MapLibreParticleLab/src/mbgl/text/get_anchors.cpp

#include <mbgl/text/get_anchors.hpp>
#include <mbgl/text/check_max_angle.hpp>
#include <mbgl/util/constants.hpp>
#include <mbgl/util/interpolate.hpp>

#include <cassert>
#include <cmath>

namespace mbgl {
  
float getAngleWindowSize(const float textLeft, const float textRight, const float glyphSize, const float boxScale) {
    return (textLeft - textRight) != 0.0f ?
        3.0f / 5.0f * glyphSize * boxScale :
        0;
}

float getLineLength(const GeometryCoordinates& line) {
    float lineLength = 0;
    for (auto it = line.begin(), end = line.end() - 1; it != end; it++) {
        lineLength += util::dist<float>(*(it), *(it + 1));
    }
    return lineLength;
}

static Anchors resample(const GeometryCoordinates& line,
                        const float offset,
                        const float spacing,
                        const float angleWindowSize,
                        const float maxAngle,
                        const float labelLength,
                        const bool continuedLine,
                        const bool placeAtMiddle) {
    const float halfLabelLength = labelLength / 2.0f;
    const float lineLength = getLineLength(line);

    float distance = 0;
    float markedDistance = offset - spacing;

    Anchors anchors;

    assert(spacing > 0.0);

    std::size_t i = 0u;
    for (auto it = line.begin(), end = line.end() - 1; it != end; it++, i++) {
        const GeometryCoordinate& a = *(it);
        const GeometryCoordinate& b = *(it + 1);

        const auto segmentDist = util::dist<float>(a, b);
        const auto angle = static_cast<float>(util::angle_to(b, a));

        while (markedDistance + spacing < distance + segmentDist) {
            markedDistance += spacing;

            float t = (markedDistance - distance) / segmentDist;
            float x = util::interpolate(static_cast<float>(a.x), static_cast<float>(b.x), t);
            float y = util::interpolate(static_cast<float>(a.y), static_cast<float>(b.y), t);

            // Check that the point is within the tile boundaries and that
            // the label would fit before the beginning and end of the line
            // if placed at this point.
            if (x >= 0 && x < util::EXTENT && y >= 0 && y < util::EXTENT &&
                    markedDistance - halfLabelLength >= 0.0f &&
                    markedDistance + halfLabelLength <= lineLength) {
                Anchor anchor(std::round(x), std::round(y), angle, i);

                if (!angleWindowSize || checkMaxAngle(line, anchor, labelLength, angleWindowSize, maxAngle)) {
                    anchors.push_back(anchor);
                }
            }
        }

        distance += segmentDist;
    }

    if (!placeAtMiddle && anchors.empty() && !continuedLine) {
        // The first attempt at finding anchors at which labels can be placed failed.
        // Try again, but this time just try placing one anchor at the middle of the line.
        // This has the most effect for short lines in overscaled tiles, since the
        // initial offset used in overscaled tiles is calculated to align labels with positions in
        // parent tiles instead of placing the label as close to the beginning as possible.
        anchors = resample(line, distance / 2, spacing, angleWindowSize, maxAngle, labelLength, continuedLine, true);
    }

    return anchors;
}

Anchors getAnchors(const GeometryCoordinates& line,
                   float spacing,
                   const float maxAngle,
                   const float textLeft,
                   const float textRight,
                   const float iconLeft,
                   const float iconRight,
                   const float glyphSize,
                   const float boxScale,
                   const float overscaling) {
    if (line.empty()) {
        return {};
    }

    // Resample a line to get anchor points for labels and check that each
    // potential label passes text-max-angle check and has enough froom to fit
    // on the line.

    const float angleWindowSize = getAngleWindowSize(textLeft, textRight, glyphSize, boxScale);

    const float shapedLabelLength = std::fmax(textRight - textLeft, iconRight - iconLeft);
    const float labelLength = shapedLabelLength * boxScale;

    // Is the line continued from outside the tile boundary?
    const bool continuedLine = (line[0].x == 0 || line[0].x == util::EXTENT || line[0].y == 0 || line[0].y == util::EXTENT);

    // Is the label long, relative to the spacing?
    // If so, adjust the spacing so there is always a minimum space of `spacing / 4` between label edges.
    if (spacing - labelLength < spacing / 4) {
        spacing = labelLength + spacing / 4;
    }

    // Offset the first anchor by:
    // Either half the label length plus a fixed extra offset if the line is not continued
    // Or half the spacing if the line is continued.

    // For non-continued lines, add a bit of fixed extra offset to avoid collisions at T intersections.
    const float fixedExtraOffset = glyphSize * 2;

    const float offset = !continuedLine ?
    std::fmod((shapedLabelLength / 2 + fixedExtraOffset) * boxScale * overscaling, spacing) :
    std::fmod(spacing / 2 * overscaling, spacing);

    return resample(line, offset, spacing, angleWindowSize, maxAngle, labelLength, continuedLine, false);
}

optional<Anchor> getCenterAnchor(const GeometryCoordinates& line,
                                 const float maxAngle,
                                 const float textLeft,
                                 const float textRight,
                                 const float iconLeft,
                                 const float iconRight,
                                 const float glyphSize,
                                 const float boxScale) {
    if (line.empty()) {
        return {};
    }
    
    const float angleWindowSize = getAngleWindowSize(textLeft, textRight, glyphSize, boxScale);
    const float labelLength = std::fmax(textRight - textLeft, iconRight - iconLeft) * boxScale;

    float prevDistance = 0;
    const float centerDistance = getLineLength(line) / 2;

    std::size_t i = 0u;
    for (auto it = line.begin(), end = line.end() - 1; it != end; it++, i++) {
        const GeometryCoordinate& a = *(it);
        const GeometryCoordinate& b = *(it + 1);
        
        const auto segmentDistance = util::dist<float>(a, b);
        
        if (prevDistance + segmentDistance > centerDistance) {
            // The center is on this segment
            float t = (centerDistance - prevDistance) / segmentDistance;
            float x = util::interpolate(static_cast<float>(a.x), static_cast<float>(b.x), t);
            float y = util::interpolate(static_cast<float>(a.y), static_cast<float>(b.y), t);

            Anchor anchor(std::round(x), std::round(y), static_cast<float>(util::angle_to(b, a)), i);

            if (!angleWindowSize || checkMaxAngle(line, anchor, labelLength, angleWindowSize, maxAngle)) {
                return anchor;
            }

            return nullopt;
        }
        
        prevDistance += segmentDistance;
    }
    return {};
}

} // namespace mbgl