MapLibreParticleLab/src/mbgl/text/collision_feature.cpp

#include <mbgl/text/collision_feature.hpp>
#include <mbgl/util/math.hpp>
#include <mbgl/math/log2.hpp>

namespace mbgl {

CollisionFeature::CollisionFeature(const GeometryCoordinates& line,
                                   const Anchor& anchor,
                                   const float top,
                                   const float bottom,
                                   const float left,
                                   const float right,
                                   const optional<Padding>& collisionPadding,
                                   const float boxScale,
                                   const float padding,
                                   const style::SymbolPlacementType placement,
                                   IndexedSubfeature indexedFeature_,
                                   const float overscaling,
                                   const float rotate)
    : indexedFeature(std::move(indexedFeature_)), alongLine(placement != style::SymbolPlacementType::Point) {
    if (top == 0 && bottom == 0 && left == 0 && right == 0) return;

    float y1 = top * boxScale - padding;
    float y2 = bottom * boxScale + padding;
    float x1 = left * boxScale - padding;
    float x2 = right * boxScale + padding;

    if (collisionPadding) {
        x1 -= collisionPadding->left * boxScale;
        y1 -= collisionPadding->top * boxScale;
        x2 += collisionPadding->right * boxScale;
        y2 += collisionPadding->bottom * boxScale;
    }

    if (alongLine) {
        float height = y2 - y1;
        const float length = x2 - x1;

        if (height <= 0.0f) return;

        height = std::max(10.0f * boxScale, height);

        GeometryCoordinate anchorPoint = convertPoint<int16_t>(anchor.point);
        bboxifyLabel(line, anchorPoint, anchor.segment.value_or(0u), length, height, overscaling);
    } else {
        if (rotate) {
            // Account for *-rotate in point collision boxes
            // Doesn't account for icon-text-fit
            const float rotateRadians = rotate * util::DEG2RAD_F;

            const Point<float> tl = util::rotate(Point<float>(x1, y1), rotateRadians);
            const Point<float> tr = util::rotate(Point<float>(x2, y1), rotateRadians);
            const Point<float> bl = util::rotate(Point<float>(x1, y2), rotateRadians);
            const Point<float> br = util::rotate(Point<float>(x2, y2), rotateRadians);
            
            // Collision features require an "on-axis" geometry,
            // so take the envelope of the rotated geometry
            // (may be quite large for wide labels rotated 45 degrees)
            const float xMin = std::min({tl.x, tr.x, bl.x, br.x});
            const float xMax = std::max({tl.x, tr.x, bl.x, br.x});
            const float yMin = std::min({tl.y, tr.y, bl.y, br.y});
            const float yMax = std::max({tl.y, tr.y, bl.y, br.y});
            
            boxes.emplace_back(anchor.point, xMin, yMin, xMax, yMax);
        } else {
            boxes.emplace_back(anchor.point, x1, y1, x2, y2);
        }
    }
}

void CollisionFeature::bboxifyLabel(const GeometryCoordinates& line,
                                    GeometryCoordinate& anchorPoint,
                                    std::size_t segment,
                                    const float labelLength,
                                    const float boxSize,
                                    const float overscaling) {
    const float step = boxSize / 2;
    const int nBoxes = std::max(static_cast<int>(std::floor(labelLength / step)), 1);

    // We calculate line collision circles out to 300% of what would normally be our
    // max size, to allow collision detection to work on labels that expand as
    // they move into the distance
    // Vertically oriented labels in the distant field can extend past this padding
    // This is a noticeable problem in overscaled tiles where the pitch 0-based
    // symbol spacing will put labels very close together in a pitched map.
    // To reduce the cost of adding extra collision circles, we slowly increase
    // them for overscaled tiles.
    const double overscalingPaddingFactor = 1 + .4 * util::log2(static_cast<double>(overscaling));
    const int nPitchPaddingBoxes = static_cast<int>(std::floor(nBoxes * overscalingPaddingFactor / 2));

    // offset the center of the first box by half a box so that the edge of the
    // box is at the edge of the label.
    const float firstBoxOffset = -boxSize / 2;

    GeometryCoordinate &p = anchorPoint;
    std::size_t index = segment + 1;
    float anchorDistance = firstBoxOffset;
    const float labelStartDistance = -labelLength / 2;
    const float paddingStartDistance = labelStartDistance - labelLength / 8;

    // move backwards along the line to the first segment the label appears on
    do {
        if (index == 0u) {
            if (anchorDistance > labelStartDistance) {
                // there isn't enough room for the label after the beginning of the line
                // checkMaxAngle should have already caught this
                return;
            } else {
                // The line doesn't extend far enough back for all of our padding,
                // but we got far enough to show the label under most conditions.
                index = 0;
                break;
            }
        }

        index--;
        anchorDistance -= util::dist<float>(line[index], p);
        p = line[index];
    } while (anchorDistance > paddingStartDistance);

    auto segmentLength = util::dist<float>(line[index], line[index + 1]);

    for (int i = -nPitchPaddingBoxes; i < nBoxes + nPitchPaddingBoxes; i++) {
        // the distance the box will be from the anchor
        const float boxOffset = i * step;
        float boxDistanceToAnchor = labelStartDistance + boxOffset;

         // make the distance between pitch padding boxes bigger
         if (boxOffset < 0) boxDistanceToAnchor += boxOffset;
         if (boxOffset > labelLength) boxDistanceToAnchor += boxOffset - labelLength;

        if (boxDistanceToAnchor < anchorDistance) {
            // The line doesn't extend far enough back for this box, skip it
            // (This could allow for line collisions on distant tiles)
            continue;
        }

        // the box is not on the current segment. Move to the next segment.
        while (anchorDistance + segmentLength < boxDistanceToAnchor) {
            anchorDistance += segmentLength;
            index++;

            // There isn't enough room before the end of the line.
            if (index + 1 >= line.size()) return;

            segmentLength = util::dist<float>(line[index], line[index + 1]);
        }

        // the distance the box will be from the beginning of the segment
        const float segmentBoxDistance = boxDistanceToAnchor - anchorDistance;

        const auto& p0 = line[index];
        const auto& p1 = line[index + 1];

        Point<float> boxAnchor = {
            p0.x + segmentBoxDistance / segmentLength * (p1.x - p0.x),
            p0.y + segmentBoxDistance / segmentLength * (p1.y - p0.y)
        };
        
        // If the box is within boxSize of the anchor, force the box to be used
        // (so even 0-width labels use at least one box)
        // Otherwise, the .8 multiplication gives us a little bit of conservative
        // padding in choosing which boxes to use (see CollisionIndex#placedCollisionCircles)
        const float paddedAnchorDistance = std::abs(boxDistanceToAnchor - firstBoxOffset) < step ?
            0.0f :
            (boxDistanceToAnchor - firstBoxOffset) * 0.8f;

        boxes.emplace_back(boxAnchor, -boxSize / 2, -boxSize / 2, boxSize / 2, boxSize / 2, paddedAnchorDistance);
    }
}

} // namespace mbgl