Afaik there is an OpenGL extension that allows to bind a vertex-stream that is not sampled per vertex, but per instance. Though atm i can’t find the extension. The functionality definitely exists in D3D11.
Though the way D3D implements it, it is mostly useless. There is a drawcall similar to glDrawElementsInstanced, only that you can not only define the number of instance to render, but also with which instance to start.
So you could have a vertex-stream that defines the world-matrices for 100 instances, and then you could just say “render instances 23 till 37”.
The problem with this approach is, that this is still very coarse. It does not work well with viewfrustum-culling.
It would be much more interesting, if i could instead specify something like an index-buffer, but for which instances to use.
So i could store all the instances on the GPU, with their modelview-matrices, etc., cull them on the CPU and just fill an index-buffer, which instances to draw. That way i could then render instances 23,26,29,33,37 in one drawcall without reverting to using dynamically filled uniforms.
I don’t know whether there are already built-in shader variables to handle such cases, but if such a drawcall is made, one would need two different variables: gl_InstanceID that just counts which instance is currently rendered, and maybe something like gl_InstanceIndex, that would then say “although you are the 3rd instance to be rendered, you are actually instance 29 in the vertex-array”.
IMO such functionality would be extremely useful, because one often renders many identical objects, only in different locations. Till now instancing is very cumbersome to use, because one usually has to dynamically manage data just to define WHICH instances to render. With such a simple method one would only need to dynamically create the index-buffer, and maybe change the instance vertex-streams, when the position of an instance changes.