Cannot get index buffers to draw a square in OpenGL - c++

Question

I'm definitely doing something extremely rookie here, but I simply cannot draw a square with index buffers. I'm following the intructions in this video. But I just don't get the expected output.
If I comment out one of the vertices in the array, it draws second triangle. Also, the drawElement() call seems to be working as intended as there is definitely linking between ELEMENT_ARRAY and ARRAY_BUFFER in my code, but I cannot get a square for the life of me. I checked and rechecked my code a lot. Does anyone know what I might be missing? Here's my code:
#include <GL\glew.h>
#include <GLFW/glfw3.h>
#include <iostream>
#define print(x) std::cout << x << std::endl
static unsigned int CompileShader(unsigned int type, const std::string& source) {
// returns an empty shader object of type specified
unsigned int shader = glCreateShader(type);
// because one of the arguments requires a double pointer
const char* src = source.c_str();
// Replaces(in this case writes) the source code of a shader object
glShaderSource(shader, 1, &src, nullptr);
// Compiles the shader
glCompileShader(shader);
// Finds compile status of shader
int result;
glGetShaderiv(shader, GL_COMPILE_STATUS, &result);
if (result == GL_FALSE) {
int length;
glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &length);
char* message = new char[length];
glGetShaderInfoLog(shader, length, &length, message);
print("Failed to compile shader");
std::cout << (type == GL_VERTEX_SHADER ? "vertex" : "fragment") << std::endl;
glDeleteShader(shader);
return 0;
}
return shader;
}
static unsigned int CreateShader(const std::string& vertexShader, const std::string& fragmentShader) {
// All shaders must be attached to a program object before executed. This returns an empyty program object
unsigned int program = glCreateProgram();
// get shaders
unsigned vs = CompileShader(GL_VERTEX_SHADER, vertexShader);
unsigned fs = CompileShader(GL_FRAGMENT_SHADER, fragmentShader);
// Attaches the shader to the program
glAttachShader(program, vs);
glAttachShader(program, fs);
// creates shader executables
glLinkProgram(program);
// validates success/failure/performance of program. stores in program's log
glValidateProgram(program);
return program;
}
int main(void)
{
GLFWwindow* window;
/* Initialize the library */
if (!glfwInit())
return -1;
/* Create a windowed mode window and its OpenGL context */
window = glfwCreateWindow(640, 480, "Hello World", NULL, NULL);
if (!window)
{
glfwTerminate();
return -1;
}
/* Make the window's context current */
glfwMakeContextCurrent(window);
if (glewInit() != GLEW_OK) {
print("Error");
}
float positions[8] = {
-0.5f, -0.5f, // 0
0.5f, -0.5f, // 1
-0.5f, 0.5f, // 2
0.5f, 0.5f // 3
};
unsigned int indices[6] = {
0, 1, 2,
1, 2, 3
};
// Assigns a memory to GL_ARRAY_BUFFER
unsigned int triangleBuffer;
glGenBuffers(1, &triangleBuffer);
glBindBuffer(GL_ARRAY_BUFFER, triangleBuffer);
// Fills in data to GL_ARRAY_BUFFER memory
glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), positions, GL_STATIC_DRAW);
// Defines where positions are located in GL_ARRAY_BUFFER and how to draw them configurations
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), 0);
// Enables this vertex to be drawn using glDrawArrays().
glEnableVertexAttribArray(0);
// Shader programs in GLSL
std::string vertexShader =
// opengl version 3.3
"#version 330 core\n"
"\n"
// select vertex attribute 0. vec4 because gl_Position requires vec4 argument. vertex shaders are 'in'
"layout(location = 0) in vec4 position;\n"
"\n"
"void main()\n"
"{\n"
" gl_Position = position;\n"
"}\n";
std::string fragmentShader =
// opengl version 3.3
"#version 330 core\n"
"\n"
// select vertex attribute 0. vec4 because color requires vec4 argument. fragment shader have 'out'
"layout(location = 0) out vec4 color;\n"
"\n"
"void main()\n"
"{\n"
// colors in graphics are usually in 0-1 range. RGBA
" color = vec4(0.2, 0.3, 0.8, 1.0);\n"
"}\n";
// generate index buffers
unsigned int ibo;
glGenBuffers(1, &ibo);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, 6 * sizeof(unsigned int), &indices, GL_STATIC_DRAW);
// Use Shaders
unsigned int shaderProgram = CreateShader(vertexShader, fragmentShader);
glUseProgram(shaderProgram);
/* Loop until the user closes the window */
while (!glfwWindowShouldClose(window))
{
/* Render here */
glClear(GL_COLOR_BUFFER_BIT);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, nullptr);
/* Swap front and back buffers */
glfwSwapBuffers(window);
/* Poll for and process events */
glfwPollEvents();
}
glfwTerminate();
return 0;
}

Answer 1

In general your code is fine, but the specification of the vertex array buffer size is wrong. The 4 verex coordinates
float positions[8] = { -0.5f, -0.5f, 0.5f, -0.5f, -0.5f, 0.5f, 0.5f, 0.5f };
consists of 8 floats, not of 6.
This means you have the sie of the vertex array is 8*sizeof(float). Change your code like this:
glBufferData(
GL_ARRAY_BUFFER,
8 * sizeof(float), // <----- 8 instead of 6
positions,
GL_STATIC_DRAW );
Note, you can use the sizeof operator to determine the size of the array in bytes:
glBufferData(GL_ARRAY_BUFFER, sizeof(positions), positions, GL_STATIC_DRAW);
In c++ I would prefer something like this:
const std::vector<float> positions{ -0.5f, -0.5f, 0.5f, -0.5f, -0.5f, 0.5f, 0.5f, 0.5f };
glBufferData(GL_ARRAY_BUFFER, positions.size()*sizeof(positions[0]), positions.data(), GL_STATIC_DRAW);