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 <GL/glew.h>
#include <SFML/Window.hpp>
#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <glm/gtc/type_ptr.hpp>
// 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
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。
