A Guide to Instanced Geometry

As stated in the title, this is more of a guide than a tutorial. It isn't for the complete beginner, some experience is necessary. Throughout this I assume you have a general knowledge of what a mesh is, basic understanding of how matrices work, basic knowledge of GLSL, and experience with C++ or another object-oriented language. I am just writing about my own personal implementation of instanced geometry, which is definitely open for comments and suggestions. The code snippets are stripped down versions directly from my basic 2D OpenGL rendering engine I have dubbed IronClad.

What Is Instanced Geometry?

Primitive rendering techniques have many copies of a single object's data. Say you wanted to draw a tiled map, with 2 unique tiles. Say, for instance (no pun intended), a floor tile and a wall tile. Now, each of these objects contains, at the very least, 8 floats for vertex positions, and 8 floats for texture coordinates. That's 8 * 4 + 8 * 4 = 64 bytes. If each instance of a tile contains this information for rendering, and you have 1000 wall tiles, that's 64 kilobytes of memory! And that's not even considering the other tile types. Obviously, this is an example of a very simple mesh with only 4 vertices. Most games have models with hundreds if not thousands of vertices, so you can see why it'd be a serious problem to have multiple copies of that data.

Of course, there's a simple solution to this problem; you keep around one copy of the data in the first object you create, and the other objects simply refer to the original, just in their own position. Well, that's exactly where instancing comes in!