OpenGL provides a built-in variable to the vertex shader named gl_InstanceID. This is simply a counter and takes on a different value for each instance that is rendered. The first instance will have an ID of zero, the second will have an ID of one, and so on. This can be useful as a way to index to texture data appropriate for each instance. Another possibility is to use the instance's ID as a way to generate some random data for that instance. For example, we could use the instance ID (or some hash) as a seed to a pseudo-random number generation routine to get a unique random stream for each instance.