OpenGL绘制多边形边框线

利用OpenGL如何在绘制多边形的时候同时绘制其变现呢？
网上一种解决方案是利用glPolygonMode，将多边形绘制两次，一次绘制面，一次绘制边。这种方案理论上是可行的（我没有试过），但是OpenGL要进行两次绘制，效率上明显是不高的。

如果以顺时针绘制则是反面,逆时针绘制则是正面 
// 设置正面为填充模式
glPolygonMode(GL_FRONT, GL_FILL);
// 设置反面为线形模式
glPolygonMode(GL_BACK, GL_LINE);
// 设置逆时针绘制一个正方形

参考了Easy wireframe display with barycentric coordinates这篇博文，参考其方法，使用Barycentric Coordinates（重心坐标），在GLSL中直接进行判断，如果离边近的像素就渲染成别的颜色。

要注意的主要有两点：
1. 在定点坐标数组中增加一个重心坐标属性，如下图。

2. 在GLSL中对重心坐标进行判断。

参考代码如下：
（使用了GLEW、SFML和GLM第三方库）

#include 
#include 
#include 
#include 
#include 

// Shader sources
const GLchar* vertexSource =
    "#version 150 core\n"
    "varying vec3 vBC;"
    "attribute vec3 position;"
    "attribute vec3 barycentric;"
    "uniform mat4 model;"
    "uniform mat4 view;"
    "uniform mat4 proj;"
    "void main() {"
    "   vBC = barycentric;"
    "   gl_Position = proj * view * model * vec4(position, 1.0);"
    "}";
const GLchar* fragmentSource =
    "#version 150 core\n"
    "varying vec3 vBC;"
    "void main() {"
    "   if(any(lessThan(vBC, vec3(0.005)))){"
    "       gl_FragColor = vec4(1.0, 1.0, 1.0, 1.0);"
    "   }"
    "   else{"
    "       gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);"
    "   }"
    "}";

int main()
{
    sf::ContextSettings settings;
    settings.depthBits = 24;
    settings.stencilBits = 8;

    sf::Window window(sf::VideoMode(800, 600, 32), "OpenGL", sf::Style::Titlebar | sf::Style::Close, settings);

    // Initialize GLEW
    glewExperimental = GL_TRUE;
    glewInit();
    glEnable(GL_DEPTH_TEST);

    // Create Vertex Array Object
    GLuint vao;
    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);

    // Create a Vertex Buffer Object and copy the vertex data to it
    GLuint vbo;
    glGenBuffers(1, &vbo);

    GLfloat vertices[] = {
        // 三维定点坐标        //重心坐标
        -0.5f, -0.5f, -0.5f, 1.0f, 0.0f, 0.0f,
        0.5f, -0.5f, -0.5f, 0.0f, 1.0f, 0.0f,
        0.5f, 0.5f, -0.5f, 0.0f, 0.0f, 1.0f,
        0.5f, 0.5f, -0.5f, 1.0f, 0.0f, 0.0f,
        -0.5f, 0.5f, -0.5f, 0.0f, 1.0f, 0.0f,
        -0.5f, -0.5f, -0.5f, 0.0f, 0.0f, 1.0f,

        -0.5f, -0.5f, 0.5f, 1.0f, 0.0f, 0.0f,
        0.5f, -0.5f, 0.5f, 0.0f, 1.0f, 0.0f,
        0.5f, 0.5f, 0.5f, 0.0f, 0.0f, 1.0f,
        0.5f, 0.5f, 0.5f, 1.0f, 0.0f, 0.0f,
        -0.5f, 0.5f, 0.5f, 0.0f, 1.0f, 0.0f,
        -0.5f, -0.5f, 0.5f, 0.0f, 0.0f, 1.0f,

        -0.5f, 0.5f, 0.5f, 1.0f, 0.0f, 0.0f,
        -0.5f, 0.5f, -0.5f, 0.0f, 1.0f, 0.0f,
        -0.5f, -0.5f, -0.5f, 0.0f, 0.0f, 1.0f,
        -0.5f, -0.5f, -0.5f, 1.0f, 0.0f, 0.0f,
        -0.5f, -0.5f, 0.5f, 0.0f, 1.0f, 0.0f,
        -0.5f, 0.5f, 0.5f, 0.0f, 0.0f, 1.0f,

        0.5f, 0.5f, 0.5f, 1.0f, 0.0f, 0.0f,
        0.5f, 0.5f, -0.5f, 0.0f, 1.0f, 0.0f,
        0.5f, -0.5f, -0.5f, 0.0f, 0.0f, 1.0f,
        0.5f, -0.5f, -0.5f, 1.0f, 0.0f, 0.0f,
        0.5f, -0.5f, 0.5f, 0.0f, 1.0f, 0.0f,
        0.5f, 0.5f, 0.5f, 0.0f, 0.0f, 1.0f,

        -0.5f, -0.5f, -0.5f, 1.0f, 0.0f, 0.0f,
        0.5f, -0.5f, -0.5f, 0.0f, 1.0f, 0.0f,
        0.5f, -0.5f, 0.5f, 0.0f, 0.0f, 1.0f,
        0.5f, -0.5f, 0.5f, 1.0f, 0.0f, 0.0f,
        -0.5f, -0.5f, 0.5f, 0.0f, 1.0f, 0.0f,
        -0.5f, -0.5f, -0.5f, 0.0f, 0.0f, 1.0f,

        -0.5f, 0.5f, -0.5f, 1.0f, 0.0f, 0.0f,
        0.5f, 0.5f, -0.5f, 0.0f, 1.0f, 0.0f,
        0.5f, 0.5f, 0.5f, 0.0f, 0.0f, 1.0f,
        0.5f, 0.5f, 0.5f, 1.0f, 0.0f, 0.0f,
        -0.5f, 0.5f, 0.5f, 0.0f, 1.0f, 0.0f,
        -0.5f, 0.5f, -0.5f, 0.0f, 0.0f, 1.0f
    };

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

    // Create and compile the vertex shader
    GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexSource, NULL);
    glCompileShader(vertexShader);

    // Create and compile the fragment shader
    GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentSource, NULL);
    glCompileShader(fragmentShader);

    // Link the vertex and fragment shader into a shader program
    GLuint shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, vertexShader);
    glAttachShader(shaderProgram, fragmentShader);
    glLinkProgram(shaderProgram);
    glUseProgram(shaderProgram);

    // Specify the layout of the vertex data
    GLint posAttrib = glGetAttribLocation(shaderProgram, "position");
    glEnableVertexAttribArray(posAttrib);
    glVertexAttribPointer(posAttrib, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), 0);

    GLint baryAttrib = glGetAttribLocation(shaderProgram, "barycentric");
    glEnableVertexAttribArray(baryAttrib);
    glVertexAttribPointer(baryAttrib, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (void*)(3 * sizeof(GLfloat)));

    // Set up projection
    glm::mat4 view = glm::lookAt(
        glm::vec3(1.5f, 1.5f, 1.5f),
        glm::vec3(0.0f, 0.0f, 0.0f),
        glm::vec3(0.0f, 0.0f, 1.0f));
    GLint uniView = glGetUniformLocation(shaderProgram, "view");
    glUniformMatrix4fv(uniView, 1, GL_FALSE, glm::value_ptr(view));

    glm::mat4 proj = glm::perspective(glm::radians(45.0f), 800.0f / 600.0f, 1.0f, 10.0f);
    GLint uniProj = glGetUniformLocation(shaderProgram, "proj");
    glUniformMatrix4fv(uniProj, 1, GL_FALSE, glm::value_ptr(proj));

    glm::mat4 model;
    GLint uniModel = glGetUniformLocation(shaderProgram, "model");
    model = glm::rotate(model, glm::radians(180.0f), glm::vec3(0.0f, 0.0f, 1.0f));
    glUniformMatrix4fv(uniModel, 1, GL_FALSE, glm::value_ptr(model));

    while (window.isOpen())
    {
        sf::Event windowEvent;
        while (window.pollEvent(windowEvent))
        {
            switch (windowEvent.type)
            {
            case sf::Event::Closed:
                window.close();
                break;
            }
        }

        // Clear the screen to black
        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        // Draw cube
        glDrawArrays(GL_TRIANGLES, 0, 36);

        // Swap buffers
        window.display();
    }

    glDeleteProgram(shaderProgram);
    glDeleteShader(fragmentShader);
    glDeleteShader(vertexShader);

    glDeleteBuffers(1, &vbo);
    glDeleteVertexArrays(1, &vao);

    return EXIT_SUCCESS;
}

最后的渲染效果如下：

如图渲染效果不是很好，有明显的锯齿。如何反锯齿，参看原始博文Easy wireframe display with barycentric coordinates。