How can I efficiently render lots of 2D quads?

Question

I have lots of 2D quads. I write their local vertex position info to a buffer (accompanied with a world transform matrix) which gets sent to the render thread.

I then pass the world transform matrix as a uniform into the vertex shader which applies itself to each vertex and works fine. I however have to make a call to glDrawElements() for each quad I want to render because each one has a different world transform.

What is the most efficient way for me to render all of these quads? Should I do what I'm already doing, or should I calculate the world position for each vertex on the CPU before I write them to the buffer, so I can attempt to batch more quads into a single glDrawElements() call? Or is there another approach?

Answer 1

You definitely want to use instancing.

Your quads are instances: constant data (e.g. 4 vertices, bound texture) but with a few different parameters (e.g. world space matrix). Your world space matrices will be the instance data: one for each instance. It's a quite elegant and standard solution.

You can have all sorts of data as instance data, in the above case this means applicable to all four vertices of the quad. Examples: size of the quad, texture index, colour tint, transparency values, etc.

Per-vertex data could be your unit quad { (0,0), (1,0), (0,1), (1,1) } with indices: { (0,1,2),(2,1,3)} for CCW ordering.

Answer 2

I second instancing.

For the sake of extra information: Converting quads to world-space on the CPU.

If you can support it, you could also offload that work to the GPU through a geometry-shader. A couple arrays, 1 for the vertices of each quad, 2 for the transform marix of each quad, and then compute the world coordinates based on that (or some similar approach.) Since the geometry shader comes before the vertex shader step, it would fit right in.

You could also use a compute-shader to offload cpu work, if it's to much.

You could also avoid local coordinates. Local coordinates are nice for many reasons. If you don't need those benefits, or use them rarely, you could just use world coordinates and there would be no conversion to worry about. Aside from the rare conversion to local. That's going to be very dependent on what you're doing though.