Simple VBO doesn't display - c++

Question

I'm using a vertex with three floats for the position (XYZ) and three other for the color (RGB):
XYZ RGB
XYZ RGB
...
I'm currently trying to plot a red triangle. Unfortunately I end up with a white window. I think there's a problem with the stride but I can't figure it out. I've tried many values for the stride and the size, still it doesn't seem to display anything.
//main.cpp
#include "data.h"
GLuint ID;
int size,el_size;
void init(){
vector<float>data_vector(18);
data_vector[0]=0; //x
data_vector[1]=0; //y
data_vector[2]=0; //z
data_vector[3]=1;
data_vector[4]=0;
data_vector[5]=0;
data_vector[6]=1; //x
data_vector[7]=0; //y
data_vector[8]=0; //z
data_vector[9]=1;
data_vector[10]=0;
data_vector[11]=0;
data_vector[12]=0; //x
data_vector[13]=1; //y
data_vector[14]=0; //z
data_vector[15]=1;
data_vector[16]=0;
data_vector[17]=0;
size=data_vector.size();
// Init GLEW
if ( glewInit() != GLEW_OK ){
cerr << "Failed to initialize GLEW." << endl;
exit(-1);
}
if ( !glewIsSupported("GL_VERSION_1_5") && !glewIsSupported( "GL_ARB_vertex_buffer_object" ) ){
cerr << "ARB_vertex_buffer_object not supported!" << endl;
exit(-2);
}
glOrtho(-1, 1,1,-1, -5.0f, 5.0f);
glClearColor(1.0f, 1.0f, 1.0f, 0.0f);
glShadeModel(GL_SMOOTH);
glEnableClientState(GL_VERTEX_ARRAY);
glGenBuffers(1,&ID);
glBindBuffer(GL_ARRAY_BUFFER, ID);
glBufferData(GL_ARRAY_BUFFER,size*sizeof(float), &data_vector[0], GL_STATIC_DRAW);
el_size=3*sizeof(data_vector[0]);
}
void reshape(int w, int h){
cout<<"reshape"<<endl;
glViewport(0,0, (GLsizei) w, (GLsizei) h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0.0f, (GLdouble) w, 0.0f, (GLdouble) h);
}
void display(){
cout<<"display"<<endl;
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glBindBuffer(GL_ARRAY_BUFFER, ID);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, el_size, 0);
glEnableClientState(GL_COLOR_ARRAY);
glColorPointer(3,GL_FLOAT, el_size,(void*)(el_size));
glDrawArrays(GL_TRIANGLES,0,size/6);
glFlush();
}
int main(int argc, char **argv){
cout<<"main"<<endl;
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(500,500);
glutInitWindowPosition(300,300);
glutCreateWindow(argv[0]);
init();
glutDisplayFunc(display);
// glutReshapeFunc(reshape);
glutMainLoop();
return 0;
}

Answer 1

You do appear to be using the incorrect stride. The stride should be the distance from the start of one vertex to the start of the next vertex, or in your case 6 floats.
You've set the stride as el_size, which is only 3 floats.
Also take care that your resize function is using an ortho matrix from 0 to screen width, and your init function is setting it from -1 to 1. If resize ever gets called your scene will become radically different.

Answer 2

One problem I see is that call to glOrtho in the init function. Whatever you intend to do, this surely doesn't do what you want. As a general rule, put all drawing state related commands only into the display function, nowhere else. Setting transformation matrices (and the viewport) should happen there. Doing so saves a lot of headaches later.
The other problem is, that the stride you define is to short. The stride is the distance from vertex to vertex in an interlaced array, not the length of a single attribute. el_size is calculated wrong.