网上很多方法，算法也比较简单，如果真的gis缓冲算法有很多其他参数
这里用比较简单的方法，把c++的改成ue的c++,顺便留下自己的学习记录

//等距离缓冲
TArray<FVector2D> UBFL_SpatialAnalysis::PolygonBuffer(TArray<FVector2D> PList , float BufferValue)
{
    // 1. vertex set
    TArray<FVector2D>out;

        // 2. edge set and normalize it
    TArray<FVector2D> dpList, ndpList;
    int count = PList.Num()-1;
    for (int i = 0; i < count; i++) {
        int next = (i == (count - 1) ? 0 : (i + 1));
        dpList.Add(PList[next] - PList[i]);
        float unitLen = 1.0f / FMath::Sqrt(FVector2D::DotProduct(dpList[i],dpList[i]));
        ndpList.Add(dpList[i] * unitLen);
    }

    // 3. compute Line
    float SAFELINE = BufferValue;//负数为内缩， 正数为外扩。 需要注意算法本身并没有检测内缩多少后折线会自相交，那不是本代码的示范意图
    for (int i = 0; i < count; i++) {
        int startIndex = (i == 0 ? (count - 1) : (i - 1));
        int endIndex = i;
        float sinTheta = ndpList[startIndex].X * ndpList[endIndex].Y - ndpList[endIndex].X * ndpList[startIndex].Y;
        FVector2D orientVector = ndpList[endIndex] - ndpList[startIndex];//i.e. PV2-V1P=PV2+PV1
        FVector2D temp_out;
        temp_out.X = PList[i].X + SAFELINE / sinTheta * orientVector.X;
        temp_out.Y = PList[i].Y + SAFELINE / sinTheta * orientVector.Y;
        out.Add(temp_out);
    }
    return out;
}

image.png

图中是生成缓冲后减去缓冲之前的模型，顺便测了下包围盒。

//添加点、线缓冲
点缓冲其实就是根据点画圆。

TArray<FVector2D> UBFL_3DAnalysis::PointBuffer(FVector2D PList, float BufferValue, float BufferNumber)
{
    // 1. vertex set
    TArray<FVector2D>out;

    int32 BufferNum = BufferNumber;
    if (BufferNumber<=1)
    {
        return out;
    }
    float a0 = 360 / BufferNumber;//计算角度步长

    for (float angle = 0; angle <360; angle += a0)
    {
        FVector2D p1 = FVector2D(BufferValue * UKismetMathLibrary::DegCos(angle),BufferValue*UKismetMathLibrary::DegSin(angle));
        out.Add(p1);
    }
    return out;
}

//线缓冲
先缓冲我更具面的算法，重新计算起始点和结束点

TArray<FVector2D> UBFL_3DAnalysis::LineBuffer(TArray<FVector2D> PList, float BufferValue)
{
    // 1. vertex set
    TArray<FVector2D>out;
    TArray<FVector2D>reserve;
    // 2. edge set and normalize it
    TArray<FVector2D> dpList, ndpList;
    int count = PList.Num();
    for (int i = 0; i < count; i++) {//获取头尾去掉的数据
        int next = (i == (count - 1) ? 0 : (i + 1));
        dpList.Add(PList[next] - PList[i]);//向量AB
        float unitLen = 1.0f / FMath::Sqrt(FVector2D::DotProduct(dpList[i], dpList[i]));//模长
        ndpList.Add(dpList[i] * unitLen);//单位
    }

    // 3. compute Line
    float SAFELINE = BufferValue;//负数为内缩， 正数为外扩。 需要注意算法本身并没有检测内缩多少后折线会自相交，那不是本代码的示范意图
    for (int i = 0; i < count; i++) {
        int startIndex = (i == 0 ? (count - 1) : (i - 1));
        int endIndex = i;
        float sinTheta = ndpList[startIndex].X * ndpList[endIndex].Y - ndpList[endIndex].X * ndpList[startIndex].Y;
        
        
        FVector2D orientVector = ndpList[endIndex] - ndpList[startIndex];//i.e. PV2-V1P=PV2+PV1 平行向量
        FVector2D temp_out, temp_in;

        if (i == 0 || i == count-1)
        {
            sinTheta = 1.0;
            orientVector = i == count - 1? ndpList[startIndex]:ndpList[endIndex];
            temp_in.X = PList[i].X + SAFELINE / sinTheta * orientVector.Y;
            temp_in.Y = PList[i].Y + SAFELINE / sinTheta * (orientVector.X*-1.0);

            temp_out.X = PList[i].X + (SAFELINE) / sinTheta * (orientVector.Y*-1.0);
            temp_out.Y = PList[i].Y + (SAFELINE) / sinTheta * orientVector.X;
        }
        else
        {
            
            temp_out.X = PList[i].X + SAFELINE / sinTheta * orientVector.X;
            temp_out.Y = PList[i].Y + SAFELINE / sinTheta * orientVector.Y;

            temp_in.X = PList[i].X + (-1.0 * SAFELINE) / sinTheta * orientVector.X;
            temp_in.Y = PList[i].Y + (-1.0 * SAFELINE) / sinTheta * orientVector.Y;
        }


        
        out.Add(temp_out);
        reserve.Insert(temp_in, 0);
        
        
    }

    out.Append(reserve);

    return out;
}

最后如果需要把原来面扣掉可能需要计算多边形的正反面，也就是顺逆时针

bool UBFL_3DAnalysis::IsClickWise(TArray<FVector2D>PList)
{
    //1. 找到横坐标最小的点，该点一定是多边形的最左侧的点，且为凸点
    float min_x = PList[0].X;
    int min_idx = 0;
    int32 size = PList.Num();
    for (int32 i = 1; i < size; i++) {
        if (PList[i].X < min_x) {
            min_x = PList[i].X;
            min_idx = i;
        }
    }

    //2. 判断该点与前一个点构成的向量 和 与后一个点构成的向量的叉积的正负
    //2.1 获取该点前一个点和后一个点的下标
    int prev_min_idx = 0, next_min_idx = 0;
    if (0 == min_idx) {     //如果该点是第一个点
        prev_min_idx = size - 1;
        next_min_idx = min_idx + 1;
    }
    else if (size - 1 == min_idx) { //如果该点是最后一个点
        prev_min_idx = min_idx - 1;
        next_min_idx = 0;
    }
    else {
        prev_min_idx = min_idx - 1;
        next_min_idx = min_idx + 1;
    }

    //2.2 计算叉积
    double cross = (PList[prev_min_idx].X - PList[min_idx].X) * (PList[next_min_idx].Y - PList[min_idx].Y) -
        (PList[next_min_idx].X - PList[min_idx].X) * (PList[prev_min_idx].Y - PList[min_idx].Y);

    if (cross > 0)
        return true;
    else
        return false;
}

111.png