OpenGL: 深度裁剪glClearDepth

glClearDepth(1.0); // 1.0是最大深度([0.0,1.0])

glClear(GL_DEPTH_BUFFER_BIT);

glEnable(GL_DEPTH_TEST); // 最好在display()中调用

以下面的代码为例：
GLfloat vertexArr1[] = { -5.0, 0.0, 0.0,
5.0, 0.0, 0.0,
0.0, 10.0, 0.0,
-5.0, 5.0, -5.0,
5.0, 5.0, -5.0,
0.0, -5.0, -5.0};
int vertArrSize = 6;
......
glClear(GL_COLOR_BUFFER_BIT);
glClear(GL_DEPTH_BUFFER_BIT);

glEnable(GL_DEPTH_TEST);

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0, 1.0, 0.1, 500.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(0.0, 0.0, 50.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);

glColor3f(1.0, 0.0, 0.0);
glBegin(GL_TRIANGLES);
for(int i=0; i<3; i++) {
int offset = i*3;
glVertex3f(vertexArr1[offset], vertexArr1[offset+1], vertexArr1[offset+2]);
}
glEnd();

glColor3f(0.0, 1.0, 0.0);
glBegin(GL_TRIANGLES);
for(int i=3; i<6; i++) {
int offset = i*3;
glVertex3f(vertexArr1[offset], vertexArr1[offset+1], vertexArr1[offset+2]);
}
glEnd();

glFlush();

如果不使用glEnable(GL_DEPTH_TEST)，则绘制的结果是绿色三角形在红色三角形之上；
这同时说明了openGL的绘制顺序与代码中的出现顺序相反。
代码如下：

#include <windows.h>
#include <GL/glut.h>
#include <GL/glaux.h>
#pragma comment(lib,"glut32.lib")
#pragma comment(lib,"glu32.lib")
#pragma comment(lib,"opengl32.lib")

#include <math.h>
#include <iostream>
using namespace std;
GLfloat vertexArr1[] = 
{
	-5.0,    0.0,    0.0,
	5.0,    0.0,    0.0,
	0.0,    10.0,    0.0,
	-5.0,    5.0,    -5.0,
	5.0,    5.0,    -5.0,
	0.0,    -5.0,    -5.0
};
int vertArrSize = 6;
double calcDist(GLdouble _x,GLdouble _y,GLdouble _z,GLdouble _wx0,GLdouble _wy0,GLdouble _wz0,GLdouble _wx1,GLdouble _wy1,GLdouble _wz1) 
{
	double tDx = _wx1 - _wx0;
	double tDy = _wy1 - _wy0;
	double tDz = _wz1 - _wz0;
	double tDisUnit = sqrt(tDx*tDx + tDy*tDy + tDz * tDz);
	tDx = tDx / tDisUnit;
	tDy = tDy / tDisUnit;
	tDz = tDz / tDisUnit;
	double tVecX = _wx0 - _x;
	double tVecY = _wy0 - _y;
	double tVecZ = _wz0 - _z;

	double tDisX = tDy*tVecZ - tDz*tVecY;
	double tDisY = -tDx*tVecZ + tDz*tVecX;
	double tDisZ = tDx*tVecY - tDy*tVecX;
	return sqrt(tDisX*tDisX + tDisY*tDisY + tDisZ*tDisZ);
}
int findClosestPt(GLdouble _wx0,GLdouble _wy0,GLdouble _wz0,GLdouble _wx1,GLdouble _wy1,GLdouble _wz1) 
{
	double minDis;
	int minDisPos;
	minDisPos = 0;
	minDis = calcDist((double)vertexArr1[0], (double)vertexArr1[1], (double)vertexArr1[2], _wx0, _wy0, _wz0, _wx1, _wy1, _wz1);
	for(int i=1; i<vertArrSize; i++)
	{
		int offset = i * 3;
		double tmpDis = calcDist(vertexArr1[offset], vertexArr1[offset+1], vertexArr1[offset+2], _wx0, _wy0, _wz0, _wx1, _wy1, _wz1);
		if(tmpDis < minDis)
		{
			minDis = tmpDis;
			minDisPos = i;
		}   
	}
	return minDisPos;
}
void init()
{
	glClearColor(0.0, 0.0, 0.0, 0.0);
	glShadeModel(GL_SMOOTH);

	glClearDepth(1.0);            // 1.0是最大深度([0.0,1.0])
	glClear(GL_DEPTH_BUFFER_BIT);

	glEnable(GL_DEPTH_TEST);    // 最好在display()中调用
}
void display()
{
	glClear(GL_COLOR_BUFFER_BIT);
	glClear(GL_DEPTH_BUFFER_BIT);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluPerspective(45.0, 1.0, 0.1, 500.0);
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	gluLookAt(0.0, 0.0, 50.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
	glColor3f(1.0, 0.0, 0.0);
	glBegin(GL_TRIANGLES);
	for(int i=0; i<3; i++)
	{
		int offset = i*3;
		glVertex3f(vertexArr1[offset], vertexArr1[offset+1], vertexArr1[offset+2]);
	}
	glEnd();
	glColor3f(0.0, 1.0, 0.0);
	glBegin(GL_TRIANGLES);
	for(int i=3; i<6; i++)
	{
		int offset = i*3;
		glVertex3f(vertexArr1[offset], vertexArr1[offset+1], vertexArr1[offset+2]);
	}
	glEnd();
	glFlush();
}
void mouse(int button, int state, int x, int y)
{
	GLint viewport[4];
	GLdouble mvmatrix[16], projmatrix[16];
	GLint realy;
	GLdouble wx0, wy0, wz0;
	GLdouble wx1, wy1, wz1;
	switch(button)
	{
	case GLUT_LEFT_BUTTON:
		if(state == GLUT_DOWN)
		{
			//printf(" %d %d \n", x, y);
			glGetIntegerv(GL_VIEWPORT, viewport);
			glGetDoublev(GL_MODELVIEW_MATRIX, mvmatrix);
			glGetDoublev(GL_PROJECTION_MATRIX, projmatrix);
			realy = viewport[3] - (GLint)y - 1;
			//printf("Coordinates at cursor are (M, M)\n", x, realy);
			gluUnProject((GLdouble)x, (GLdouble)realy, 0.0, mvmatrix, projmatrix, viewport, &wx0, &wy0, &wz0);
			printf("World coords at z=0.0 are (%.3f, %.3f, %.3f)\n", wx0, wy0, wz0);
			gluUnProject((GLdouble)x, (GLdouble)realy, 1.0, mvmatrix, projmatrix, viewport, &wx1, &wy1, &wz1);
			printf("World coords at z=1.0 are (%.3f, %.3f, %.3f)\n", wx1, wy1, wz1);
			int i = findClosestPt(wx0, wy0, wz0, wx1, wy1, wz1);
			int offset = i*3;
			printf("Choose Point (%.3f, %.3f, %.3f)\n\n", vertexArr1[offset], vertexArr1[offset+1], vertexArr1[offset+2]);
		}
		break;
	default:
		break;
	}
}
int main(int argc, char** argv)
{
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
	glutInitWindowPosition(100, 100);
	glutInitWindowSize(300, 300);
	glutCreateWindow(argv[0]);
	init();
	glutDisplayFunc(display);
	glutMouseFunc(mouse);
	glutMainLoop();
	return 0;
}

运行结果如下：
Enable Depth



Not Enable Depth



http://blog.sina.com.cn/s/blog_8e6bfecf0100v0nr.html