Add an option to calculate feature density as a feature attribute

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 1.11.8
+
+* Add an option to calculate the density of features as a feature attribute
+
 ## 1.11.7
 
 * Keep metadata together with geometry for features that don't span many tiles,

README.md

@@ -118,6 +118,7 @@ resolution is obtained than by using a smaller _maxzoom_ or _detail_.
  * -ao or --reorder: Reorder features to put ones with the same properties in sequence, to try to get them to coalesce
  * -al or --drop-lines: Let "dot" dropping at lower zooms apply to lines too
  * -ap or --drop-polygons: Let "dot" dropping at lower zooms apply to polygons too
+ * -ag or --calculate-feature-density: Add a new attribute, `tippecanoe_feature_density`, to each feature, to record how densely features are spaced in that area of the tile. You can use this attribute in the style to produce a glowing effect where points are densely packed. It can range from 0 in the sparsest areas to 255 in the densest.
 
 ### Doing less
 

main.cpp

@@ -987,6 +987,13 @@ int read_input(std::vector<source> &sources, char *fname, const char *layername,
 		std::string reading;
 		int fd;
 
+		if (additional[A_CALCULATE_FEATURE_DENSITY]) {
+			type_and_string tas;
+			tas.type = VT_NUMBER;
+			tas.string = "tippecanoe_feature_density";
+			file_keys[source].insert(tas);
+		}
+
 		if (source >= sources.size()) {
 			reading = "standard input";
 			fd = 0;
@@ -1771,6 +1778,7 @@ int main(int argc, char **argv) {
 		{"check-polygons", no_argument, &additional[A_DEBUG_POLYGON], 1},
 		{"drop-polygons", no_argument, &additional[A_POLYGON_DROP], 1},
 		{"prefer-radix-sort", no_argument, &additional[A_PREFER_RADIX_SORT], 1},
+		{"calculate-feature-density", no_argument, &additional[A_CALCULATE_FEATURE_DENSITY], 1},
 
 		{"no-line-simplification", no_argument, &prevent[P_SIMPLIFY], 1},
 		{"simplify-only-low-zooms", no_argument, &prevent[P_SIMPLIFY_LOW], 1},

man/tippecanoe.1

@@ -145,6 +145,8 @@ which may not be what you want.
 \-al or \-\-drop\-lines: Let "dot" dropping at lower zooms apply to lines too
 .IP \(bu 2
 \-ap or \-\-drop\-polygons: Let "dot" dropping at lower zooms apply to polygons too
+.IP \(bu 2
+\-ag or \-\-calculate\-feature\-density: Add a new attribute, \fB\fCtippecanoe_feature_density\fR, to each feature, to record how densely features are spaced in that area of the tile. You can use this attribute in the style to produce a glowing effect where points are densely packed. It can range from 0 in the sparsest areas to 255 in the densest.
 .RE
 .SS Doing less
 .RS

options.hpp

@@ -5,6 +5,7 @@
 #define A_DEBUG_POLYGON ((int) 'd')
 #define A_POLYGON_DROP ((int) 'p')
 #define A_PREFER_RADIX_SORT ((int) 'R')
+#define A_CALCULATE_FEATURE_DENSITY ((int) 'g')
 
 #define P_SIMPLIFY ((int) 's')
 #define P_SIMPLIFY_LOW ((int) 'S')

tile.cpp

@@ -73,6 +73,7 @@ struct coalesce {
 	int type;
 	int m;
 	bool coalesced;
+	double spacing;
 
 	bool operator<(const coalesce &o) const {
 		int cmp = coalindexcmp(this, &o);
@@ -387,6 +388,7 @@ struct partial {
 	int z;
 	int line_detail;
 	int maxzoom;
+	double spacing;
 	signed char t;
 };
 
@@ -599,9 +601,10 @@ long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *s
 
 		double fraction_accum = 0;
 
-		unsigned long long previndex = 0;
+		unsigned long long previndex = 0, density_previndex = 0;
 		double scale = (double) (1LL << (64 - 2 * (z + 8)));
-		double gap = 0;
+		double gap = 0, density_gap = 0;
+		double spacing = 0;
 
 		long long original_features = 0;
 		long long unclipped_features = 0;
@@ -796,6 +799,11 @@ long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *s
 				continue;
 			}
 
+			unsigned long long index = 0;
+			if (additional[A_CALCULATE_FEATURE_DENSITY] || gamma > 0) {
+				index = encode(bbox[0] / 2 + bbox[2] / 2, bbox[1] / 2 + bbox[3] / 2);
+			}
+
 			if (gamma >= 0 && (t == VT_POINT ||
 					   (additional[A_LINE_DROP] && t == VT_LINE) ||
 					   (additional[A_POLYGON_DROP] && t == VT_POLYGON))) {
@@ -806,14 +814,25 @@ long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *s
 					continue;
 				}
 
-				if (gamma > 0) {
-					unsigned long long index = encode(bbox[0] / 2 + bbox[2] / 2, bbox[1] / 2 + bbox[3] / 2);
+				if (gamma > 0 || additional[A_CALCULATE_FEATURE_DENSITY]) {
 					if (manage_gap(index, &previndex, scale, gamma, &gap)) {
 						continue;
 					}
 				}
 			}
 
+			if (additional[A_CALCULATE_FEATURE_DENSITY]) {
+				// Gamma is always 1 for this calculation so there is a reasonable
+				// interpretation when no features are being dropped.
+				// The spacing is only calculated if a feature would be retained by
+				// that standard, so that duplicates aren't reported as infinitely dense.
+
+				double o_density_previndex = density_previndex;
+				if (!manage_gap(index, &density_previndex, scale, 1, &density_gap)) {
+					spacing = (index - o_density_previndex) / scale;
+				}
+			}
+
 			fraction_accum += fraction;
 			if (fraction_accum < 1) {
 				continue;
@@ -840,6 +859,7 @@ long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *s
 				p.maxzoom = maxzoom;
 				p.keys = metakeys;
 				p.values = metavals;
+				p.spacing = spacing;
 				partials.push_back(p);
 			}
 		}
@@ -900,6 +920,7 @@ long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *s
 					c.stringpool = stringpool + pool_off[partials[i].segment];
 					c.keys = partials[i].keys;
 					c.values = partials[i].values;
+					c.spacing = partials[i].spacing;
 
 					features[layer].push_back(c);
 				}
@@ -995,6 +1016,21 @@ long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *s
 				features[k][x].geom.clear();
 
 				decode_meta(features[k][x].m, features[k][x].keys, features[k][x].values, features[k][x].stringpool, layer, feature);
+
+				if (additional[A_CALCULATE_FEATURE_DENSITY]) {
+					int glow = 255;
+					if (features[k][x].spacing > 0) {
+						glow = (1 / features[k][x].spacing);
+						if (glow > 255) {
+							glow = 255;
+						}
+					}
+					mvt_value v;
+					v.type = mvt_sint;
+					v.numeric_value.sint_value = glow;
+					layer.tag(feature, "tippecanoe_feature_density", v);
+				}
+
 				layer.features.push_back(feature);
 			}
 

version.hpp

@@ -1 +1 @@
-#define VERSION "tippecanoe v1.11.7\n"
+#define VERSION "tippecanoe v1.11.8\n"