The marriage of computers and animation began modestly in the late 1970s, when producers realized that Personal Computer–aided drawing provided a fast, inexpensive shortcut to hand painting individual animation cels. Later, innovations in 3-D rendering that originated with military-based flight simulators became incorporated into cinematic special-effects work, helping filmmakers animate realistic, textured environments for science fiction and action-adventure films. As computer microprocessors and data management grew faster and more powerful, 3-D animation became more accurate, more fluid, and more economical. Techniques such as keyframing (specifying critical positions for graphic objects and letting the computer fill in the frames between those positions), motion capture (using sensors to record the actions of live subjects, then interpolating the data into 3-D models), and simulation (extrapolating motion based on the laws of physics) made it possible to create naturalistic movement and facial expressions, paving the way for 3-D character simulation.