Render multiple models in OpenGL with a single draw call

Question

I built a 2D graphical engine, and I created a batching system for it, so, if I have 1000 sprites with the same texture, I can draw them with one single call to openGl.

This is achieved by putting in a single vbo vertex array all the vertices of all the sprites with the same texture.

Instead of "print these vertices, print these vertices, print these vertices", I do "put all the vertices toghether, print", just to be very clear. Easy enough, but now I'm trying to achieve the same thing in 3D, and I'm having a big problem.

The problem is that I'm using a Model View Projection matrix to place and render my models, which is the common approach to render a model in 3D space.

For each model on screen, I need to pass the MVP matrix to the shader, so that I can use it to transform each vertex to the correct position.

If I would do the transformation outside the shader, it would be executed by the cpu, which I not a good idea, for obvious reasons.

But the problem lies there. I need to pass the matrix to the shader, but for each model the matrix is different.

So I cannot do the same I did with 2d sprites, because changing a shader uniform requires a draw every time.

I hope I've been clear, maybe you have a good idea I didn't have or you already had the same problem. I know for a fact that there is a solution somewhere, because in engine like Unity, you can use the same shader for multiple models, and get away with one draw call

Answer 1

There exists a feature exactly like what you're looking for, and it's called instancing. With instancing, you store n matrices (or whatever else you need) in a Uniform Buffer and call glDrawElementsInstanced to draw n copies. In the shader, you get an extra input gl_InstanceID, with which you index into the Uniform Buffer to fetch the matrix you need for that particular instance.

You can read more about instancing here: https://www.opengl.org/wiki/Vertex_Rendering#Instancing