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

Besides instancing and adding another vertex attribute as some object ID, I'd like to also mention another strategy (which requires modern OpenGL, though):

The extension ARB_multi_draw_indirect (in core since GL 4.3) adds indirect drawing commands. These commands do source their parameters (number of vertices, starting index and so on) directly from another buffer object. With these functions, many different objects can be drawn with a single draw call.

However, as you still want some per-object state like transformation matrices, that feature is not enough. But in combination with ARB_shader_draw_parameters (not in core GL yet), you get the gl_DrawID parameter, which will be incremented by one for each single object in one mult draw indirect call. That way, you can index into some UBO, or TBO, or SSBO (or whatever) where you store per-object data.

Answer 2

The answer depends on whether the vertex data for each item is identical or not. If it is, you can use instancing as in @orost's answer, using glDrawElementsInstanced, and gl_InstanceID within the vertex shader, and that method should be preferred.

However, if each 3D model requires different vertex data (which is frequently the case), you can still render them using a single draw call. To do this, you would add another stream into your vertex data with glVertexAttribPointer (and glEnableVertexAttribArray). This extra stream would contain the index of the matrix within the uniform buffer that vertex should use when rendering - so each mesh within the VBO would have an identical index in the extra stream. The uniform buffer contains the same data as in the instancing setup.

Note this method may require some extra CPU processing, if you need to redo the batching - for example, an object within a batch should not be rendered anymore. If this process is required frequently, it should be determined whether batching items is actually beneficial or not.

Answer 3

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