OpenGL编程逐步深入（三）在窗口中显示一个三角形

这一节教程的内容会比较少，我们仅仅是对上一节教程中的代码进行扩展，在窗口中渲染一个三角形出来。
本节我们以下图所示正方形来讲解OpenGl中的坐标系统。当沿着Z轴负方向看时，可见顶点的坐标必须在这个正方形内，这样视口变换才可以將它们映射到窗口中的可见区域。

点（-1.0，-1.0）对应窗口的左下角，（-1.0，1.0）对应窗口的左上角，依此类推。如果指定一个顶点在这个正方形之外，构成的三角形在这个正方形之外的部分会被裁剪掉，将会看不到这部分。

显示三角形代码

/*    Copyright 2010 Etay Meiri    This program is free software: you can redistribute it and/or modify    it under the terms of the GNU General Public License as published by    the Free Software Foundation, either version 3 of the License, or    (at your option) any later version.    This program is distributed in the hope that it will be useful,    but WITHOUT ANY WARRANTY; without even the implied warranty of    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the    GNU General Public License for more details.    You should have received a copy of the GNU General Public License    along with this program.  If not, see <http://www.gnu.org/licenses/>.    Tutorial 03 - Hello triangle!*/#include "stdafx.h"#include <stdio.h>#include <GL/glew.h>#include <GL/freeglut.h>GLuint VBO;//创建顶点结构体，用于表示OpenGL中的顶点struct Vector3f{    float x;    float y;    float z;    Vector3f(){}    Vector3f(float _x, float _y, float _z)    {        x = _x;        y = _y;        z = _z;    }};static void RenderSceneCB(){    glClear(GL_COLOR_BUFFER_BIT);    glEnableVertexAttribArray(0);    glBindBuffer(GL_ARRAY_BUFFER, VBO);    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);    glDrawArrays(GL_TRIANGLES, 0, 3);    glDisableVertexAttribArray(0);    glutSwapBuffers();}static void InitializeGlutCallbacks(){    glutDisplayFunc(RenderSceneCB);}static void CreateVertexBuffer(){    Vector3f Vertices[3];    Vertices[0] = Vector3f(-1.0f, -1.0f, 0.0f);    Vertices[1] = Vector3f(1.0f, -1.0f, 0.0f);    Vertices[2] = Vector3f(0.0f, 1.0f, 0.0f);    glGenBuffers(1, &VBO);    glBindBuffer(GL_ARRAY_BUFFER, VBO);    glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices,GL_STATIC_DRAW);}int _tmain(int argc, _TCHAR* argv[]){    glutInit(&argc, argv);    glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGBA);    glutInitWindowSize(800, 600);    glutInitWindowPosition(100, 100);    glutCreateWindow("Tutorial 02");    InitializeGlutCallbacks();    // Must be done after glut is initialized!    GLenum res = glewInit();    if (res != GLEW_OK) {        fprintf(stderr, "Error: '%s'\n", glewGetErrorString(res));        return 1;    }    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);    CreateVertexBuffer();    glutMainLoop();    return 0;}

代码解读

本节代码在上节代码中稍作修改。

Vector3f Vertices[3];Vertices[0] = Vector3f(-1.0f, -1.0f, 0.0f);Vertices[1] = Vector3f(1.0f, -1.0f, 0.0f);Vertices[2] = Vector3f(0.0f, 1.0f, 0.0f);

扩展数组容量，存放三角形的三个顶点。

glDrawArrays(GL_TRIANGLES, 0, 3);

调用绘图函数时將绘制模式改为GL_TRIANGLES（表示绘制三角形），第三个参数改为3，表示使用3个顶点绘制。

编译运行

可以看到窗口中显示一个白色的三角形。