Start implementing `CollisionIndex`, the global/viewport-based replacement for `CollisionTile`.

No support for line-labels/circles yet.
Compiles but not tested.
[skip ci]

1 files changed, 171 insertions, 0 deletions

diff --git a/src/mbgl/text/collision_index.cpp b/src/mbgl/text/collision_index.cpp
new file mode 100644
index 0000000000..4504459f05
--- /dev/null
+++ b/src/mbgl/text/collision_index.cpp
@@ -0,0 +1,171 @@
+#include <mbgl/text/collision_index.hpp>
+#include <mbgl/geometry/feature_index.hpp>
+#include <mbgl/math/log2.hpp>
+#include <mbgl/util/constants.hpp>
+#include <mbgl/util/math.hpp>
+#include <mbgl/math/minmax.hpp>
+#include <mbgl/util/intersection_tests.hpp>
+
+#include <mbgl/layout/symbol_projection.hpp>
+
+#include <mapbox/geometry/envelope.hpp>
+#include <mapbox/geometry/multi_point.hpp>
+
+#include <cmath>
+
+namespace mbgl {
+
+// When a symbol crosses the edge that causes it to be included in
+// collision detection, it will cause changes in the symbols around
+// it. This constant specifies how many pixels to pad the edge of
+// the viewport for collision detection so that the bulk of the changes
+// occur offscreen. Making this constant greater increases label
+// stability, but it's expensive.
+static const float viewportPadding = 100;
+
+CollisionIndex::CollisionIndex(const TransformState& transformState_) : transformState(transformState_) {
+    pitchFactor = std::cos(transformState.getPitch()) * transformState.getCameraToCenterDistance();
+}
+
+float CollisionIndex::approximateTileDistance(const TileDistance& tileDistance, const float lastSegmentAngle, const float pixelsToTileUnits, const float cameraToAnchorDistance, const bool pitchWithMap) {
+    // This is a quick and dirty solution for chosing which collision circles to use (since collision circles are
+    // laid out in tile units). Ideally, I think we should generate collision circles on the fly in viewport coordinates
+    // at the time we do collision detection.
+
+    // incidenceStretch is the ratio of how much y space a label takes up on a tile while drawn perpendicular to the viewport vs
+    //  how much space it would take up if it were drawn flat on the tile
+    // Using law of sines, camera_to_anchor/sin(ground_angle) = camera_to_center/sin(incidence_angle)
+    // Incidence angle 90 -> head on, sin(incidence_angle) = 1, no stretch
+    // Incidence angle 1 -> very oblique, sin(incidence_angle) =~ 0, lots of stretch
+    // ground_angle = u_pitch + PI/2 -> sin(ground_angle) = cos(u_pitch)
+    // incidenceStretch = 1 / sin(incidenceAngle)
+
+    const float incidenceStretch = pitchWithMap ? 1 : cameraToAnchorDistance / pitchFactor;
+    const float lastSegmentTile = tileDistance.lastSegmentViewportDistance * pixelsToTileUnits;
+    return tileDistance.prevTileDistance +
+        lastSegmentTile +
+        (incidenceStretch - 1) * lastSegmentTile * std::abs(std::sin(lastSegmentAngle));
+}
+
+Box CollisionIndex::getTreeBox(const CollisionBox& box, const mat4& posMatrix, const float textPixelRatio) const {
+    const auto projectedPoint = projectAndGetPerspectiveRatio(posMatrix, box.anchor);
+    const float tileToViewport = textPixelRatio * projectedPoint.second;
+    const float tlX = box.x1 / tileToViewport + projectedPoint.first.x;
+    const float tlY = box.y1 / tileToViewport + projectedPoint.first.y;
+    const float brX = box.x2 / tileToViewport + projectedPoint.first.x;
+    const float brY = box.y2 / tileToViewport + projectedPoint.first.y;
+
+    return Box{
+        CollisionPoint{ tlX, tlY },
+        CollisionPoint{ brX, brY }
+    };
+}
+
+// TODO: add circle support
+bool CollisionIndex::placeFeature(const CollisionFeature& feature, bool allowOverlap, const mat4& posMatrix, const float textPixelRatio) {
+    for (auto& box : feature.boxes) {
+        const auto projectedBox = getTreeBox(box, posMatrix, textPixelRatio);
+
+        if (!allowOverlap) {
+            for (auto it = tree.qbegin(bgi::intersects(projectedBox)); it != tree.qend(); ++it) {
+                return false;
+            }
+        }
+    }
+
+    return true;
+}
+
+// TODO: don't duplicate projection that just happened in placeFeature
+void CollisionIndex::insertFeature(CollisionFeature& feature, bool ignorePlacement, const mat4& posMatrix, const float textPixelRatio) {
+    std::vector<CollisionTreeBox> treeBoxes;
+    for (auto& box : feature.boxes) {
+        treeBoxes.emplace_back(getTreeBox(box, posMatrix, textPixelRatio), box, feature.indexedFeature);
+    }
+    
+    if (ignorePlacement) {
+        ignoredTree.insert(treeBoxes.begin(), treeBoxes.end());
+    } else {
+        tree.insert(treeBoxes.begin(), treeBoxes.end());
+    }
+}
+
+std::vector<IndexedSubfeature> CollisionIndex::queryRenderedSymbols(const GeometryCoordinates& queryGeometry, const UnwrappedTileID& tileID, const float textPixelRatio) const {
+    std::vector<IndexedSubfeature> result;
+    if (queryGeometry.empty() || (tree.empty() && ignoredTree.empty())) {
+        return result;
+    }
+
+    mat4 posMatrix;
+    transformState.matrixFor(posMatrix, tileID);
+
+    GeometryCoordinates polygon;
+    for (const auto& point : queryGeometry) {
+        auto projected = projectPoint(posMatrix, convertPoint<float>(point));
+        polygon.push_back(convertPoint<int16_t>(projected));
+    }
+
+    // Predicate for ruling out already seen features.
+    std::unordered_map<std::string, std::unordered_set<std::size_t>> sourceLayerFeatures;
+    auto seenFeature = [&] (const CollisionTreeBox& treeBox) -> bool {
+        const IndexedSubfeature& feature = std::get<2>(treeBox);
+        const auto& seenFeatures = sourceLayerFeatures[feature.sourceLayerName];
+        return seenFeatures.find(feature.index) == seenFeatures.end();
+    };
+
+
+    // Check if query polygon intersects with the feature box at current scale.
+    auto intersectsAtScale = [&] (const CollisionTreeBox& treeBox) -> bool {
+        const CollisionBox& collisionBox = std::get<1>(treeBox);
+        const auto projectedAnchor = projectAndGetPerspectiveRatio(posMatrix, collisionBox.anchor);
+        const float tileToViewport = textPixelRatio * projectedAnchor.second;
+
+        const int16_t x1 = projectedAnchor.first.x + (collisionBox.x1 / tileToViewport);
+        const int16_t y1 = projectedAnchor.first.y + (collisionBox.y1 / tileToViewport);
+        const int16_t x2 = projectedAnchor.first.x + (collisionBox.x2 / tileToViewport);
+        const int16_t y2 = projectedAnchor.first.y + (collisionBox.y2 / tileToViewport);
+        auto bbox = GeometryCoordinates {
+            { x1, y1 }, { x2, y1 }, { x2, y2 }, { x1, y2 }
+        };
+        return util::polygonIntersectsPolygon(polygon, bbox);
+    };
+
+    auto predicates = bgi::satisfies(seenFeature) && bgi::satisfies(intersectsAtScale);
+
+    auto queryTree = [&](const auto& tree_) {
+        for (auto it = tree_.qbegin(predicates); it != tree_.qend(); ++it) {
+            const IndexedSubfeature& feature = std::get<2>(*it);
+            auto& seenFeatures = sourceLayerFeatures[feature.sourceLayerName];
+            seenFeatures.insert(feature.index);
+            result.push_back(feature);
+        }
+    };
+
+    queryTree(tree);
+    queryTree(ignoredTree);
+
+    return result;
+}
+
+std::pair<Point<float>,float> CollisionIndex::projectAndGetPerspectiveRatio(const mat4& posMatrix, const Point<float>& point) const {
+    vec4 p = {{ point.x, point.y, 0, 1 }};
+    matrix::transformMat4(p, p, posMatrix);
+    return std::make_pair(
+        Point<float>(
+            (((p[0]  / p[3] + 1) / 2) * transformState.getSize().width) + viewportPadding,
+            (((-p[1] / p[3] + 1) / 2) * transformState.getSize().height) + viewportPadding
+        ),
+        0.5 + 0.5 * (p[3] / transformState.getCameraToCenterDistance())
+    );
+}
+
+Point<float> CollisionIndex::projectPoint(const mat4& posMatrix, const Point<float>& point) const {
+    vec4 p = {{ point.x, point.y, 0, 1 }};
+    matrix::transformMat4(p, p, posMatrix);
+    return Point<float>(
+        (((p[0]  / p[3] + 1) / 2) * transformState.getSize().width) + viewportPadding,
+        (((-p[1] / p[3] + 1) / 2) * transformState.getSize().height) + viewportPadding
+    );
+}
+
+} // namespace mbgl