ZバッファをＯＮにして、「glPushAttrib()」、「glPopAttrib()」を使う事で、部分的にｚバッファをＯＦＦにして描画せよ。

#include <GL/glut.h>

//------------- プロトタイプ宣言 ------------------//
void display();
void reshape(int w, int h);
void myMouseFunc(int button,int state,int x,int y);
void myMouseMotion(int x,int y);

void DRAW_XYZ();
void DRAW_TRI();

//------------- 各種データ構造 ------------------------//
//マウスの状態
struct MOUSE
{
	int Xstart,Ystart;
	bool flag;
	double weight;
};
MOUSE MouseStatus={0,0,false,0.5};


//回転関係
struct ObjectRotate
{
	double xAngle,yAngle;
};
ObjectRotate ObjRot={0,0};

//------------- OpenGLの初期設定 -------------------------//
void GLUT_INIT()
{
	glutInitDisplayMode(GLUT_RGBA| GLUT_DOUBLE | GLUT_DEPTH);
	glutInitWindowSize(640,480);
}

void GLUT_CALL_FUNC()
{
	glutDisplayFunc(display);
	glutReshapeFunc(reshape);
	glutMouseFunc(myMouseFunc);
	glutMotionFunc(myMouseMotion);
}

void MY_INIT()
{
	glClearColor(1.0, 1.0, 1.0, 1.0);
	
}

//----------- メイン関数 -----------------//
int main(int argc, char **argv)
{
	glutInit(&argc,argv);
	GLUT_INIT();
	glutCreateWindow("test");
	GLUT_CALL_FUNC();
	MY_INIT();
	glutMainLoop();

	return 0;
}

//------------- ここから各種コールバック -----------------//
void display()
{
	glEnable(GL_DEPTH_TEST);

	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glLoadIdentity();
	gluLookAt(3.0, 4.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);

	glRotated(ObjRot.xAngle,1,0,0);
	glRotated(ObjRot.yAngle,0,1,0);

	DRAW_XYZ();

	
	glPushAttrib(GL_DEPTH_BUFFER_BIT);
	glDisable(GL_DEPTH_TEST);
	glPushMatrix();
	glTranslated(1.5,0,0);
	DRAW_TRI();
	glPopMatrix();
	glPopAttrib();


	DRAW_TRI();

	glutSwapBuffers();

}

void reshape(int w, int h)
{
	glViewport(0, 0, w, h);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluPerspective(30.0, (double)w / (double)h, 1.0, 100.0);
	glMatrixMode(GL_MODELVIEW);
}

void myMouseFunc(int button,int state,int x,int y)
{
	if(button == GLUT_LEFT_BUTTON &&state == GLUT_DOWN){
		MouseStatus.Xstart = x;
		MouseStatus.Ystart = y;
		MouseStatus.flag = true;
	}
	else{
		MouseStatus.flag =false;
	}

}

void myMouseMotion(int x,int y)
{
	if(MouseStatus.flag == false)return;

	static int xdir,ydir;

	xdir = x - MouseStatus.Xstart;
	ydir = y - MouseStatus.Ystart;

	ObjRot.xAngle += (double)ydir * MouseStatus.weight;
	ObjRot.yAngle += (double)xdir * MouseStatus.weight;

	MouseStatus.Xstart = x;
	MouseStatus.Ystart = y;
	
	glutPostRedisplay();
}


//------------ ここから各種関数 ---------------//
void DRAW_XYZ()
{
	glBegin(GL_LINES);

	glColor3d(0,1,0);//x
	glVertex2d(-100,0);
	glVertex2d(100, 0);

	glColor3d(1,0,0);//y
	glVertex2d(0,0);
	glVertex2d(0,100);

	glColor3d(0,0,1);//z
	glVertex3d(0,0,-100);
	glVertex3d(0,0, 100);
	glEnd();
}

void DRAW_TRI()
{
	glColor3d(0,0,1);
	glBegin(GL_TRIANGLES);
	glVertex3d(0,1,0);
	glVertex3d(-0.5, 0,0);
	glVertex3d(0.5, 0,0);
	glEnd();

	glColor3d(1,0,0);
	glBegin(GL_TRIANGLES);
	glVertex3d(0.2,0.7,-0.3);
	glVertex3d(-0.3, -0.3,-0.3);
	glVertex3d(0.7, -0.3,-0.3);
	glEnd();

	glColor3d(1,1,1);
}

【メモ】
実行結果で、右側ポリゴン２つがＺバッファＯＦＦで描画される(赤いポリゴンが手前に来る)