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88 lines
2.9 KiB
C++
88 lines
2.9 KiB
C++
#include <cmath>
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#include "drop.hpp"
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#include "options.hpp"
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#include "geometry.hpp"
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unsigned long long preserve_point_density_threshold = 0;
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int calc_feature_minzoom(struct index *ix, struct drop_state ds[], int maxzoom, double gamma) {
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int feature_minzoom = 0;
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if (gamma >= 0 && (ix->t == VT_POINT ||
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(additional[A_LINE_DROP] && ix->t == VT_LINE) ||
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(additional[A_POLYGON_DROP] && ix->t == VT_POLYGON))) {
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for (ssize_t i = 0; i <= maxzoom; i++) {
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// This zoom level is now lighter on features
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ds[i].error -= 1.0;
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}
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ssize_t chosen = maxzoom + 1;
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for (ssize_t i = 0; i <= maxzoom; i++) {
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if (ds[i].error < 0) {
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// this zoom level is too light, so it is time to emit a feature.
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feature_minzoom = i;
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// this feature now appears in this zoom level and all higher zoom levels,
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// so each of them has this feature as its last feature, and each of them
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// is now one feature (which has a weight of `interval`) heavier than before.
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for (ssize_t j = i; j <= maxzoom; j++) {
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ds[j].previndex = ix->ix;
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ds[j].error += ds[j].interval;
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}
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chosen = i;
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break;
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}
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}
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// If this feature has been chosen only for a high zoom level,
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// check whether at a low zoom level it is nevertheless too far
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// from the last feature chosen for that low zoom, in which case
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// we will go ahead and push it out.
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if (preserve_point_density_threshold > 0) {
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for (ssize_t i = 0; i < chosen && i < maxzoom; i++) {
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if (ix->ix - ds[i].previndex > ((1LL << (32 - i)) / preserve_point_density_threshold) * ((1LL << (32 - i)) / preserve_point_density_threshold)) {
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feature_minzoom = i;
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// this feature now appears in this zoom level and all higher zoom levels
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// (below `chosen`, beyond which were already credited with this feature)
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// so each of them has this feature as its last feature, and each of them
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// is now one feature (which has a weight of `interval`) heavier than before.
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for (ssize_t j = i; j < chosen; j++) {
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ds[j].previndex = ix->ix;
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ds[j].error += ds[j].interval;
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}
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break;
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}
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}
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}
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}
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return feature_minzoom;
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}
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void prep_drop_states(struct drop_state ds[], int maxzoom, int basezoom, double droprate) {
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// Needs to be signed for interval calculation
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for (ssize_t i = 0; i <= maxzoom; i++) {
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ds[i].previndex = 0;
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ds[i].interval = 1; // every feature appears in every zoom level at or above the basezoom
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if (i < basezoom) {
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// at zoom levels below the basezoom, the fraction of points that are dropped is
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// the drop rate to the power of the number of zooms this zoom is below the basezoom
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//
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// for example:
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// basezoom: 1 (droprate ^ 0)
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// basezoom - 1: 2.5 (droprate ^ 1)
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// basezoom - 2: 6.25 (droprate ^ 2)
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// ...
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// basezoom - n: (droprate ^ n)
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ds[i].interval = std::pow(droprate, basezoom - i);
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}
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ds[i].error = 0;
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}
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}
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