实现3D引擎（五）- 绘制五边形

 本文绘制一个颜色随时间变化的五边形。

 在片元着色器中设置uniform变量。uniform变量可以接收CPU中发送到着色器的数据，用以设置图形的颜色。片元着色器的定义如下：

#version 330 core
uniform vec4 vertexColor; 
out vec4 color;
void main()
{
    color = vertexColor;
}
           

 设置随着时间改变的颜色，代码如下：

float timeValue = glfwGetTime();
float greenValue = (sin(timeValue) / 2.0f) + 0.5f;
int vertexColorLocation = glGetUniformLocation(shaderProgram, "vertexColor");
//启用着色器程序
glUseProgram(shaderProgram);
glUniform4f(vertexColorLocation, 0, greenValue, 0, 1.0f);
           

  首先，通过glfwGetTime()获取运行的秒数，使用sin函数让颜色在0.0到1.0之间改变，将结果储存到greenValue里。

  然后，用glGetUniformLocation查询uniform 

vertexColor

的位置值。

  最后，通过glUniform4f函数设置uniform值。

 程序完整的代码如下：

#include<iostream>
#define GLEW_STATIC
#include<GL/glew.h>
#include<GLFW/glfw3.h>

const char* vertexShaderSource =
"#version 330 core                     \n"
"layout(location = 0) in vec3 position; \n"
"void main() {                          \n"
"		gl_Position = vec4(position.x, position.y, position.z, 1.0);  \n"
"} \n ";

const char* fragmentShaderSource =
"#version 330 core                   \n"
"uniform vec4 vertexColor;                \n"
"out vec4 color;                     \n"
"void main(){                        \n"
"	color = vertexColor;             \n"
"}  \n";

GLfloat vertices[] = {
	-0.5f, -0.5f, 0.0f,   //0
	0.5f, -0.5f, 0.0f,	  //1
	0.0f,  0.5f, 0.0f,	  //2	  
	0.7f, 0.0f, 0.0f,	  //3
	0.8f, 0.8f, 0.0f	  //4
};

unsigned int indices[] = {
	0,1,2,
	2,1,3,
	2,3,4
};

//key表示按下的键，action表示键按下/释放，mode表示是否有Ctrl、Shift、Alt等按钮操作
void key_callback(GLFWwindow * window, int key, int scancode, int action, int mode)
{
	//用户按下ESC键，设置窗口WindowShouldClose属性为true,关闭应用程序
	if (key == GLFW_KEY_ESCAPE && action == GLFW_PRESS)
		glfwSetWindowShouldClose(window, GL_TRUE);
}

int main()
{
	glfwInit();
	glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);    //主版本号
	glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);    //次版本号
	glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);//核心模式
	
	GLFWwindow* window = glfwCreateWindow(800,600,"OpenGL Game",NULL,NULL); //创建窗口
	if (window == NULL)
	{
		std::cout << "Open window failed!" << std::endl;
		glfwTerminate();
		return -1;
	}
	//将窗口的上下文设置为当前线程的主上下文
	glfwMakeContextCurrent(window);
	
	//GLEW是用来管理OpenGL的函数指针,调用任何OpenGL的函数之前都需要初始化GLEW
	glewExperimental = GL_TRUE;
	if (glewInit() != GLEW_OK)
	{
		std::cout << "Init Glew failed！" << std::endl;
		glfwTerminate();
		return -1;
	}

	int width, height;
	//从GLFW中获取视口大小
	glfwGetFramebufferSize(window,&width,&height);
	//窗口左下角的位置、宽度、高度
	glViewport(0,0, width, height);
	//glEnable(GL_CULL_FACE);
	//glCullFace(GL_BACK);
	//glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
	//注册按键回调函数
	glfwSetKeyCallback(window, key_callback);

	//顶点数组对象
	unsigned int VAO;
	glGenVertexArrays(1,&VAO);
	glBindVertexArray(VAO);

	//顶点缓冲对象
	unsigned int VBO;
	glGenBuffers(1, &VBO);
	glBindBuffer(GL_ARRAY_BUFFER,VBO);
	//将顶点数据复制到缓冲中
	glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
	
	//索引缓冲对象
	unsigned int EBO;
	glGenBuffers(1,&EBO);
	glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
	glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

	//创建顶点着色器
	unsigned int vertexShader = glCreateShader(GL_VERTEX_SHADER);
	glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
	glCompileShader(vertexShader);

	//创建片段着色器
	unsigned int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
	glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
	glCompileShader(fragmentShader);

	//创建着色器程序
	unsigned int shaderProgram = glCreateProgram();
	glAttachShader(shaderProgram, vertexShader);
	glAttachShader(shaderProgram, fragmentShader);
	glLinkProgram(shaderProgram);
	
	//顶点属性配置,告诉OpenGL该如何解析顶点数据
	glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3*sizeof(float), (void*)0);
	glEnableVertexAttribArray(0);

	//检查GLFW是否要求退出
	while (!glfwWindowShouldClose(window))
	{
		//检查是否触发事件（鼠标移动、键盘输入等），如果有则调用相应的回调函数
		glfwPollEvents();
		//渲染操作放入循环中
		//清屏
		glClearColor(0.2f, 0.3f, 0.3f, 1.0);
		glClear(GL_COLOR_BUFFER_BIT);

		glBindVertexArray(VAO);
		glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);

		float timeValue = glfwGetTime();
		float greenValue = (sin(timeValue) / 2.0f) + 0.5f;
		int vertexColorLocation = glGetUniformLocation(shaderProgram, "vertexColor");
		//启用着色器程序
		glUseProgram(shaderProgram);
		glUniform4f(vertexColorLocation, 0, greenValue, 0, 1.0f);

		//绘制
		glDrawElements(GL_TRIANGLES, 9,GL_UNSIGNED_INT,0);
		//glDrawArrays(GL_TRIANGLES, 0, 6);
		//交换颜色缓冲
		glfwSwapBuffers(window);
	}
	//结束时释放资源
	glfwTerminate();
	return 0;
}
           

  输出结果：