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Imported from P4

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <GL/glut.h>


GLenum doubleBuffer;


static void Init(void)
{

    glClearColor(0.0, 0.0, 0.0, 0.0);

    glClearStencil(0);
    glStencilMask(1);
    glEnable(GL_STENCIL_TEST);
}

static void Reshape(int width, int height)
{

    glViewport(0, 0, width, height);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-5.0, 5.0, -5.0, 5.0, -5.0, 5.0);
    glMatrixMode(GL_MODELVIEW);
}

static void Key(unsigned char key, int x, int y)
{

    switch (key) {
      case 27:
	exit(0);
    }
}

static void Draw(void)
{

    glClear(GL_COLOR_BUFFER_BIT|GL_STENCIL_BUFFER_BIT);

    glStencilFunc(GL_ALWAYS, 1, 1);
    glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);

    glColor3ub(200, 0, 0);
    glBegin(GL_POLYGON);        
	glVertex3i(-4, -4, 0);
	glVertex3i( 4, -4, 0);
	glVertex3i( 0,  4, 0);
    glEnd();

    glStencilFunc(GL_EQUAL, 1, 1);
    glStencilOp(GL_INCR, GL_KEEP, GL_DECR);

    glColor3ub(0, 200, 0);
    glBegin(GL_POLYGON);
	glVertex3i(3, 3, 0);
	glVertex3i(-3, 3, 0);
	glVertex3i(-3, -3, 0);
	glVertex3i(3, -3, 0);
    glEnd();

    glStencilFunc(GL_EQUAL, 1, 1);
    glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);

    glColor3ub(0, 0, 200);
    glBegin(GL_POLYGON);
	glVertex3i(3, 3, 0);
	glVertex3i(-3, 3, 0);
	glVertex3i(-3, -3, 0);
	glVertex3i(3, -3, 0);
    glEnd();


    if (doubleBuffer) {
	glutSwapBuffers();
    } else {
	glFlush();
    }
}

static void Args(int argc, char **argv)
{
    GLint i;

    doubleBuffer = GL_FALSE;

    for (i = 1; i < argc; i++) {
	if (strcmp(argv[i], "-sb") == 0) {
	    doubleBuffer = GL_FALSE;
	} else if (strcmp(argv[i], "-db") == 0) {
	    doubleBuffer = GL_TRUE;
	}
    }
}

int main(int argc, char **argv)
{
    GLenum type;

    glutInit(&argc, argv);
    Args(argc, argv);

    type = GLUT_RGB | GLUT_STENCIL;
    type |= (doubleBuffer) ? GLUT_DOUBLE : GLUT_SINGLE;
    glutInitDisplayMode(type);
    glutInitWindowSize(300, 300);
    glutCreateWindow("Stencil Test");

    Init();

    glutReshapeFunc(Reshape);
    glutKeyboardFunc(Key);
    glutDisplayFunc(Draw);
    glutMainLoop();
}