Dynamically Packing Data for Vertex Buffer Objects of OpenGL

Question

I am trying to write a wrapper for VBOs in OOP which consists of addVertex, addNormal.. addX, flush() and render() functions. At first, I am holding vertices, normals, indices in separate vectors like :

std::vector<glm::vec4> vertexBuffer;
std::vector<glm::vec4> colorBuffer;
std::vector<glm::vec3> normalBuffer;
std::vector<glm::vec2> texCoordBuffer;
std::vector<unsigned int> indexBuffer;

But as I read somewhere, holding different VBO IDs for each one is totally inefficient so it is better to pack them in order like VertexNormalTexCoordColor - VNTC - VNTC..., and represent them in a single VBO ID. Therefore the buffer can be used efficiently when uploaded to GPU. So this time I am still holding them in vectors but when I call flush() I want to pack them with structs, then upload that struct vector to GPU :

struct VBOData {
    glm::vec4 vert;
    glm::vec3 normal;
    glm::vec4 color;
    glm::vec2 texcoord;
};
std::vector<VBOData> vboBuffer;

and then i upload everything in flush() :

vboBuffer.reserve(vertexBuffer.size());

for (int i = 0; i < vertexBuffer.size(); ++i) {
    VBOData data;
    data.vert = vertexBuffer[i];
    data.color = colorBuffer[i];
    data.normal = normalBuffer[i];
    data.texcoord = texCoordBuffer[i];
    vboBuffer.push_back(data);
}

glGenBuffers(1, &vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, vboBuffer.size() * sizeof(VBOData), (GLchar*) &vboBuffer[0], GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, 0);

Now there is this problem, what if my object doesn't contain colors, normals, or tex coords and only consists of vertices ? Or only about vertices and normals ? Or different combinations of data that will always have vertices ? Can I dynamically pack them in V(N)(T)(C) order for efficiency ?

Answer 1

Now there is this problem, what if my object doesn't contain colors, normals, or tex coords and only consists of vertices ? Or only about vertices and normals ?

Then you are going to set the offset to components you got with glVertexAttribPointer, and not set of components you are not using.

BTW you have a nice tutorial on opengl.org about VBOs.