OpenGL VBO batch best practices

Question

I am creating a 2d game engine as a side project. I've been doing some experimenting and research and have come across a lot of people suggesting that you batch (store multiple objects to draw) in the same VBO. For example, if I my scene had a lot of trees I could put all the trees in the same VBO because they have the same memory footprint and then use a single glDrawArrays to draw all of them.

This is fine and it makes total sense... but then I started to wonder how I can send the different transforms for each tree? How do I get that to the shader? Or is this approach assuming I do the calculations on CPU and send the entire VBO each draw?

Here are two of the main questions i've been looking at:

OpenGL VAO best practices

OpenGL How Many VAOs

Answer 1

The term you're looking for is Instancing .

You create a VAO that contains the model of the tree, and then instead of passing the model matrix through the uniforms, you put it as an instanced vertex attribute in the VAO. Then each instance of your tree will get drawn with a different model matrix.

You can utilize compute shaders or transform feedback to update and store the model-view product in the VAO once per instance per frame (rather than calculating it per each vertex of each instance).