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Optimize encoding of speed values. Only store 0-127, so we can efficiently pack into a byte. Speed and is_tiny values are now shared, so tiles should be much smaller.

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Daniel Patterson 2016-02-20 07:02:19 -08:00 committed by Patrick Niklaus
parent 36adeec220
commit 6a2592421c
1 changed files with 23 additions and 10 deletions
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27
include/engine/plugins/tile.hpp
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@ -9,6 +9,7 @@
#include <string>
#include <cmath>
#include <algorithm>
/*
* This plugin generates Mapbox Vector tiles that show the internal
@ -304,12 +305,14 @@ template <class DataFacadeT> class TilePlugin final : public BasePlugin
// When adding attributes to a feature, we have to write
// pairs of numbers. The first value is the index in the
// keys array (written later), and the second value is the
// index into the "values" array (also written later).
// index into the "values" array (also written later). We're
// not writing the actual speed or bool value here, we're saving
// an index into the "values" array. This means many features
// can share the same value data, leading to smaller tiles.
protozero::packed_field_uint32 field(feature_writer, 2);
field.add_element(0); // "speed" tag key offset
field.add_element(std::min(speed, 127)); // save the speed value, or
field.add_element(std::min(speed, 127u)); // save the speed value, or
field.add_element(1); // "is_small" tag key offset
field.add_element(edge.component.is_tiny ? 0 : 1); // is_small feature
}
@ -348,7 +351,7 @@ template <class DataFacadeT> class TilePlugin final : public BasePlugin
{
protozero::packed_field_uint32 field(feature_writer, 2);
field.add_element(0); // "speed" tag key offset
field.add_element(speed < 140 ? speed : 139);
field.add_element(std::min(speed, 127u)); // save the speed value, or
field.add_element(1); // "is_small" tag key offset
field.add_element(edge.component.is_tiny ? 0 : 1); // is_small feature
}
@ -361,22 +364,32 @@ template <class DataFacadeT> class TilePlugin final : public BasePlugin
}
}
// Now, we add two "key" fields, these are referred to with 0 and 1 (their array indexes)
// Field id 3 is the "keys" attribute
// We need two "key" fields, these are referred to with 0 and 1 (their array indexes)
// earlier
layer_writer.add_string(3,"speed");
layer_writer.add_string(3,"is_small");
// Now, we write out the possible speed value arrays and possible is_tiny
// values.
// values. Field type 4 is the "values" field. It's a variable type field,
// so requires a two-step write (create the field, then write its value)
for (size_t i=0; i<speeds.size(); i++) {
{
// Writing field type 4 == variant type
protozero::pbf_writer values_writer(layer_writer,4);
// Attribute value 5 == uin64 type
values_writer.add_uint64(5, speeds[i]);
}
}
{
protozero::pbf_writer values_writer(layer_writer,4);
values_writer.add_bool(7, is_smalls[i]);
// Attribute value 7 == bool type
values_writer.add_bool(7, true);
}
{
protozero::pbf_writer values_writer(layer_writer,4);
// Attribute value 7 == bool type
values_writer.add_bool(7, false);
}
}