Blank Screen after compiling openGL

Question

I am really new to OpenGL and I am trying to just make a surface from two triangles. I don't know where I am going wrong with this code. I know that all the positions and colors are getting into the triangles class and that the Triangles are being made, but it's not getting outputted. Can someone help?

I tried to get just the output from the Triangle class but it doesn't seem to be working. I don't think there's anything wrong with the way I am calling the Display function.

Code:

#include<GL/gl.h>
#include<GL/glu.h>
#include<GL/glut.h>
#include<iostream>
#include<vector>
using namespace std;

class Triangle
{
public:
  float position[9],color[3];
  Triangle()
  {}
  Triangle(float position_t[], float color_t[])
  {
    for(int i=0;i<9;i++)
      {position[i] = position_t[i];}
    for(int i=0;i<3;i++)
          {color[i]= color_t[i];}
  }
  void makeTriangle()
  {
    glBegin(GL_TRIANGLES);
        glColor3f(color[0],color[1],color[2]);glVertex3f(position[0],position[1],position[2]);
            glColor3f(color[0],color[1],color[2]);glVertex3f(position[3],position[4],position[5]);
        glColor3f(color[0],color[1],color[2]);glVertex3f(position[6],position[7],position[8]);
    glEnd();}
};

class Mesh
{
public:
  /*float center[3],position[9],color[3];
    float size;*/
  vector<Triangle> elements;
  float center[3],position[9],color[3];
  float size;

  Mesh(){}
  Mesh(float center_in[3], float color_in[3])
  {
    for (int i=0;i<3;i++)
      {
        color[i] = color_in[i];
        center[i] = center_in[i];
      }
}

  void getPositions()
  {
    position[0] = 1;position[1] = 1; position[2] = 1;
    position[3] = -1;position[4] = -1; position[5] = 1;
    position[6] = 1;position[7] = -1; position[8] = 1;
  }

  void getColor()
  {
    color[0] = 1; color[1]=0; color[2]=0;
  }

  static Mesh makeMesh()
  {
    Mesh a;
    a.elements.resize(2);
    a.getPositions();
    a.getColor();
    Triangle T(a.position,a.color);
    a.elements[0] = T;
    //Triangle O(2);
    //a.elements[1] = 0;
    return a;
  }

};

void render()
{
  glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
  Mesh a;
  a.elements.resize(2);
  a.getPositions();
  a.getColor();
  Triangle T(a.position,a.color);  
  //vector<Mesh> m;
  //m.push_back(Mesh::makeMesh());
  glPushMatrix();
  T.makeTriangle();
  glPopMatrix();
  glFlush();
  glutSwapBuffers();
  glutPostRedisplay();
}

Full Code: http://pastebin.com/xa3B7166

Answer 1

Your code does not specify any transformations. Therefore, your coordinates need to be within the default view volume, which is [-1, 1] in all coordinate directions.

Or more technically, the model/view/projection transformations (or all the transformations applied in your vertex shader if you use the programmable pipeline) transform the coordinates into the clip coordinate space, and after perspective division into the normalized device coordinate (aka NDC) space. The range of the NDC space is [-1, 1] for all coordinates.

If you don't apply any transformations, like is the case in your code, your original coordinates already have to be in NDC space.

With your current coordinates:

position[0] = 1;position[1] = 1; position[2] = 1;
position[3] = -1;position[4] = -1; position[5] = 1;
position[6] = 1;position[7] = -1; position[8] = 1;

all the z-coordinates have values of 1, which means that the whole triangle is right on the boundary of the clip volume. To make it visible, you can simply set the z-coordinates to 0:

position[0] = 1;position[1] = 1; position[2] = 0;
position[3] = -1;position[4] = -1; position[5] = 0;
position[6] = 1;position[7] = -1; position[8] = 0;

This centers it within the NDC space in z-direction, with the vertices being on 3 of the corners in the xy-plane. You will therefore see half of your window covered by the triangle, cutting it in half along the diagonal.

It's of course common in OpenGL to have the original coordinates in a different coordinate space, and then apply transformations to place them within the view volume.

You're probably already aware of this, but I thought I'd mention it anyway: If you're just starting to learn OpenGL, I would suggest that you learn what people often call "modern OpenGL". This includes the OpenGL Core Profile, or OpenGL ES 2.0 or later. The calls you are using now are mostly deprecated in newer versions of OpenGL, and not available anymore in the Core Profile and ES. The initial hurdle is somewhat higher for "modern OpenGL", particularly since you have to write your own shaders, but you will get on the path to acquiring knowledge that is still current.

Answer 2

As I suggested you in the comments, you are not setting the gluLookat() function. Everything is being drawn but you are just not looking at it!

Docs: https://www.opengl.org/sdk/docs/man2/xhtml/gluLookAt.xml