Treat tiny polygon holes as dust again, but accumulated separately now

geometry.cpp

@@ -186,7 +186,7 @@ void check_polygon(drawvec &geom) {
 	}
 }
 
-drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *reduced, double *accum_area, serial_feature *this_feature, serial_feature *tiny_feature) {
+drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *reduced, double *accum_area, double *accum_hole, serial_feature *this_feature, serial_feature *tiny_feature) {
 	drawvec out;
 	const double pixel = (1LL << (32 - detail - z)) * (double) tiny_polygon_size;
 	bool includes_real = false;
@@ -222,7 +222,7 @@ drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *reduced, double
 				// OR it is an inner ring and we haven't output an outer ring for it to be
 				// cut out of, so we are just subtracting its area from the tiny polygon
 				// rather than trying to deal with it geometrically
-				if ((area > 0 && area <= pixel * pixel) || (area < 0 && !included_last_outer)) {
+				if ((area > 0 && area <= pixel * pixel) || (area < 0 && -area <= pixel * pixel && !included_last_outer)) {
 					// printf("area is only %f vs %lld so using square\n", area, pixel * pixel);
 
 					*accum_area += area;
@@ -243,10 +243,30 @@ drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *reduced, double
 						included_last_outer = false;
 					}
 				}
+
+				// this is a tiny hole, and we have a real outer ring that we can cut it out of,
+				// so accumulate some hole, and if we have enough to be worth cutting, cut it out.
+				//
+				// these hole areas are accumulated separately rather than just decrementing
+				// the accumulated area, because otherwise it is possible to accumulate enough
+				// negative area to lose entire outer rings in the process of getting back up to 0.
+				else if (area < 0 && -area <= pixel * pixel && included_last_outer) {
+					*accum_hole -= area;
+					if (*accum_hole > pixel * pixel) {
+						// XXX use centroid;
+
+						out.push_back(draw(VT_MOVETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2));
+						out.push_back(draw(VT_LINETO, geom[i].x - pixel / 2 + pixel, geom[i].y - pixel / 2));
+						out.push_back(draw(VT_LINETO, geom[i].x - pixel / 2 + pixel, geom[i].y - pixel / 2 + pixel));
+						out.push_back(draw(VT_LINETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2 + pixel));
+						out.push_back(draw(VT_LINETO, geom[i].x - pixel / 2, geom[i].y - pixel / 2));
+						includes_dust = true;
+
+						*accum_hole -= pixel * pixel;
+					}
+				}
+
 				// i.e., this ring is large enough that it gets to represent itself
-				// or it is a tiny hole out of a real polygon, which we are still treating
-				// as a real geometry because otherwise we can accumulate enough tiny holes
-				// that we will drop the next several outer rings getting back up to 0.
 				else {
 					// printf("area is %f so keeping instead of %lld\n", area, pixel * pixel);
 

geometry.hpp

@@ -66,7 +66,7 @@ drawvec clip_point(drawvec &geom, int z, long long buffer);
 drawvec clean_or_clip_poly(drawvec &geom, int z, int buffer, bool clip);
 drawvec simple_clip_poly(drawvec &geom, int z, int buffer);
 drawvec close_poly(drawvec &geom);
-drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *reduced, double *accum_area, serial_feature *this_feature, serial_feature *tiny_feature);
+drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *reduced, double *accum_area, double *accum_hole, serial_feature *this_feature, serial_feature *tiny_feature);
 int clip(double *x0, double *y0, double *x1, double *y1, double xmin, double ymin, double xmax, double ymax);
 drawvec clip_lines(drawvec &geom, int z, long long buffer);
 drawvec stairstep(drawvec &geom, int z, int detail);

overzoom.cpp

@@ -11,7 +11,7 @@ extern char *optarg;
 extern int optind;
 
 int detail = 12;  // tippecanoe-style: mvt extent == 1 << detail
-int buffer = 5;  // tippecanoe-style: mvt buffer == extent * buffer / 256;
+int buffer = 5;	  // tippecanoe-style: mvt buffer == extent * buffer / 256;
 
 std::set<std::string> keep;
 

tile.cpp

@@ -1914,7 +1914,7 @@ long long write_tile(decompressor *geoms, std::atomic<long long> *geompos_in, ch
 	int line_detail;
 	for (line_detail = detail; line_detail >= min_detail || line_detail == detail; line_detail--, oprogress = 0) {
 		long long count = 0;
-		double accum_area = 0;
+		double accum_area = 0, accum_hole = 0;
 
 		double fraction_accum = 0;
 
@@ -2185,7 +2185,7 @@ long long write_tile(decompressor *geoms, std::atomic<long long> *geompos_in, ch
 			bool reduced = false;
 			if (sf.t == VT_POLYGON) {
 				if (!prevent[P_TINY_POLYGON_REDUCTION] && !additional[A_GRID_LOW_ZOOMS]) {
-					sf.geometry = reduce_tiny_poly(sf.geometry, z, line_detail, &reduced, &accum_area, &sf, &tiny_feature);
+					sf.geometry = reduce_tiny_poly(sf.geometry, z, line_detail, &reduced, &accum_area, &accum_hole, &sf, &tiny_feature);
 					if (reduced) {
 						strategy->tiny_polygons++;
 					}