;

xb = b.y;
yb = b.x;
} else if (kind == 2) { // diagonal
xa = a.x + a.y;
ya = a.x - a.y;

xb = b.x + b.y;
yb = b.x - b.y;
} else { // other diagonal
xa = a.x - a.y;
ya = a.x + a.y;

xb = b.x - b.y;
yb = b.x + b.y;
}

if (ya < yb) {
return true;
} else if (ya == yb && xa < xb) {
return true;
} else {
return false;
}
};
};

struct candidate {
long long x;
long long y;
double dist;

bool operator<(const candidate &c) const {
// largest distance sorts first
return dist > c.dist;
};
};

// Generate a label point for a polygon feature.
//
// A good label point will be near the center of the feature and far from any border.
Expand All @@ -1521,6 +1614,10 @@ double label_goodness(const drawvec &dv, size_t start, size_t count, long long x
drawvec polygon_to_anchor(const drawvec &geom) {
size_t start = 0, end = 0;
size_t best_area = 0;
std::vector<sorty> points;

// find the largest outer ring, which will be the best thing
// to label if we can do it.

for (size_t i = 0; i < geom.size(); i++) {
if (geom[i].op == VT_MOVETO) {
Expand All @@ -1529,6 +1626,11 @@ drawvec polygon_to_anchor(const drawvec &geom) {
if (geom[j].op != VT_LINETO) {
break;
}

sorty sy;
sy.x = geom[j].x;
sy.y = geom[j].y;
points.push_back(sy);
}

double area = get_area(geom, i, j);
Expand All @@ -1542,13 +1644,15 @@ drawvec polygon_to_anchor(const drawvec &geom) {
}
}

// If there are no outer rings, don't generate a label point

if (best_area > 0) {
long long xsum = 0;
long long ysum = 0;
size_t count = 0;
long long xmin = LLONG_MAX, ymin = LLONG_MAX, xmax = LLONG_MIN, ymax = LLONG_MIN;

// Calculate centroid and bounding box.
// Calculate centroid and bounding box of biggest ring.
// start + 1 to exclude the first point, which is duplicated as the last
for (size_t k = start + 1; k < end; k++) {
xsum += geom[k].x;
Expand All @@ -1562,17 +1666,79 @@ drawvec polygon_to_anchor(const drawvec &geom) {
}

if (count > 0) {
draw centroid(VT_MOVETO, xsum / count, ysum / count);
draw d = centerOfMass(geom, start, end, centroid);
// We want label points that are at least a moderate distance from
// the edge of the feature. The threshold for what is too close
// is derived from the area of the feature.

double radius = sqrt(best_area / M_PI);
double goodness_threshold = radius / 5;

double goodness = label_goodness(geom, start, end - start - 1, d.x, d.y);
// First choice: Turf's center of mass.

draw centroid(VT_MOVETO, xsum / count, ysum / count);
draw d = centerOfMass(geom, start, end, centroid);
double goodness = label_goodness(geom, d.x, d.y);
const char *kind = "mass";

if (goodness < goodness_threshold) {
// Label is too close to the border or outside it,
// so try some other possible points
// so try some other possible points. Sort the vertices
// both by Y and X coordinate and then by diagonals,
// and walk through each set
// in sorted order. Adjacent pairs of coordinates should
// tend to bounce back and forth between rings, so the
// midpoint of each pair will hopefully be somewhere in the
// interior of the polygon.

std::vector<candidate> candidates;

for (size_t pass = 0; pass < 4; pass++) {
std::sort(points.begin(), points.end(), sorty_sorter(pass));

for (size_t i = 1; i < points.size(); i++) {
double dx = points[i].x - points[i - 1].x;
double dy = points[i].y - points[i - 1].y;

double dist = sqrt(dx * dx + dy * dy);
if (dist > 2 * goodness_threshold) {
candidate c;

c.x = (points[i].x + points[i - 1].x) / 2;
c.y = (points[i].y + points[i - 1].y) / 2;
c.dist = dist;

candidates.push_back(c);
}
}
}

// Now sort the accumulate list of segment midpoints by the lengths
// of the segments. Starting from the longest
// segment, if we find one whose midpoint is inside the polygon and
// far enough from any edge to be good enough, stop looking.

std::sort(candidates.begin(), candidates.end());
// only check the top 50 stride midpoints, since this list can be quite large
for (size_t i = 0; i < candidates.size() && i < 50; i++) {
double maybe_goodness = label_goodness(geom, candidates[i].x, candidates[i].y);

if (maybe_goodness > goodness) {
d.x = candidates[i].x;
d.y = candidates[i].y;

goodness = maybe_goodness;
kind = "diagonal";
if (goodness > goodness_threshold) {
break;
}
}
}
}

// We may still not have anything decent, so the next thing to look at
// is points from gridding the bounding box of the largest ring.

if (goodness < goodness_threshold) {
for (long long sub = 2;
sub < 32 && (xmax - xmin) > 2 * sub && (ymax - ymin) > 2 * sub;
sub *= 2) {
Expand All @@ -1582,11 +1748,12 @@ drawvec polygon_to_anchor(const drawvec &geom) {
xmin + x * (xmax - xmin) / sub,
ymin + y * (ymax - ymin) / sub);

double maybe_goodness = label_goodness(geom, start, end - start, maybe.x, maybe.y);
double maybe_goodness = label_goodness(geom, maybe.x, maybe.y);
if (maybe_goodness > goodness) {
// better than the previous
d = maybe;
goodness = maybe_goodness;
kind = "grid";
}
}
}
Expand All @@ -1598,8 +1765,17 @@ drawvec polygon_to_anchor(const drawvec &geom) {

// There is nothing really good. Is the centroid maybe better?
// If not, we're stuck with whatever the best we found was.
if (label_goodness(geom, start, end - start, centroid.x, centroid.y) > goodness) {
double maybe_goodness = label_goodness(geom, centroid.x, centroid.y);
if (maybe_goodness > goodness) {
d = centroid;
goodness = maybe_goodness;
kind = "centroid";
}

if (goodness <= 0) {
double lon, lat;
tile2lonlat(d.x, d.y, 32, &lon, &lat);
fprintf(stderr, "could not find label point: %s %f,%f\n", kind, lat, lon);
}
}

Expand Down Expand Up @@ -1676,31 +1852,18 @@ drawvec checkerboard_anchors(drawvec const &geom, int tx, int ty, int z, unsigne
continue;
}

for (size_t a = 0; a < geom.size(); a++) {
if (geom[a].op == VT_MOVETO) {
size_t b;
for (b = a + 1; b < geom.size(); b++) {
if (geom[b].op != VT_LINETO) {
break;
}
}

// If it's the central label, it's the best we've got,
// so accept it in any case. If it's from the outer spiral,
// don't use it if it's too close to a border.
if (lx == 0 && ly == 0) {
out.push_back(draw(VT_MOVETO, x - tx1, y - ty1));
break;
} else {
double tilesize = 1LL << (32 - z);
double goodness_threshold = tilesize / 100;
if (label_goodness(geom, a, b - a, x - tx1, y - ty1) > goodness_threshold) {
out.push_back(draw(VT_MOVETO, x - tx1, y - ty1));
break;
}
}

a = b - 1;
// If it's the central label, it's the best we've got,
// so accept it in any case. If it's from the outer spiral,
// don't use it if it's too close to a border.
if (lx == 0 && ly == 0) {
out.push_back(draw(VT_MOVETO, x - tx1, y - ty1));
break;
} else {
double tilesize = 1LL << (32 - z);
double goodness_threshold = tilesize / 100;
if (label_goodness(geom, x - tx1, y - ty1) > goodness_threshold) {
out.push_back(draw(VT_MOVETO, x - tx1, y - ty1));
break;
}
}
}
Expand Down
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