Even now, the industry measures rendered frames per second using software-based tools like Fraps and in-game counters that capture frames from the graphics card's memory every second. On the surface, that sounds like a great way to generate accurate data.
The problem is that there can be a significant disconnect between the game engine's output and what you see on your monitor. And, when you move beyond the issues that might affect a single-GPU setup and start considering the complexity of multiple graphics processors working cooperatively, two additional variables surface, affecting your experience: dropped frames and what Nvidia is introducing to us as runt frames.
Briefly, even after a frame is loaded into memory, there's still a lot of work that goes on in the graphics pipeline; this takes time. As a result, some frames are dropped before they ever show up on-screen. Other frames show up, but only on a very small part of the screen. Nvidia's definition of a runt is any frame that shows up on fewer than 21 scan lines on a monitor.
