plese give me reply (fading to the display screen)

his is naveen … how to map Pixel buffer object into openGL address space…if my display resolution is 1280 X 800 .thing is display screen should be fadding … i written code for this using openGL … but i want use PBO object and this should be mapeed into openGL buffer object… if i mapped PBO into openGL … can i decrement RGB value and how to decrement RGB values from PBO(pixel buffer object) … doubt is how to access(decrement) RGB value from Pixel buffer object …can any one help my question

const int SCREEN_WIDTH =1280;
const int SCREEN_HEIGHT = 800;

void display(void);
void init(void);

void display(void)
{
static float i;
for (float i=0.0f;i<256.0f;i++)
{
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glTranslatef( 0,0, 0 );
glLoadIdentity();
glBegin( GL_QUADS );
glColor4ub(i,i,i,i);
glVertex3f( 0,0,0 );
glVertex3f( SCREEN_WIDTH, 0,0 );
glVertex3f( SCREEN_WIDTH, SCREEN_HEIGHT,0 );
glVertex3f( 0,SCREEN_HEIGHT, 0 );
glEnd();
glutSwapBuffers();

}

}

void init(void)
{
glClearColor (0.0, 0.0, 0.0, 0.0);
GLfloat _div = ( GLfloat ) SCREEN_WIDTH / ( GLfloat )SCREEN_HEIGHT;
glViewport( 0, 0, ( GLint ) SCREEN_WIDTH , ( GLint )SCREEN_HEIGHT );
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
gluOrtho2D (0.0, (GLdouble) SCREEN_WIDTH, 0.0, (GLdouble) SCREEN_HEIGHT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

}
int main(int argc, char** argv)
{

while(1)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize (1280, 800);
//glutInitWindowPosition (100, 100);
glutCreateWindow (“hello”);
init ();
glutDisplayFunc(display);
glutMainLoop();
}
return 0;

}

can any one give reply yaarrrrrrr…