What is the best way to store Vertex Buffer Object data?

Question

Until now, I have been using a vertexData structure to store data for a Vertex Buffer Object (VBO); vertexData holds a static array of 6 vertices (2 triangles). I then save them to a vector of the vertexData type, before finally using this vertexData in the buffering method:

struct vertexData {

    Vertex vertices[6]; // position, color, UVs
};

std::vector<vertexData> _DATA;

void SpriteBatch::createVBO() {

    glBindBuffer(GL_ARRAY_BUFFER,_VBO);

    glBufferData(GL_ARRAY_BUFFER,_DATA.size()*sizeof(vertexData),nullptr,GL_DYNAMIC_DRAW);
    glBufferSubData(GL_ARRAY_BUFFER,0,_DATA.size()*sizeof(vertexData),_DATA.data());

    glBindBuffer(GL_ARRAY_BUFFER,0);
}

---

This works good, as everything in my vector is contiguous. However, I want to work with other vertex sizes, for different shapes. I tried a vector of vector types, in oder to work with dynamically allocated arrays, but it did not work, as it was not contiguous.

At first, I thought about using a polymorphic class to be stored as a vector, where every child would have a different array size; but then I recognized that this vertex buffer does not work with pointers

The second idea was to create a generic SpriteBatch. For example, a batch for 6 vertices, and then a batch for 2 vertices, etc. etc.

What is the best way to store Vertex Buffer Object data?

Answer 1

My knee jerk (operative work jerk?) reaction:

Leave your contiguous vector as it is but make it just 3 or 1 vertices (depending on the rest of your code), store all your different shapes into the same contiguous space as you do now. Then use an index (map) to record the offset of each polygon/shape.

Then when you use the VBO, just remember to offset your pointers by the index depending on what shape you want.

Example (warning, haven't touched C++ in years)...

std::map<int,int> _shapeOffsets;
_shapeOffsets[_myPolygonsCustomIdentifier] = _theOffsetOfMyPolygonInTheVertexData;
MyPolygonDrawingFunc(_DATA.data() + _shapeOffsets[_myPolygonsCustomIdentifier]);

Pro: Not much of a change from what you're doing now Con: Could have complications if you wanted to 'delete' a shape and remove vertices. If this is an issue, we can brainstorm an alternative solution.

Hope that was clear enough, hope I didn't miss the point somewhere :)