Well, each additional object state does not have a big cost by itself, but it adds up and we already have lots of stuff in C4Object. Whether it's in C4Object or another class is not really important. Another thing is that we probably want to store the vertex data on the GPU, because that's the most efficient way to use contemporary GPUs, and preferably neither once for every object nor recalculated every frame. Interpolating between two vertex sets can be done with a vertex shader. I think some games also do the bone animation stuff with vertex shaders, but that is more complex. I think without that, the two or more animations cannot share triangles, so that the drawing function can use one set of precalculated vertex data for one animation and one set for the other.