OpenGL Program Does Not Render - c++

Question

I have a Mac running OS X Yosemite with GLFW 3 and OpenGL 4.1. I've got this program:
#include <glm/gtc/matrix_transform.hpp>
#include <glm/gtx/string_cast.hpp>
#include <stdio.h>
#include "myglutils.hpp"
int main() {
if (!setupGLFW()) return 1;
setupApple();
GLFWwindow* window = glfwCreateWindow(640, 480, "Perspective", 0, NULL);
if (!window) {
fprintf(stderr, "Failed to create window.\n");
glfwTerminate();
return 0;
}
glfwMakeContextCurrent(window);
if (!setupGLEW()) return 1;
glClearColor(0.2, 0.0, 0.8, 1.0);
const GLfloat vertices[] = {
-1.0, -1.0,
1.0, -1.0,
0.0, 1.0
};
const GLchar* vert_shader =
"#version 410 core\n"
"in vec2 pos;"
"uniform mat4 MVP;"
"void main() {"
" gl_Position = MVP * vec4(pos, 0.0, 1.0);"
"}";
const GLchar* frag_shader =
"#version 410 core\n"
"out vec4 color;"
"void main() {"
" color = vec4(1.0, 0.0, 0.0, 1.0);"
"}";
GLuint shader = getShader(vert_shader, frag_shader);
GLuint vbo;
glGenBuffers(1, &vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(GLfloat), vertices, GL_STATIC_DRAW);
GLuint vao;
glGenVertexArrays(1, &vao);
glBindVertexArray(vao);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, NULL);
GLuint MVPID = glGetUniformLocation(shader, "MVP");
glm::mat4 perspective = glm::perspective(45.0, 4.0 / 3.0, 0.1, 100.0);
glm::mat4 view = glm::lookAt(glm::vec3(4.0, 3.0, 3.0), glm::vec3(0.0, 0.0, 0.0), glm::vec3(0.0, 1.0, 0.0));
glm::mat4 MVP = perspective * view;
while (!glfwWindowShouldClose(window) && glfwGetKey(window, GLFW_KEY_ESCAPE) != GLFW_PRESS) {
glClear(GL_COLOR_BUFFER_BIT);
glUseProgram(shader);
glUniformMatrix4fv(MVPID, 1, GL_FALSE, &MVP[0][0]);
glBindVertexArray(vao);
glDrawArrays(GL_TRIANGLES, 0, 6);
glfwSwapBuffers(window);
glfwPollEvents();
}
glDeleteVertexArrays(1, &vao);
glDeleteProgram(shader);
glfwTerminate();
return 0;
}
The functions setupGLFW(), setupApple(), and setupGLEW() aren't the problem. Neither is getShader(). I feel like my error is something silly like forgetting a simple OpenGL function call. What I'm trying to do is draw a triangle at a 3D angle.
Note: this program compiles perfectly, but only displays an empty blue screen.

Answer 1

There are a number of issues in this code. The main one is that you never enable the vertex attribute array. You will need to add a glEnableVertexAttribArray() call in the setup code:
glBindVertexArray(vao);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray(0);
Also, the arguments to glDrawArrays() are not correct:
glDrawArrays(GL_TRIANGLES, 0, 6);
The 3rd argument is the number of vertices. Since you only have 3 vertices, it should be:
glDrawArrays(GL_TRIANGLES, 0, 3);
Also, unless this is taken care of in code you did not post, you need to make sure that the attribute location you are using matches the program. You use 0 in the code, but you can't count on the location being zero without any further action. The easiest way to fix this is to use a location layout qualifier in the shader code:
"layout(location=0) in vec2 pos;"
This will set the location to 0, matching your code.