OpenGL - Drawing multiple meshes at once using VBOs and IBOs

Question

I have been learning OpenGL 2.1 but using shaders, VBOs, IBOs, etc. I have gotten a rendering engine that can load and draw meshes, materials, forward lighting (no shadows yet), SceneNodes, and NodeComponents. There are no optimizations yet (for obvious reasons) such as occlusion culling, only Face culling at the moment.

A mesh (simplified) class looks like this:

struct Mesh
{
    GLuint vbo;
    GLuint ibo;
    GLuint size;

    void draw(Program* shaderProgram) const
    {
        glEnableVertexAttribArray(shaderProgram->attrib("vertPos"));
        glEnableVertexAttribArray(shaderProgram->attrib("vertTexCoord"));
        ...

        glBindBuffer(GL_ARRAY_BUFFER, vbo);

        glVertexAttribPointer(shaderProgram->attrib("vertPos"), 3, GL_FLOAT, false, 
                              sizeof(Vertex), (const GLvoid*)(0 * sizeof(float)));
        ...

        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
        glDrawElements(GL_TRIANGLES, size, GL_UNSIGNED_INT, 0);

        glBindBuffer(GL_ARRAY_BUFFER, 0);
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

        glDisableVertexAttribArray(shaderProgram->attrib("vertPos"));
        ...
    }
}

When I have been drawing multiple objects with the same mesh (or multiple meshes), I notice a frame drop compared to a single mesh with the same number of triangles.

Stanford Models were used.
(1 Face = Triangle)
| Model  | Num Models  | Total Faces | With Lighting | Without Lighting |
| -      | -           | -           | ms / frame    | ms / frame       |
|--------|-------------|-------------|---------------|------------------|
|        | 0 (control) |        0    |  0.47         |  0.41            |
| bunny  | 1           |    69630    |  1.2          |  0.64            |
| bunny  | 14          |   974820    | 12.4          |  3.50            |
| buddha | 1           |  1087474    |  7.87         |  2.89            |
| buddha | 10          | 10874740    | 71.4          | 23.8             |

I does look like that time/frame is ~O(n). However, drawing one vbo with 12% more faces, the frame time is decreased by 37%.

So my questions are:

• Is drawing multiple VBOs actually worse than drawing one big one?
• How would I combine these VBOs and IBOs into one big one if this is true?

Answer 1

Yes, doing several draw calls is slower than one. This is why state changes are expensive. There are techniques (like instancing) that fight this, but they're still more expensive than simply doing a single draw call.

To combine several VBOs into one, they must have the same state (i.e, same shader, same textures, etc), and they must be static (i.e, not moving independently). Then simply transform the vertex data into the same space (for instance world space, or possibly the object space of a reference object) and handle the result as a single object.

The first question you have to ask, however, is whether all of this is worth it. First make it work, then make it fast.