使用Shapefile C Library读取shp文件并使用OpenGL绘制

1. 概述

坐标数据是空间数据文件的核心，空间数据的数据量往往是很大的。数据可视化是GIS的一个核心应用，绘制海量的坐标数据始终是一个考验设备性能的难题，使用GPU进行绘制可有效减少CPU的负载，提升绘制时的速度

shapefile是空间数据文件常用的格式，Shapefile C Library（也叫shapelib）提供了编写简单的 C/C++ 程序以读取、写入和更新ESRI Shapefile 以及关联的属性文件（.dbf）的功能

shapelib的官网：Shapefile C Library (maptools.org)

shapelib的GitHub地址：OSGeo/shapelib: Official repository of shapelib (github.com)

本文基于C++语言，使用shapelib库来读取shp文件，并使用基于GLFW和GLAD来使用OpenGL绘制空间数据

2. 环境准备

本文的系统环境为Ubuntu 20.04.3 LTS，关于C++语言的OpenGL环境搭建可参考：

基于Ubuntu搭建OpenGL开发环境 - 当时明月在曾照彩云归 - 博客园 (cnblogs.com)

shapelib在Linux上安装参考其GitHub指导：

OSGeo/shapelib: Official repository of shapelib (github.com)

具体步骤为：

$ git clone https://github.com/OSGeo/shapelib.git

```
$ cmake build .
```
```
$ make
```
```
$ make install
```

Windows平台上的安装可以参考：

shapelib库 VS2017X64编译并调用_moneymyone的博客-CSDN博客

本文使用的数据为云南的县界，数据信息如图所示：

注意：

本文数据编码是UTF-8
本文数据Geometry是Polygon
本文数据坐标系是投影坐标系

3. 读取shp文件

参考博客：

官方API文档：

.SHP File API (maptools.org)

这里笔者需要读取shp文件的四至范围和每个Geometry的坐标数据

代码如下：

#include <shapefil.h>

int main()

{

    //读取shp

    const char * pszShapeFile = "../云南县界/云南县界.shp";

    SHPHandle hShp= SHPOpen(pszShapeFile, "r");

    int nShapeType, nVertices;

    int nEntities = 0;

    double* minB = new double[4];

    double* maxB = new double[4];

    SHPGetInfo(hShp, &nEntities, &nShapeType, minB, maxB);

    printf("ShapeType:%d\n", nShapeType);

    printf("Entities:%d\n", nEntities);

    printf("Xmin, Ymin: %f,%f\n", minB[0], minB[1]);

    printf("Xmax, Ymax: %f,%f\n", maxB[0], maxB[1]);

    for (int i = 0; i < nEntities;i++)

    {

        int iShape = i;

        SHPObject *obj = SHPReadObject(hShp, iShape);

        printf("--------------Feature:%d------------\n",iShape);

        int parts = obj->nParts;

        int verts=obj->nVertices;

        printf("nParts:%d\n", parts);

        printf("nVertices:%d\n", verts);

        for (size_t i = 0; i < verts; i++)

        {

            double x=obj->padfX[i];

            double y = obj->padfY[i];

            printf("%f,%f;", x,y);

        }

        printf("\n");

    }

    SHPClose(hShp);

}

结果输出：

Xmin, Ymin: 348122 2.34118e+06

Xmax, Ymax: 1.23349e+06 3.23468e+06

ShapeType:5

Entities:125

......

4. OpenGL绘制

OpenGL的绘制流程参考：

你好，三角形 - LearnOpenGL CN (learnopengl-cn.github.io)

该网站上绘制两个三角形的示例代码：

Code Viewer. Source code: src/1.getting_started/2.4.hello_triangle_exercise2/hello_triangle_exercise2.cpp (learnopengl.com)

本文所使用的方法是每个Geometry绑定一个VAO和VBO，然后进行绘制

全部代码如下：

#include <glad/glad.h>

#include <GLFW/glfw3.h>

#include <shapefil.h>

#include <vector>

#include <iostream>

void framebuffer_size_callback(GLFWwindow *window, int width, int height);

void processInput(GLFWwindow *window);

// settings

const unsigned int SCR_WIDTH = 800;

const unsigned int SCR_HEIGHT = 600;

double XMAX, YMAX, XMIN, YMIN;

std::vector<int> size;

const char *vertexShaderSource = "#version 330 core\n"

                                 "layout (location = 0) in vec3 aPos;\n"

                                 "void main()\n"

                                 "{\n"

                                 "   gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);\n"

                                 "}\0";

const char *fragmentShaderSource = "#version 330 core\n"

                                   "out vec4 FragColor;\n"

                                   "void main()\n"

                                   "{\n"

                                   "   FragColor = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n"

                                   "}\n\0";

int main()

{

    // glfw: initialize and configure

    // ------------------------------

    glfwInit();

    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);

    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);

    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

#ifdef __APPLE__

    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);

#endif

    // glfw window creation

    // --------------------

    GLFWwindow *window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "LearnOpenGL", NULL, NULL);

    if (window == NULL)

    {

        std::cout << "Failed to create GLFW window" << std::endl;

        glfwTerminate();

        return -1;

    }

    glfwMakeContextCurrent(window);

    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    // glad: load all OpenGL function pointers

    // ---------------------------------------

    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))

    {

        std::cout << "Failed to initialize GLAD" << std::endl;

        return -1;

    }

    // build and compile our shader program

    // ------------------------------------

    // vertex shader

    unsigned int vertexShader = glCreateShader(GL_VERTEX_SHADER);

    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);

    glCompileShader(vertexShader);

    // check for shader compile errors

    int success;

    char infoLog[512];

    glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success);

    if (!success)

    {

        glGetShaderInfoLog(vertexShader, 512, NULL, infoLog);

        std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n"

                  << infoLog << std::endl;

    }

    // fragment shader

    unsigned int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);

    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);

    glCompileShader(fragmentShader);

    // check for shader compile errors

    glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);

    if (!success)

    {

        glGetShaderInfoLog(fragmentShader, 512, NULL, infoLog);

        std::cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n"

                  << infoLog << std::endl;

    }

    // link shaders

    unsigned int shaderProgram = glCreateProgram();

    glAttachShader(shaderProgram, vertexShader);

    glAttachShader(shaderProgram, fragmentShader);

    glLinkProgram(shaderProgram);

    // check for linking errors

    glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);

    if (!success)

    {

        glGetProgramInfoLog(shaderProgram, 512, NULL, infoLog);

        std::cout << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n"

                  << infoLog << std::endl;

    }

    glDeleteShader(vertexShader);

    glDeleteShader(fragmentShader);

    const char *pszShapeFile = "../云南县界/云南县界.shp";

    SHPHandle hShp = SHPOpen(pszShapeFile, "r");

    int nShapeType, nVertices;

    int nEntities = 0;

    double *minB = new double[4];

    double *maxB = new double[4];

    SHPGetInfo(hShp, &nEntities, &nShapeType, minB, maxB);

    std::cout <<"Xmin, Ymin: "<< minB[0] << " " << minB[1] << std::endl;

    std::cout <<"Xmax, Ymax: "<< maxB[0] << " " << maxB[1] << std::endl;

    XMAX = maxB[0];

    XMIN = minB[0];

    YMAX = maxB[1];

    YMIN = minB[1];

    printf("ShapeType:%d\n", nShapeType);

    printf("Entities:%d\n", nEntities);

    unsigned int VBOs[nEntities], VAOs[nEntities];

    glGenVertexArrays(nEntities, VAOs); // we can also generate multiple VAOs or buffers at the same time

    glGenBuffers(nEntities, VBOs);

    for (int i = 0; i < nEntities; i++)

    {

        std::vector<double> arr;

        int iShape = i;

        SHPObject *obj = SHPReadObject(hShp, iShape);

        // printf("--------------Feature:%d------------\n",iShape);

        int parts = obj->nParts;

        int verts = obj->nVertices;

        // printf("nParts:%d\n", parts);

        // printf("nVertices:%d\n", verts);

        for (size_t i = 0; i < verts; i++)

        {

            double x = (obj->padfX[i] - XMIN) / (XMAX - XMIN) * 2 - 1;

            double y = (obj->padfY[i] - YMIN) / (YMAX - YMIN) * 2 - 1;

            // double x = obj->padfX[i];

            // double y = obj->padfY[i];

            // printf("%f,%f;", x, y);

            arr.push_back(x);

            arr.push_back(y);

            arr.push_back(0.0f);

        }

        size.push_back(arr.size()/3);

        double vertices[arr.size()];

        std::copy(arr.begin(), arr.end(), vertices);

        glBindVertexArray(VAOs[i]);

        glBindBuffer(GL_ARRAY_BUFFER, VBOs[i]);

        glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

        glVertexAttribPointer(0, 3, GL_DOUBLE, GL_FALSE, 3 * sizeof(double), (void *)0); // Vertex attributes stay the same

        glEnableVertexAttribArray(0);

    }

    SHPClose(hShp);

    // uncomment this call to draw in wireframe polygons.

    // glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

    // render loop

    // -----------

    while (!glfwWindowShouldClose(window))

    {

        // input

        // -----

        processInput(window);

        // render

        // ------

        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);

        glClear(GL_COLOR_BUFFER_BIT);

        glUseProgram(shaderProgram);

        for (int i = 0; i < size.size(); i++)

        {

            glBindVertexArray(VAOs[i]);

            glDrawArrays(GL_LINE_STRIP, 0, size[i]);

            // std::cout<<VAOs[i]<<"  "<<size[i]<<std::endl;

        }

        // glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.)

        // -------------------------------------------------------------------------------

        glfwSwapBuffers(window);

        glfwPollEvents();

    }

    // optional: de-allocate all resources once they've outlived their purpose:

    // ------------------------------------------------------------------------

    glDeleteVertexArrays(size.size(), VAOs);

    glDeleteBuffers(size.size(), VBOs);

    glDeleteProgram(shaderProgram);

    // glfw: terminate, clearing all previously allocated GLFW resources.

    // ------------------------------------------------------------------

    glfwTerminate();

    return 0;

}

// process all input: query GLFW whether relevant keys are pressed/released this frame and react accordingly

// ---------------------------------------------------------------------------------------------------------

void processInput(GLFWwindow *window)

{

    if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)

        glfwSetWindowShouldClose(window, true);

}

// glfw: whenever the window size changed (by OS or user resize) this callback function executes

// ---------------------------------------------------------------------------------------------

void framebuffer_size_callback(GLFWwindow *window, int width, int height)

{

    // make sure the viewport matches the new window dimensions; note that width and

    // height will be significantly larger than specified on retina displays.

    glViewport(0, 0, width, height);

}

本文基于CMake构建，CMakeLists.txt代码如下：

# CMake 最低版本号要求

cmake_minimum_required (VERSION 2.8)

# 项目信息

project (Demo)

# 查找当前目录下的所有源文件

# 并将名称保存到 DIR_SRCS 变量

# aux_source_directory(.. DIR_SRCS)

find_package(shapelib)

find_package(glfw3 REQUIRED)

find_package( OpenGL REQUIRED )

include_directories( ${OPENGL_INCLUDE_DIRS} )

# 指定生成目标

# add_executable(Demo ${DIR_SRCS})

add_executable(Demo ../glad.c ../shapelib_opengl.cpp)

target_link_libraries(${PROJECT_NAME} ${shapelib_LIBRARIES} ${OPENGL_LIBRARIES} glfw)

注意：