Region.IsVisible(PointF) 对于大浮点值的性能非常慢

Region.IsVisible(PointF) has very slow performance for large floating point values

我 运行 遇到了一个奇怪的性能问题,最好能解释一下我遇到的行为。

我正在使用 System.Drawing.Region.IsVisible(PointF) 来确定点是否在多边形内。这通常工作得很好,但昨天我注意到如果多边形很复杂并且包含大的 x 和 y 值,IsVisible 方法的性能会变得非常慢。

下面是一些重现该问题的代码(以及显示多边形形状的图像),对于较大的数组大小感到抱歉,但多边形需要非常复杂才能出现问题。

在原始点上调用 IsVisible 时,我的机器需要 460 651 毫秒才能完成,而当我首先将所有点除以 1000,然后调用该方法时,需要 1 毫秒。为什么我在时间上看到如此大的差异?我不认为 float 的实际值会影响性能。

using System;
using System.Diagnostics;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Linq;

namespace PerformanceTest
{
    class Program
    {
        static void Main(string[] args)
        {

            // Create complex polygon with large x and y values
            float[] xValues = {1.014498E+07f, 1.016254E+07f, 1.019764E+07f, 1.021519E+07f, 1.023274E+07f, 1.026785E+07f, 1.026785E+07f, 1.02854E+07f, 1.02854E+07f, 1.030295E+07f, 1.03205E+07f, 1.033805E+07f, 1.035561E+07f, 1.037316E+07f, 1.039071E+07f, 1.040826E+07f, 1.042581E+07f, 1.044337E+07f, 1.046092E+07f, 1.047847E+07f, 1.049602E+07f, 1.051357E+07f, 1.054868E+07f, 1.056623E+07f, 1.058378E+07f, 1.060133E+07f, 1.061888E+07f, 1.061888E+07f, 1.063644E+07f, 1.065399E+07f, 1.068909E+07f, 1.068909E+07f, 1.070664E+07f, 1.07242E+07f, 1.074175E+07f, 1.074175E+07f, 1.07593E+07f, 1.07593E+07f, 1.077685E+07f, 1.07944E+07f, 1.07944E+07f, 1.081196E+07f, 1.081196E+07f, 1.081196E+07f, 1.082951E+07f, 1.084706E+07f, 1.084706E+07f, 1.086461E+07f, 1.086461E+07f, 1.088216E+07f, 1.089971E+07f, 1.091727E+07f, 1.093482E+07f, 1.098747E+07f, 1.100503E+07f, 1.102258E+07f, 1.104013E+07f, 1.105768E+07f, 1.107523E+07f, 1.107523E+07f, 1.109279E+07f, 1.109279E+07f, 1.109279E+07f, 1.109279E+07f, 1.109279E+07f, 1.111034E+07f, 1.111034E+07f, 1.111034E+07f, 1.111034E+07f, 1.111034E+07f, 1.112789E+07f, 1.112789E+07f, 1.112789E+07f, 1.114544E+07f, 1.116299E+07f, 1.118054E+07f, 1.11981E+07f, 1.12332E+07f, 1.125075E+07f, 1.12683E+07f, 1.128586E+07f, 1.130341E+07f, 1.135606E+07f, 1.137361E+07f, 1.139117E+07f, 1.140872E+07f, 1.144382E+07f, 1.146137E+07f, 1.147893E+07f, 1.149648E+07f, 1.151403E+07f, 1.153158E+07f, 1.154913E+07f, 1.156669E+07f, 1.156669E+07f, 1.158424E+07f, 1.158424E+07f, 1.158424E+07f, 1.158424E+07f, 1.158424E+07f, 1.158424E+07f, 1.158424E+07f, 1.158424E+07f, 1.158424E+07f, 1.158424E+07f, 1.158424E+07f, 1.156669E+07f, 1.156669E+07f, 1.151403E+07f, 1.149648E+07f, 1.149648E+07f, 1.149648E+07f, 1.149648E+07f, 1.149648E+07f, 1.149648E+07f, 1.149648E+07f, 1.149648E+07f, 1.149648E+07f, 1.153158E+07f, 1.154913E+07f, 1.156669E+07f, 1.156669E+07f, 1.158424E+07f, 1.160179E+07f, 1.160179E+07f, 1.161934E+07f, 1.165444E+07f, 1.1672E+07f, 1.168955E+07f, 1.17071E+07f, 1.172465E+07f, 1.17422E+07f, 1.175976E+07f, 1.177731E+07f, 1.179486E+07f, 1.181241E+07f, 1.182996E+07f, 1.184752E+07f, 1.186507E+07f, 1.188262E+07f, 1.190017E+07f, 1.190017E+07f, 1.191772E+07f, 1.191772E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.193528E+07f, 1.195283E+07f, 1.197038E+07f, 1.198793E+07f, 1.200548E+07f, 1.202303E+07f, 1.204059E+07f, 1.205814E+07f, 1.207569E+07f, 1.209324E+07f, 1.211079E+07f, 1.212835E+07f, 1.21459E+07f, 1.216345E+07f, 1.2181E+07f, 1.219855E+07f, 1.221611E+07f, 1.221611E+07f, 1.223366E+07f, 1.225121E+07f, 1.226876E+07f, 1.226876E+07f, 1.228631E+07f, 1.230386E+07f, 1.230386E+07f, 1.230386E+07f, 1.232142E+07f, 1.232142E+07f, 1.232142E+07f, 1.232142E+07f, 1.232142E+07f, 1.232142E+07f, 1.232142E+07f, 1.232142E+07f, 1.235652E+07f, 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9793945f, 9776394f, 9741290f, 9723738f, 9688635f, 9653531f, 9653531f, 9618427f, 9618427f, 9600875f, 9600875f, 9600875f, 9583323f, 9565771f, 9565771f, 9530667f, 9530667f, 9530667f, 9530667f, 9530667f, 9530667f, 9530667f, 9530667f, 9548219f, 9565771f, 9583323f, 9618427f, 9653531f, 9671083f, 9688635f, 9706186f, 9741290f, 9758842f, 9811497f, 9829050f, 9864154f, 9881705f, 9916809f, 9934361f, 9951913f, 9987016f, 1.000457E+07f, 1.003967E+07f, 1.005722E+07f, 1.007478E+07f, 1.010988E+07f, 1.014498E+07f, 1.016254E+07f, 1.016254E+07f, 1.018009E+07f, 1.019764E+07f, 1.021519E+07f, 1.023274E+07f, 1.023274E+07f, 1.023274E+07f, 1.023274E+07f, 1.023274E+07f, 1.023274E+07f, 1.023274E+07f, 1.023274E+07f, 1.021519E+07f, 1.019764E+07f, 1.016254E+07f, 1.014498E+07f, 1.012743E+07f, 1.009233E+07f, 1.003967E+07f, 1.000457E+07f, 9951913f, 9934361f, 9899257f, 9881705f, 9864154f, 9846602f, 9829050f, 9793945f, 9758842f, 9723738f, 9688635f, 9653531f, 9635979f, 9618427f, 9583323f, 9565771f, 9530667f, 9513116f, 9495564f, 9478012f, 9460460f, 9460460f, 9442908f, 9425357f, 9425357f, 9407805f, 9390253f, 9390253f, 9372701f, 9372701f, 9372701f, 9372701f, 9372701f, 9372701f, 9372701f, 9372701f, 9372701f, 9372701f, 9372701f, 9372701f, 9390253f, 9407805f, 9425357f, 9460460f, 9495564f, 9513116f, 9583323f, 9600875f, 9635979f, 9653531f, 9688635f, 9706186f, 9723738f, 9758842f, 9793945f, 9811497f, 9846602f};
            float[] yValues = { 7286825f, 7286825f, 7269351f, 7269351f, 7269351f, 7269351f, 7251876f, 7251876f, 7234401f, 7234401f, 7234401f, 7234401f, 7234401f, 7234401f, 7234401f, 7234401f, 7234401f, 7234401f, 7234401f, 7234401f, 7234401f, 7216927f, 7199453f, 7181979f, 7181979f, 7164504f, 7164504f, 7147029f, 7129555f, 7112081f, 7077132f, 7042183f, 7024709f, 7007235f, 6972285f, 6954811f, 6937337f, 6919863f, 6902388f, 6884913f, 6867439f, 6867439f, 6832491f, 6815016f, 6797541f, 6780067f, 6762593f, 6762593f, 6745119f, 6745119f, 6727644f, 6727644f, 6710169f, 6710169f, 6710169f, 6710169f, 6710169f, 6710169f, 6710169f, 6727644f, 6762593f, 6780067f, 6832491f, 6849965f, 6867439f, 6902388f, 6937337f, 6954811f, 6972285f, 6989760f, 7024709f, 7042183f, 7077132f, 7094607f, 7112081f, 7129555f, 7129555f, 7129555f, 7129555f, 7129555f, 7129555f, 7147029f, 7147029f, 7147029f, 7147029f, 7147029f, 7147029f, 7147029f, 7147029f, 7147029f, 7147029f, 7147029f, 7147029f, 7147029f, 7129555f, 7112081f, 7077132f, 7059657f, 7007235f, 6972285f, 6954811f, 6937337f, 6902388f, 6867439f, 6832491f, 6815016f, 6797541f, 6780067f, 6710169f, 6710169f, 6692695f, 6675221f, 6640272f, 6622797f, 6605323f, 6587849f, 6570375f, 6535425f, 6517951f, 6500477f, 6483003f, 6465528f, 6448053f, 6448053f, 6430579f, 6430579f, 6413105f, 6413105f, 6413105f, 6413105f, 6413105f, 6413105f, 6413105f, 6413105f, 6413105f, 6413105f, 6413105f, 6413105f, 6413105f, 6413105f, 6430579f, 6448053f, 6483003f, 6500477f, 6552900f, 6570375f, 6622797f, 6640272f, 6657747f, 6675221f, 6692695f, 6710169f, 6745119f, 6762593f, 6780067f, 6797541f, 6849965f, 6867439f, 6902388f, 6954811f, 6972285f, 6989760f, 7007235f, 7042183f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7059657f, 7042183f, 7042183f, 7024709f, 7007235f, 6989760f, 6989760f, 6972285f, 6954811f, 6937337f, 6919863f, 6902388f, 6867439f, 6832491f, 6815016f, 6797541f, 6780067f, 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6762593f, 6762593f, 6745119f, 6745119f, 6745119f, 6745119f, 6745119f, 6745119f, 6762593f, 6762593f, 6797541f, 6832491f, 6867439f, 6884913f, 6884913f, 6919863f, 6954811f, 6954811f, 6972285f, 6989760f, 7007235f, 7042183f, 7042183f, 7059657f, 7077132f, 7094607f, 7094607f, 7112081f, 7112081f, 7112081f, 7112081f, 7112081f, 7112081f, 7112081f, 7112081f, 7112081f, 7112081f, 7112081f, 7094607f, 7077132f, 7042183f, 7024709f, 7024709f, 7007235f, 6989760f, 6989760f, 6972285f, 6954811f, 6937337f, 6937337f, 6919863f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6902388f, 6919863f, 6919863f, 6954811f, 6972285f, 6972285f, 6989760f, 7024709f, 7042183f, 7059657f, 7094607f, 7112081f, 7129555f, 7164504f, 7181979f, 7199453f, 7234401f, 7251876f, 7269351f, 7286825f, 7321773f, 7321773f, 7339248f, 7339248f, 7356723f, 7374197f, 7374197f, 7374197f, 7374197f, 7374197f, 7391671f, 7391671f, 7409145f, 7409145f, 7426620f, 7426620f, 7426620f, 7444095f, 7444095f, 7461569f, 7479043f, 7496517f, 7513992f, 7513992f, 7513992f, 7531467f, 7548941f, 7548941f, 7566415f, 7601364f, 7618839f, 7636313f, 7653787f, 7671261f, 7688736f, 7706211f, 7741159f, 7793583f, 7828531f, 7846005f, 7880955f, 7880955f, 7898429f, 7898429f, 7915903f, 7933377f, 7933377f, 7950852f, 7950852f, 7950852f, 7950852f, 7950852f, 7950852f, 7950852f, 7968327f, 7985801f, 8003275f, 8020749f, 8038224f, 8055699f, 8073173f, 8090647f, 8108121f, 8125596f, 8160545f, 8178019f, 8195493f, 8212968f, 8212968f, 8230443f, 8247917f, 8265391f, 8282865f, 8282865f, 8282865f, 8282865f, 8282865f, 8282865f, 8282865f, 8282865f, 8265391f, 8230443f, 8212968f, 8195493f, 8178019f, 8160545f, 8160545f, 8143071f, 8143071f, 8125596f, 8108121f, 8108121f, 8090647f, 8090647f, 8073173f, 8038224f, 8038224f, 8020749f, 8020749f, 8003275f, 7985801f, 7985801f, 7968327f, 7950852f, 7950852f, 7933377f, 7933377f, 7933377f, 7933377f, 7915903f, 7898429f, 7898429f, 7898429f, 7898429f, 7898429f, 7898429f, 7880955f, 7880955f, 7880955f, 7880955f, 7880955f, 7880955f, 7880955f, 7880955f, 7880955f, 7880955f, 7880955f, 7898429f, 7898429f, 7933377f, 7968327f, 7985801f, 8003275f, 8020749f, 8055699f, 8073173f, 8108121f, 8108121f, 8143071f, 8178019f, 8178019f, 8212968f, 8212968f, 8230443f, 8247917f, 8265391f, 8282865f, 8282865f, 8317815f, 8335289f, 8352763f, 8387712f, 8405186f, 8422661f, 8440134f, 8457609f, 8475084f, 8510033f, 8527506f, 8544981f, 8562456f, 8614878f, 8632353f, 8667302f, 8684777f, 8737200f, 8772149f, 8789622f, 8824572f, 8842046f, 8859521f, 8876994f, 8894469f, 8929418f, 8929418f, 8946893f, 8946893f, 8946893f, 8946893f, 8946893f, 8946893f, 8946893f, 8946893f, 8946893f, 8946893f, 8911944f, 8894469f, 8859521f, 8824572f, 8789622f, 8772149f, 8702250f, 8684777f, 8667302f, 8632353f, 8614878f, 8597405f, 8562456f, 8544981f, 8510033f, 8492558f, 8475084f, 8457609f, 8440134f, 8422661f, 8405186f, 8387712f, 8370237f, 8352763f, 8352763f, 8352763f, 8352763f, 8352763f, 8352763f, 8352763f, 8352763f, 8352763f, 8352763f, 8370237f, 8370237f, 8387712f, 8387712f, 8405186f, 8422661f, 8422661f, 8440134f, 8457609f, 8492558f, 8527506f, 8544981f, 8562456f, 8579930f, 8597405f, 8614878f, 8632353f, 8649828f, 8667302f, 8702250f, 8719725f, 8737200f, 8772149f, 8789622f, 8807097f, 8824572f, 8842046f, 8876994f, 8894469f, 8911944f, 8929418f, 8946893f, 8964366f, 8964366f, 8981841f, 8999316f, 9016790f, 9034265f, 9034265f, 9051738f, 9051738f, 9051738f, 9051738f, 9051738f, 9051738f, 9051738f, 9016790f, 8999316f, 8964366f, 8946893f, 8929418f, 8911944f, 8876994f, 8859521f, 8842046f, 8824572f, 8807097f, 8789622f, 8772149f, 8754674f, 8702250f, 8667302f, 8649828f, 8632353f, 8614878f, 8597405f, 8579930f, 8562456f, 8544981f, 8510033f, 8492558f, 8492558f, 8492558f, 8492558f, 8492558f, 8492558f, 8492558f, 8492558f, 8492558f, 8492558f, 8492558f, 8492558f, 8510033f, 8544981f, 8544981f, 8562456f, 8562456f, 8597405f, 8614878f, 8632353f, 8667302f, 8702250f, 8719725f, 8754674f, 8789622f, 8824572f, 8842046f, 8859521f, 8876994f, 8911944f, 8929418f, 8964366f, 8964366f, 8999316f, 9016790f, 9034265f, 9086688f, 9104162f, 9139110f, 9156585f, 9174060f, 9209009f, 9243957f, 9261432f, 9261432f, 9261432f, 9261432f, 9261432f, 9261432f, 9261432f, 9261432f, 9261432f, 9261432f, 9261432f, 9243957f, 9226482f, 9191534f, 9174060f, 9156585f, 9086688f, 9051738f, 9034265f, 8999316f, 8981841f, 8964366f, 8929418f, 8894469f, 8876994f, 8842046f, 8824572f, 8807097f, 8789622f, 8772149f, 8754674f, 8702250f, 8684777f, 8667302f, 8667302f, 8649828f, 8614878f, 8597405f, 8579930f, 8562456f, 8562456f, 8527506f, 8510033f, 8492558f, 8475084f, 8440134f, 8422661f, 8422661f, 8387712f, 8370237f, 8370237f, 8370237f, 8370237f, 8370237f, 8335289f, 8335289f, 8335289f, 8335289f, 8335289f, 8335289f, 8335289f, 8335289f, 8335289f, 8335289f, 8335289f, 8352763f, 8370237f, 8405186f, 8440134f, 8457609f, 8492558f, 8510033f, 8527506f, 8562456f, 8579930f, 8597405f, 8597405f, 8632353f, 8632353f, 8667302f, 8702250f, 8719725f, 8754674f, 8754674f, 8754674f, 8754674f, 8754674f, 8754674f, 8754674f, 8754674f, 8737200f, 8719725f, 8702250f, 8684777f, 8667302f, 8649828f, 8632353f, 8614878f, 8562456f, 8527506f, 8440134f, 8422661f, 8387712f, 8352763f, 8317815f, 8300340f, 8282865f, 8265391f, 8247917f, 8212968f, 8212968f, 8160545f, 8143071f, 8125596f, 8108121f, 8108121f, 8090647f, 8090647f, 8073173f, 8073173f, 8055699f, 8055699f, 8055699f, 8038224f, 8038224f, 8038224f, 8038224f, 8038224f, 8020749f, 8020749f, 8003275f, 8003275f, 7985801f, 7950852f, 7933377f, 7915903f, 7915903f, 7880955f, 7863480f, 7846005f, 7828531f, 7793583f, 7776108f, 7758633f, 7741159f, 7723685f, 7706211f, 7706211f, 7706211f, 7706211f, 7706211f, 7706211f, 7706211f, 7706211f, 7706211f, 7706211f, 7706211f, 7706211f, 7723685f, 7723685f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7741159f, 7723685f, 7723685f, 7688736f, 7671261f, 7653787f, 7583889f, 7566415f, 7513992f, 7461569f, 7444095f, 7409145f, 7374197f, 7356723f, 7321773f, 7304299f, 7286825f, 7269351f, 7251876f, 7234401f, 7199453f, 7181979f, 7164504f, 7164504f, 7164504f, 7164504f, 7164504f, 7164504f, 7164504f, 7164504f, 7164504f, 7164504f, 7164504f, 7164504f, 7164504f, 7181979f, 7199453f };

            PointF[] points = xValues.Zip(yValues, (x, y) => new PointF(x, y)).ToArray();


            // Create a region with the original values 
            GraphicsPath pathWithOriginalPoints = new GraphicsPath();
            pathWithOriginalPoints.AddPolygon(points);
            Region regionFromOriginalPoints = new Region(pathWithOriginalPoints);

            // Create a region with the values divided by 1000
            GraphicsPath pathDividedBy1000 = new GraphicsPath();
            pathDividedBy1000.AddPolygon(points.Select(p => new PointF(p.X/1000f, p.Y/1000f)).ToArray());
            Region regionDividedby1000 = new Region(pathDividedBy1000);


            // Time call to Region.IsVisible(PointF)
            var stopwatch = Stopwatch.StartNew();
            Console.WriteLine("Computing region.IsVisible for points divided by 1000:");
            regionDividedby1000.IsVisible(new PointF(0f, 0f));
            var dividedBy1000Timing = stopwatch.ElapsedMilliseconds;
            Console.WriteLine($"Elapsed time: {dividedBy1000Timing} ms");

            stopwatch.Restart();
            Console.WriteLine("Computing region.IsVisible for original points");
            regionFromOriginalPoints.IsVisible(new PointF(0f, 0f));
            var originalTiming = stopwatch.ElapsedMilliseconds;
            Console.WriteLine($"Elapsed time: {originalTiming} ms");
        }
    }
}

TLTR

进行一次 Region.IsVisible(Point) 调用所花费的时间根本不取决于点数。相反,它取决于完全和精确地 覆盖区域 所需的 矩形数量 。这取决于..:[=​​54=]

  • 这些点描述的形状通常
  • 形状的大小。

示例 1:一个矩形区域有四个点,总是需要 一个矩形 来覆盖它。如果您要添加大量点,只要它们都位于矩形的边上,这不会改变!

示例 2:圆形区域也 (!) 有四个点 () 但是完全覆盖它需要的矩形数量 ** 取决于该圆的直径。请参阅底部的图形!


全文

Region 不是我所说的 'well-documented' class.

你可以看到内部工作似乎依赖于RegionData,可以通过调用var RData = your Region.GetRegionData()来访问它。

同样的道理:至少可以说没有很好的记录。也许我只是找不到它,但似乎没有关于这些字节的结构的信息..

(下面的数字表明需要的每个点需要 1 个字节作为类型指示符加上 2+4 个字节作为两个浮点坐标。这就像 PathPoints&PathTypes GraphicsPath。在你的例子中有 924+1 个点和 1+2*4 个字节,这构成了 8325 个字节;还有 27 个字节;8 个可以保持缩放..)

有一件事很有趣:从 both 查看 RegionsRegionData 可以很容易地看到他们有 相同 大小..: 8352。这表明 RegionData 无论如何与额外时间没有直接关系..

但是还有 另一个 ,有点同样神秘,阅读:记录不完整的电话:GetRegionScans。 MSDN 是这样说的:

Returns an array of RectangleF structures that approximate this Region after the specified matrix transformation is applied.

首先 让我们看看如果我们为你们的两个区域提取这些 'scans' 会发生什么:

Matrix M = new Matrix();

var scans1 = regionFromOriginalPoints.GetRegionScans(M);
var scans2 = regionDividedby1000.GetRegionScans(M);

我使用简单的单位矩阵 (*),即没有转换,结果如下:

scans1.Length = 5.960.690
scans2.Length = 5.956

因此它创建了 1000 倍以上的 矩形来近似 Region,而且,你瞧,这样做也需要很多时间..:

这并不奇怪:未缩放的路径覆盖了很大的区域,为了近似它,我预计需要更多的矩形。我不知道近似值是如何工作的,但它可能会决定,即使返回 RectangleF,也不需要任何边小于 1 像素的矩形。

所以我得出结论,IsVisible 调用在内部需要创建那些近似矩形,并且它需要足够多的矩形来覆盖每个完整像素区域内。 (请注意 Region 不支持 partial/anti-aliased 像素!)区域边界越大,适合的像素越多,扫描过程所需的时间也越长。

我还假设完全是扫描过程耗尽了时间。

Next:执行 RectangleF.Contains(Point) 应该非常快,即使是一百万次调用。让我们来看看;当我测量这个时:

int hits = 0;
PointF pt = new PointF(123.456f, 789.012f);
foreach (RectangleF r in scans1) if (r.Contains(pt)) hits++;

..只需 42ms 即可完成。因此,一旦您 缓存 扫描矩形,您就可以轻松地对它们进行大量命中测试 而不是 进行单独的 IsVisible 调用。 .

以下是数字摘要:

Your original path:

number of polygon points = 924
regionData1.Length = 8.352
scans1.Length = 5.960.690
Elapsed time1: 453.187 ms

The path scaled down by 1000:

number of polygon points = 924
regionData2.Length = 8.352
scans2.Length= 5.956
Elapsed time2: 2 ms

Elapsed time for full set of scans: 41 ms


最后:为了更好地理解扫描矩形,我用交替颜色近似为一个圆:

for (int i = 0; i < scans.Length; i++)
     g.FillRectangle(i%2 == 0 ? Brushes.ForestGreen : Brushes.Salmon, scans[i]);

可以看到上下每一行都是一个矩形,因为这里曲线是平的,x坐标向外移动很快。我们越靠近中间,矩形就越大(即越高)...所以我们可以看到近似值严格遵循 rows..

即使边界矩形比宽度高并且使用列,也可以使用更少的矩形,这也是正确的:

此处两个椭圆(222x111111x222)近似为85水平条纹。因为曲率不同所以小于圆..


(*) - 我们跳过了 Matrix;但它可用于通过放大或缩小数据来修改结果。 圆形有 Height222 像素,它需要 135 个扫描矩形来近似它。 如果我们放大 Matrix ,(即用于表示 RegionGraphics 对象):m.Scale(10,10); 它需要 1315 个扫描矩形来覆盖放大的圆。

因此可以使用按比例缩小 0.001f 的矩阵来代替缩放数据点;但它只会测试按比例缩小的版本,并且可能会错过边界周围的像素或包含错误的像素。所以最好只使用你实际需要的正确比例..

(* *)注意循环GraphicsPath确实有13PathPoints;但这些实际上只是四个实际点(加上第一个重复以关闭图形)加上每对点之间的 两个控制点;所以 5 + 4*2 = 13。还要注意 GraphicsPath 中的 任何 线是 直线或贝塞尔曲线。 这包括圆弧、椭圆和圆!

我们一直在为同一个问题而苦苦挣扎:对于一个 50k 点的多边形,在 IsVisible() 方法后面构建矩形需要 45 秒。我们试图缓存这些数据,但最终我们有超过 100 万个矩形;对于多个区域,我们有数百兆字节的数据要缓存。

最后我们转向了 PNPoly 算法,它只需要几毫秒:

https://wrf.ecse.rpi.edu/Research/Short_Notes/pnpoly.html

这里是 c# 版本:

    public bool IsVisible(Point p, List<Point> points)
    {
        int i, j = points.Count - 1;
        bool isVisible = false;
        for (i = 0; i < points.Count; i++)
        {
            if (points[i].Y < p.Y && points[j].Y >= p.Y 
                || points[j].Y < p.Y && points[i].Y >= p.Y)
            {
                if (points[i].X + (p.Y - points[i].Y) / (points[j].Y - points[i].Y) 
                    * (points[j].X - points[i].X) < p.X)
                {
                    isVisible = !isVisible;
                }
            }
            j = i;
        }
        return isVisible;
    }