[core] Prefetch low resolution tiles

7 files changed, 56 insertions, 4 deletions

diff --git a/include/mbgl/map/map.hpp b/include/mbgl/map/map.hpp
index 1b45c87c28..158e9d733d 100644
--- a/include/mbgl/map/map.hpp
+++ b/include/mbgl/map/map.hpp
@@ -156,6 +156,15 @@ public:
 
     AnnotationIDs queryPointAnnotations(const ScreenBox&);
 
+    // Tile prefetching
+    //
+    // When loading a map, if `PrefetchZoomDelta` is set to any number greater than 0, the map will
+    // first request a tile for `zoom = getZoom() - delta` in a attempt to display a full map at
+    // lower resolution as quick as possible. It will get clamped at the tile source minimum zoom.
+    // The default `delta` is 4.
+    void setPrefetchZoomDelta(uint8_t delta);
+    uint8_t getPrefetchZoomDelta() const;
+
     // Memory
     void onLowMemory();
 
diff --git a/include/mbgl/util/constants.hpp b/include/mbgl/util/constants.hpp
index 14aaa752bc..c2250c6f2e 100644
--- a/include/mbgl/util/constants.hpp
+++ b/include/mbgl/util/constants.hpp
@@ -38,6 +38,8 @@ constexpr double MAX_ZOOM = 25.5;
 constexpr float  MIN_ZOOM_F = MIN_ZOOM;
 constexpr float  MAX_ZOOM_F = MAX_ZOOM;
 
+constexpr uint8_t DEFAULT_PREFETCH_ZOOM_DELTA = 4;
+
 constexpr uint64_t DEFAULT_MAX_CACHE_SIZE = 50 * 1024 * 1024;
 
 constexpr Duration DEFAULT_TRANSITION_DURATION = Milliseconds(300);
diff --git a/src/mbgl/map/map.cpp b/src/mbgl/map/map.cpp
index 034e43f260..565d39c515 100644
--- a/src/mbgl/map/map.cpp
+++ b/src/mbgl/map/map.cpp
@@ -13,6 +13,7 @@
 #include <mbgl/renderer/render_source.hpp>
 #include <mbgl/renderer/render_style.hpp>
 #include <mbgl/renderer/render_style_observer.hpp>
+#include <mbgl/util/constants.hpp>
 #include <mbgl/util/exception.hpp>
 #include <mbgl/util/math.hpp>
 #include <mbgl/util/exception.hpp>
@@ -92,6 +93,9 @@ public:
     std::unique_ptr<RenderStyle> renderStyle;
 
     bool cameraMutated = false;
+
+    uint8_t prefetchZoomDelta = util::DEFAULT_PREFETCH_ZOOM_DELTA;
+
     bool loading = false;
 
     util::AsyncTask asyncInvalidate;
@@ -245,7 +249,8 @@ void Map::Impl::render(View& view) {
         style->impl->getLayerImpls(),
         scheduler,
         fileSource,
-        annotationManager
+        annotationManager,
+        prefetchZoomDelta
     });
 
     bool loaded = style->impl->isLoaded() && renderStyle->isLoaded();
@@ -823,6 +828,14 @@ bool Map::isFullyLoaded() const {
     return impl->style->impl->isLoaded() && impl->renderStyle && impl->renderStyle->isLoaded();
 }
 
+void Map::setPrefetchZoomDelta(uint8_t delta) {
+    impl->prefetchZoomDelta = delta;
+}
+
+uint8_t Map::getPrefetchZoomDelta() const {
+    return impl->prefetchZoomDelta;
+}
+
 void Map::onLowMemory() {
     if (impl->painter) {
         BackendScope guard(impl->backend);
diff --git a/src/mbgl/renderer/render_style.cpp b/src/mbgl/renderer/render_style.cpp
index bae879dca7..aacabe563d 100644
--- a/src/mbgl/renderer/render_style.cpp
+++ b/src/mbgl/renderer/render_style.cpp
@@ -105,7 +105,8 @@ void RenderStyle::update(const UpdateParameters& parameters) {
         parameters.mode,
         parameters.annotationManager,
         *imageManager,
-        *glyphManager
+        *glyphManager,
+        parameters.prefetchZoomDelta
     };
 
     glyphManager->setURL(parameters.glyphURL);
diff --git a/src/mbgl/renderer/tile_parameters.hpp b/src/mbgl/renderer/tile_parameters.hpp
index cf7a5b100a..9769ae6897 100644
--- a/src/mbgl/renderer/tile_parameters.hpp
+++ b/src/mbgl/renderer/tile_parameters.hpp
@@ -22,6 +22,7 @@ public:
     AnnotationManager& annotationManager;
     ImageManager& imageManager;
     GlyphManager& glyphManager;
+    const uint8_t prefetchZoomDelta = 0;
 };
 
 } // namespace mbgl
diff --git a/src/mbgl/renderer/tile_pyramid.cpp b/src/mbgl/renderer/tile_pyramid.cpp
index caf55d64e8..a8e6c128ba 100644
--- a/src/mbgl/renderer/tile_pyramid.cpp
+++ b/src/mbgl/renderer/tile_pyramid.cpp
@@ -88,14 +88,30 @@ void TilePyramid::update(const std::vector<Immutable<style::Layer::Impl>>& layer
     // Determine the overzooming/underzooming amounts and required tiles.
     int32_t overscaledZoom = util::coveringZoomLevel(parameters.transformState.getZoom(), type, tileSize);
     int32_t tileZoom = overscaledZoom;
+    int32_t panZoom = zoomRange.max;
 
     std::vector<UnwrappedTileID> idealTiles;
+    std::vector<UnwrappedTileID> panTiles;
+
     if (overscaledZoom >= zoomRange.min) {
         int32_t idealZoom = std::min<int32_t>(zoomRange.max, overscaledZoom);
 
         // Make sure we're not reparsing overzoomed raster tiles.
         if (type == SourceType::Raster) {
             tileZoom = idealZoom;
+
+            // FIXME: Prefetching is only enabled for raster
+            // tiles until we fix #7026.
+
+            // Request lower zoom level tiles (if configure to do so) in an attempt
+            // to show something on the screen faster at the cost of a little of bandwidth.
+            if (parameters.prefetchZoomDelta) {
+                panZoom = std::max<int32_t>(tileZoom - parameters.prefetchZoomDelta, zoomRange.min);
+            }
+
+            if (panZoom < tileZoom) {
+                panTiles = util::tileCover(parameters.transformState, panZoom);
+            }
         }
 
         idealTiles = util::tileCover(parameters.transformState, idealZoom);
@@ -108,8 +124,10 @@ void TilePyramid::update(const std::vector<Immutable<style::Layer::Impl>>& layer
     std::set<OverscaledTileID> retain;
 
     auto retainTileFn = [&](Tile& tile, Resource::Necessity necessity) -> void {
-        retain.emplace(tile.id);
-        tile.setNecessity(necessity);
+        if (retain.emplace(tile.id).second) {
+            tile.setNecessity(necessity);
+        }
+
         if (needsRelayout) {
             tile.setLayers(layers);
         }
@@ -137,6 +155,12 @@ void TilePyramid::update(const std::vector<Immutable<style::Layer::Impl>>& layer
     };
 
     renderTiles.clear();
+
+    if (!panTiles.empty()) {
+        algorithm::updateRenderables(getTileFn, createTileFn, retainTileFn,
+                [](const UnwrappedTileID&, Tile&) {}, panTiles, zoomRange, panZoom);
+    }
+
     algorithm::updateRenderables(getTileFn, createTileFn, retainTileFn, renderTileFn,
                                  idealTiles, zoomRange, tileZoom);
 
diff --git a/src/mbgl/renderer/update_parameters.hpp b/src/mbgl/renderer/update_parameters.hpp
index ce79a4f31b..8ebcd11e21 100644
--- a/src/mbgl/renderer/update_parameters.hpp
+++ b/src/mbgl/renderer/update_parameters.hpp
@@ -33,6 +33,8 @@ public:
     Scheduler& scheduler;
     FileSource& fileSource;
     AnnotationManager& annotationManager;
+
+    const uint8_t prefetchZoomDelta = 0;
 };
 
 } // namespace mbgl