MapLibreParticleLab/src/mbgl/text/collision_tile.cpp

#include <mbgl/text/collision_tile.hpp>
#include <mbgl/util/constants.hpp>
#include <cmath>

namespace mbgl {

auto infinity = std::numeric_limits<float>::infinity();

CollisionTile::CollisionTile(PlacementConfig config_) : config(config_),
    edges({{
        // left
        CollisionBox(vec2<float>(0, 0), 0, -infinity, 0, infinity, infinity),
        // right
        CollisionBox(vec2<float>(util::EXTENT, 0), 0, -infinity, 0, infinity, infinity),
        // top
        CollisionBox(vec2<float>(0, 0), -infinity, 0, infinity, 0, infinity),
        // bottom
        CollisionBox(vec2<float>(0, util::EXTENT), -infinity, 0, infinity, 0, infinity),
    }}) {
    tree.clear();

    // Compute the transformation matrix.
    const float angle_sin = std::sin(config.angle);
    const float angle_cos = std::cos(config.angle);
    rotationMatrix = { { angle_cos, -angle_sin, angle_sin, angle_cos } };
    reverseRotationMatrix = { { angle_cos, angle_sin, -angle_sin, angle_cos } };

    // Stretch boxes in y direction to account for the map tilt.
    const float _yStretch = 1.0f / std::cos(config.pitch);

    // The amount the map is squished depends on the y position.
    // Sort of account for this by making all boxes a bit bigger.
    yStretch = std::pow(_yStretch, 1.3);
}


float CollisionTile::findPlacementScale(float minPlacementScale, const vec2<float>& anchor,
        const CollisionBox& box, const vec2<float>& blockingAnchor, const CollisionBox& blocking) {


    // Find the lowest scale at which the two boxes can fit side by side without overlapping.
    // Original algorithm:
    float s1 = (blocking.x1 - box.x2) / (anchor.x - blockingAnchor.x); // scale at which new box is to the left of old box
    float s2 = (blocking.x2 - box.x1) / (anchor.x - blockingAnchor.x); // scale at which new box is to the right of old box
    float s3 = (blocking.y1 - box.y2) * yStretch / (anchor.y - blockingAnchor.y); // scale at which new box is to the top of old box
    float s4 = (blocking.y2 - box.y1) * yStretch / (anchor.y - blockingAnchor.y); // scale at which new box is to the bottom of old box

    if (std::isnan(s1) || std::isnan(s2)) s1 = s2 = 1;
    if (std::isnan(s3) || std::isnan(s4)) s3 = s4 = 1;

    float collisionFreeScale = ::fmin(::fmax(s1, s2), ::fmax(s3, s4));

    if (collisionFreeScale > blocking.maxScale) {
        // After a box's maxScale the label has shrunk enough that the box is no longer needed to cover it,
        // so unblock the new box at the scale that the old box disappears.
        collisionFreeScale = blocking.maxScale;
    }

    if (collisionFreeScale > box.maxScale) {
        // If the box can only be shown after it is visible, then the box can never be shown.
        // But the label can be shown after this box is not visible.
        collisionFreeScale = box.maxScale;
    }

    if (collisionFreeScale > minPlacementScale &&
            collisionFreeScale >= blocking.placementScale) {
        // If this collision occurs at a lower scale than previously found collisions
        // and the collision occurs while the other label is visible

        // this this is the lowest scale at which the label won't collide with anything
        minPlacementScale = collisionFreeScale;
    }

    return minPlacementScale;
}

float CollisionTile::placeFeature(const CollisionFeature &feature, const bool allowOverlap, const bool avoidEdges) {

    float minPlacementScale = minScale;

    for (auto& box : feature.boxes) {
        const auto anchor = box.anchor.matMul(rotationMatrix);

        if (!allowOverlap) {
            std::vector<CollisionTreeBox> blockingBoxes;
            tree.query(bgi::intersects(getTreeBox(anchor, box)), std::back_inserter(blockingBoxes));

            for (auto& blockingTreeBox : blockingBoxes) {
                const auto& blocking = std::get<1>(blockingTreeBox);
                auto blockingAnchor = blocking.anchor.matMul(rotationMatrix);

                minPlacementScale = findPlacementScale(minPlacementScale, anchor, box, blockingAnchor, blocking);
                if (minPlacementScale >= maxScale) return minPlacementScale;
            }
        }

        if (avoidEdges) {
            const vec2<float> tl = { box.x1, box.y1 };
            const vec2<float> tr = { box.x2, box.y1 };
            const vec2<float> bl = { box.x1, box.y2 };
            const vec2<float> br = { box.x2, box.y2 };
            const vec2<float> rtl = tl.matMul(reverseRotationMatrix);
            const vec2<float> rtr = tr.matMul(reverseRotationMatrix);
            const vec2<float> rbl = bl.matMul(reverseRotationMatrix);
            const vec2<float> rbr = br.matMul(reverseRotationMatrix);
            CollisionBox rotatedBox(box.anchor,
                    ::fmin(::fmin(rtl.x, rtr.x), ::fmin(rbl.x, rbr.x)),
                    ::fmin(::fmin(rtl.y, rtr.y), ::fmin(rbl.y, rbr.y)),
                    ::fmax(::fmax(rtl.x, rtr.x), ::fmax(rbl.x, rbr.x)),
                    ::fmax(::fmax(rtl.y, rtr.y), ::fmax(rbl.y, rbr.y)),
                    box.maxScale);

            for (auto& blocking : edges) {
                minPlacementScale = findPlacementScale(minPlacementScale, box.anchor, rotatedBox, blocking.anchor, blocking);

                if (minPlacementScale >= maxScale) return minPlacementScale;
            }
        }
    }

    return minPlacementScale;
}

void CollisionTile::insertFeature(CollisionFeature &feature, const float minPlacementScale) {
    for (auto& box : feature.boxes) {
        box.placementScale = minPlacementScale;
    }

    if (minPlacementScale < maxScale) {
        std::vector<CollisionTreeBox> treeBoxes;
        for (auto& box : feature.boxes) {
            treeBoxes.emplace_back(getTreeBox(box.anchor.matMul(rotationMatrix), box), box);
        }
        tree.insert(treeBoxes.begin(), treeBoxes.end());
    }

}

Box CollisionTile::getTreeBox(const vec2<float> &anchor, const CollisionBox &box) {
    return Box{
        CollisionPoint{
            anchor.x + box.x1,
            anchor.y + box.y1 * yStretch
        },
        CollisionPoint{
            anchor.x + box.x2,
            anchor.y + box.y2 * yStretch
        }
    };
}

} // namespace mbgl