Having issues drawing 3D cube's edges with OpenGL 3 - c++

Question

I'm trying to draw 3D cube's vertices (edges only) using OpenGL (4.3 core profile), I know the glPolygonMode function but I'd like not to draw the intermediate diagonal lines. I declare my vertices and my indices like so :
struct Vertex {
glm::vec3 pos;
glm::vec3 color;
glm::vec3 normal;
glm::vec2 uv;
};
Vertex vertices[8];
// Front vertices
vertices[0].pos = glm::vec3(-0.5f, -0.5f, +0.5f);
vertices[1].pos = glm::vec3(+0.5f, -0.5f, +0.5f);
vertices[2].pos = glm::vec3(+0.5f, +0.5f, +0.5f);
vertices[3].pos = glm::vec3(-0.5f, +0.5f, +0.5f);
// Back vertices
vertices[4].pos = glm::vec3(-0.5f, -0.5f, -0.5f);
vertices[5].pos = glm::vec3(+0.5f, -0.5f, -0.5f);
vertices[6].pos = glm::vec3(+0.5f, +0.5f, -0.5f);
vertices[7].pos = glm::vec3(-0.5f, +0.5f, -0.5f);
GLuint indices[36] = {
0, 1, 2, 2, 3, 0, // Front
1, 5, 6, 6, 2, 1, // Right
7, 6, 5, 5, 4, 7, // Back
4, 0, 3, 3, 7, 4, // Left
4, 5, 1, 1, 0, 4, // Bottom
3, 2, 6, 6, 7, 3 // Top
};
My buffer is updated accordingly :
// Bind Vertex Array
glBindVertexArray(_VAO);
// Bind VBO to GL_ARRAY_BUFFER type so that all calls to GL_ARRAY_BUFFER use VBO
glBindBuffer(GL_ARRAY_BUFFER, _VBO);
// Upload vertices to VBO
glBufferData(GL_ARRAY_BUFFER, verticesNb * sizeof(Vertex), vertices, GL_STATIC_DRAW);
// Bind EBO to GL_ARRAY_BUFFER type so that all calls to GL_ARRAY_BUFFER use EBO
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _EBO);
// Updload indices to EBO
glBufferData(GL_ELEMENT_ARRAY_BUFFER, indicesNb * sizeof(GLuint), indices, GL_STATIC_DRAW);
I'm using glDrawElements(GL_LINES, 36, GL_UNSIGNED_INT, 0); to draw my cube's edges, but for some reason it doesn't draw some edges and I don't understand why.
If I use GL_TRIANGLES, it works pretty well though when I want to render my 3D cube in fill-mode. Does anyone know what I'm missing here ? Is it an issue with the indices ?
(The "cube" below has a custom size of (1.0f, 2.0f, 3.0f))

Answer 1

Your indices form GL_TRIANGLES primitives rather than GL_LINES primitives. See GL_LINES:
Vertices 0 and 1 are considered a line. Vertices 2 and 3 are considered a line. And so on.
The indices form the primitives. Change the indices:
GLuint indices[] = {
0, 1, 1, 2, 2, 3, 3, 0, // Front
4, 5, 5, 6, 6, 7, 7, 4, // Back
0, 4, 1, 5, 2, 6, 3, 7
};
glDrawElements(GL_LINES, 24, GL_UNSIGNED_INT, 0);