Fix gridding of small squares; use less memory for small features

serial.cpp

@@ -318,6 +318,7 @@ static long long scale_geometry(struct serialization_state *sst, long long *bbox
 	long long offset = 0;
 	long long prev = 0;
 	bool has_prev = false;
+	double scale = 1.0 / (1 << geometry_scale);
 
 	for (size_t i = 0; i < geom.size(); i++) {
 		if (geom[i].op == VT_MOVETO || geom[i].op == VT_LINETO) {
@@ -365,8 +366,17 @@ static long long scale_geometry(struct serialization_state *sst, long long *bbox
 				*(sst->initialized) = 1;
 			}
 
-			geom[i].x = x >> geometry_scale;
-			geom[i].y = y >> geometry_scale;
+			if (additional[A_GRID_LOW_ZOOMS]) {
+				// If we are gridding, snap to the maxzoom grid in case the incoming data
+				// is already supposed to be aligned to tile boundaries (but is not, exactly,
+				// because of rounding error during projection).
+
+				geom[i].x = std::round(x * scale);
+				geom[i].y = std::round(y * scale);
+			} else {
+				geom[i].x = x >> geometry_scale;
+				geom[i].y = y >> geometry_scale;
+			}
 		}
 	}