White noize(very slow and strange grid appeared)

whitenoize.c

#include <stdio.h>
#include <stdlib.h>
#include <GLUT/glut.h> 

#define SCREEN_WIDTH 640
#define SCREEN_HEIGHT 360

static int curr_width = 0;
static int curr_height = 0;

static void
keyboard(unsigned char c, int x, int y)
{
    switch (c) {
    case 27:  /* Esc key */
        exit(0);
        break;
    }
}

void
idle()
{
    glutPostRedisplay();
}

static void
reshape(int w, int h)
{
    curr_width = w;
    curr_height = h;
}

// Okay... Iknow this is stupid way to draw a pixel.
static void
draw_pixel(int x, int y, float r, float g, float b)
{
    glColor3f(r, g, b);
    glPointSize(1);
    glPushMatrix(); {
        glBegin(GL_POINTS);
        glVertex2f(x, y);
        glEnd();
    } glPopMatrix();
}

static void
display()
{
    int x, y;

    glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
    glViewport(0, 0, curr_width, curr_height);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0, SCREEN_WIDTH, 0, SCREEN_HEIGHT);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    for (y = 0; y < SCREEN_HEIGHT; y++) {
        for (x = 0; x < SCREEN_WIDTH; x++) {
            float v = (float)rand() / RAND_MAX;
            draw_pixel(x, y, v, v, v);
        }
    }

    glutSwapBuffers();
}

int
main(int argc, char** argv)
{
    srand(0);

    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH );
    glutInitWindowSize(SCREEN_WIDTH, SCREEN_HEIGHT);
    glutInitWindowPosition(100, 100);
    glutCreateWindow(argv[0]);
    glutKeyboardFunc(keyboard);
    glutIdleFunc(idle);
    glutReshapeFunc(reshape);
    glutDisplayFunc(display);
    glutFullScreen();

    glClearColor(0.0, 0.0, 0.0, 0.0);
    glShadeModel(GL_SMOOTH);
    glDisable(GL_DEPTH_TEST);
    glDisable(GL_LIGHTING);
    glCullFace(GL_BACK);
    glEnable(GL_CULL_FACE);

    glutMainLoop();
    return 0;
}