Why is OpenGL GLFW Rendering shapes not working? - c++

Question

I am trying to get started with openGL and GLFW in c++. My expectation is that the following code will simply render a square on the screen, but I just get a black screen.
The actual setup is working though, because if I add glClearColor(0.1f, 0.4f, 0.8f, 1.0f);, I do indeed get a blue screen. But what is wrong with my use of GL_QUADS?
#include <iostream>
// GLEW
#define GLEW_STATIC
#include <GL/glew.h>
//GLFW
#include <GLFW/glfw3.h>
const GLint WIDTH = 800, HEIGHT = 600;
int main() {
glfwInit();
glfwWindowHint( GLFW_CONTEXT_VERSION_MAJOR, 3);
glfwWindowHint( GLFW_CONTEXT_VERSION_MINOR, 3);
glfwWindowHint( GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
glfwWindowHint( GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
glfwWindowHint( GLFW_RESIZABLE, GL_TRUE);
GLFWwindow *window = glfwCreateWindow(WIDTH, HEIGHT, "Isosurface Stuffing", nullptr, nullptr);
int screenWidth, screenHeight;
glfwGetFramebufferSize(window, &screenWidth, &screenHeight);
if (window == nullptr) {
std::cout << "Failed to create GLFW window" << std::endl;
glfwTerminate();
return -1;
}
glfwMakeContextCurrent(window);
glewExperimental = GL_TRUE;
if (GLEW_OK != glewInit()) {
std::cout << "Failed to create GLEW" << std::endl;
return -1;
}
while (!glfwWindowShouldClose(window)) {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glViewport(0, 0, screenWidth, screenHeight);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0f, screenWidth, 0.0f, screenHeight, 0.0f, 1.0f);
glMatrixMode (GL_MODELVIEW);
glLoadIdentity();
glfwPollEvents();
glColor3f(0.5f, 0.5f, 0.5f);
glBegin(GL_QUADS);
glVertex2f(100, 100);
glVertex2f(200, 100);
glVertex2f(200, 200);
glVertex2f(100, 200);
glEnd();
glfwSwapBuffers(window);
}
glfwTerminate();
return 0;
}

Answer 1

You cannot use the legacy fixed-function pipeline instructions (glBegin, glEnd, glVertex2f, glMatrixMode, glLoadIdentity, glOrtho) in a Core profile OpenGL context (GLFW_OPENGL_CORE_PROFILE). You have to use Compatibility profile OpenGL Context:
glfwWindowHint( GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_COMPAT_PROFILE);
If your system does not allow you to create a compatibility profile context (e.g. Mac), you must use either an OpenGL 2.0 context or a modern OpenGL with Shaders and Vertex Array Objects. A nice tutorial for that is LearnOpenGL