Opengl超級寶典——顯示一個正方形並移動。

#include "GLtools.h"	// OpenGL toolkit
#include <gl/glut.h>


// 初始化正方形的位置和大小。
GLfloat x = 0.0f;
GLfloat y = 0.0f;
GLfloat rsize = 25;

// Step size in x and y directions每一步在x，y方向上走的距離
// (number of pixels to move each time)
GLfloat xstep = 1.0f;
GLfloat ystep = 1.0f;

// Keep track of windows changing width and height
GLfloat windowWidth;
GLfloat windowHeight;

///////////////////////////////////////////////////////////
// Called to draw scene
void RenderScene(void)//RC（渲染環境）設置
	{
	// Clear the window with current clearing color
	glClear(GL_COLOR_BUFFER_BIT);

   	// Set current drawing color to red
	//		   R	 G	   B
	glColor3f(1.0f, 0.0f, 0.0f);

	// Draw a filled rectangle with current color
	glRectf(x, y, x + rsize, y - rsize);

    // Flush drawing commands and swap
    glutSwapBuffers();
	}

///////////////////////////////////////////////////////////
// Called by GLUT library when idle (window not being
// resized or moved)
void TimerFunction(int value)
    {
    // Reverse direction when you reach left or right edge
    if(x > windowWidth-rsize || x < -windowWidth)
        xstep = -xstep;

    // Reverse direction when you reach top or bottom edge
    if(y > windowHeight || y < -windowHeight + rsize)
        ystep = -ystep;

	// Actually move the square
    x += xstep;
    y += ystep;

    // Check bounds. This is in case the window is made
    // smaller while the rectangle is bouncing and the 
	// rectangle suddenly finds itself outside the new
    // clipping volume
    if(x > (windowWidth-rsize + xstep))
        x = windowWidth-rsize-1;
	else if(x < -(windowWidth + xstep))
		x = -windowWidth -1;

    if(y > (windowHeight + ystep))
        y = windowHeight-1; 
	else if(y < -(windowHeight - rsize + ystep))
		y = -windowHeight + rsize - 1;



     // Redraw the scene with new coordinates
    glutPostRedisplay();
    glutTimerFunc(33,TimerFunction, 1);
    }


///////////////////////////////////////////////////////////
// Setup the rendering state
void SetupRC(void)
    {
    // Set clear color to blue
    glClearColor(0.0f, 0.0f, 1.0f, 1.0f);
    }


///////////////////////////////////////////////////////////
// Called by GLUT library when the window has chanaged size
void ChangeSize(int w, int h)
    {
    GLfloat aspectRatio;

    // Prevent a divide by zero
    if(h == 0)
        h = 1;
		
    // Set Viewport to window dimensions
    glViewport(0, 0, w, h);

    // Reset coordinate system
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    // Establish clipping volume (left, right, bottom, top, near, far)
    aspectRatio = (GLfloat)w / (GLfloat)h;
    if (w <= h) 
        {
        windowWidth = 100;
        windowHeight = 100 / aspectRatio;
        glOrtho (-100.0, 100.0, -windowHeight, windowHeight, 1.0, -1.0);
        }
    else 
        {
        windowWidth = 100 * aspectRatio;
        windowHeight = 100;
        glOrtho (-windowWidth, windowWidth, -100.0, 100.0, 1.0, -1.0);
        }

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    }

///////////////////////////////////////////////////////////
// Main program entry point
int main(int argc, char* argv[])
	{
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA);
	glutInitWindowSize(800,600);
    glutCreateWindow("Bounce");
	glutDisplayFunc(RenderScene);
    glutReshapeFunc(ChangeSize);
	glutTimerFunc(33, TimerFunction, 1);

	SetupRC();

	glutMainLoop();
        
    return 0;
    }

1.glutSwapBuffers（）函數，當設置Opengl窗口是，我們要指定一個雙緩衝區渲染環境，這就意味着將在後臺緩衝區渲染，然後結束時交換到前臺。這種形式能夠防止觀察者看到閃爍。

2.glutPostRedisplay()函數。刷新顯示。

3.glMatrixMode(GL_PROJECTION);切換當前矩陣。三種模式：GL_PROJECTION 投影, GL_MODELVIEW 模型視圖, GL_TEXTURE 紋理。接下要進行什麼操作。

如果參數是GL_PROJECTION，這個是投影的意思，就是要對投影相關進行操作，也就是把物體投影到一個平面上，就像我們照相一樣，把3維物體投到2維的平面上。這樣，接下來的語句可以是跟透視相關的函數，比如glFrustum()或gluPerspective()；

在操作投影矩陣以前，需要調用函數：

glMatrixMode(GL_PROJECTION); //將當前矩陣指定爲投影矩陣

然後把矩陣設爲單位矩陣：
glLoadIdentity();
然後調用glFrustum()或gluPerspective(),它們生成的矩陣會與當前的矩陣相乘,生成透視的效果；

4.glutDisplayFunc（）顯示回調函數。

5.glutReshapeFunc（）窗口改變回調函數。

總結：GLUT內部運行一個本地消息循環，攔截適當的消息，調用我們爲不同事件註冊的毀掉函數。

我自己總結了下流程：