OpenGL glDrawElements only Black Screen - c++

Question

i was trying to draw a cube, using the glDrawElements function, but even the simple code below only gives me a black screen. If helps, i'm programming on XCode 6.
//
// main.cpp
// Copyright (c) 2014 Guilherme Cardoso. All rights reserved.
//
#include <iostream>
#include <OpenGL/OpenGL.h>
#include <GLUT/GLUT.h>
#include <vector>
#include <math.h>
const GLfloat width = 500;
const GLfloat height = 500;
GLubyte cubeIndices[24] = {0,3,2,1,2,3,7,6
,0,4,7,3,1,2,6,5,4,5,6,7,0,1,5,4};
GLfloat vertices[][3] =
{{-1.0,-1.0,-1.0},{1.0,-1.0,-1.0},
{1.0,1.0,-1.0}, {-1.0,1.0,-1.0}, {-1.0,-1.0,1.0},
{1.0,-1.0,1.0}, {1.0,1.0,1.0}, {-1.0,1.0,1.0}};
GLfloat colors[][3] =
{{0.0,0.0,0.0},{1.0,0.0,0.0},
{1.0,1.0,0.0}, {0.0,1.0,0.0}, {0.0,0.0,1.0},
{1.0,0.0,1.0}, {1.0,1.0,1.0}, {0.0,1.0,1.0}};
void display(){
glEnableClientState(GL_COLOR_ARRAY);
glEnableClientState(GL_VERTEX_ARRAY);
glColorPointer(3, GL_FLOAT, 0, colors);
glVertexPointer(3, GL_FLOAT, 0, vertices);
//glDrawArrays(GL_QUADS, 0, 24);
glDrawElements(GL_QUADS, 24,GL_UNSIGNED_BYTE, cubeIndices);
glDisableClientState(GL_VERTEX_ARRAY);
glutSwapBuffers();
}
void mouse(int button, int state, int x, int y){
}
void keyboard(unsigned char key, int x, int y){
if(key=='q' || key == 'Q') exit(0);
}
void init(){
glEnable(GL_DEPTH_TEST);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0,width ,height, 0);
glMatrixMode(GL_MODELVIEW);
glClearColor (1.0, 1.0, 1.0,1.0);
//glColor3f(0.0,0.0,0.0);
}
void idle(){
}
int main(int argc, char **argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(width, height);
glutCreateWindow("Assignment 3");
glutPositionWindow(0, 0);
glutDisplayFunc(display);
glutMouseFunc(mouse);
glutKeyboardFunc(keyboard);
glutIdleFunc(idle);
init();
glutMainLoop();
}
i already checked some tutorials, and is no much different of what i'm doing.

Answer 1

You never clear the color and especially the depth buffer. You should do that at the beginning of your display() function.
Also, your matrix setup is a bit weird. You set up some ortho projection for the pixels, but try to draw a cube in the range [-1,1], so it will be 2 pixel wide on the screen.

Answer 2

Actually your code is drawing the cube just fine. Look closely :P
The main issue is your projection. The initial GL viewing volume is a -1 to 1 cube, but the cube you're drawing is -1 to 1. The call to gluOrtho2D defines the projection in OpenGL coordinates, which you make the same as pixels, but since your cube is -1 to 1 it is only two pixels big, the rest being offscreen.
Instead, drop the gluOrtho2D, which sets the Z dimension -1 to 1 and only allows you to set X/Y, and create a slightly bigger projection...
glOrtho(-2, 2, -2, 2, -2, 2);
Note: As #derhass suggests, calling glClear is important especially with depth testing enabled (without it, the cube from the last draw call will hide the updated cube).