KIRUPA.CHINNATHAMBI

In order to view stutter free movies or animations, you are often told to stick with at least 25 fps (frames per second). Think of a frame as a static picture, and each subsequent frame contains the same picture modified slightly. Playback all of the frames really quickly, and you get the illusion of something moving – a moving picture!

The speed at which the frames are played back determine whether the movement conveyed is smooth or jittery. Whether you should use 20 fps, 25 fps, 31 fps, etc. has been debated extensively, so in this post I will instead provide an explanation for why you perceive relatively smooth motion when the frame rate is over 20+ frames per second.

A simplified approach of how we perceive and store visual information was articulated by Stuart Card, Thomas Moran, and Allen Newell. They compare how our eyes/mind are similar to a computer with their Model Human Processor (http://www.isrc.umbc.edu/HCIHandbook/Figures/05-01.html). I have drawn an abbreviated version of their model:

I will be focusing only on the Perceptual processor, for it is most relevant for understanding frame rates. The graphic presented above also omits key features. The original model presented in the Model Human Processor also deals with memory and the feedback loop that tells your muscles what to do. While they are important for understanding the bigger picture (such as reaction times in video games!), they are not directly relevant to learning why the frame rates are what they are. I might cover them at a later time.

Much like real processors, our Perceptual Processor has a propagation time associated with it. The propagation time is the time it takes for a computer processor to process an output given an input, and that time for the Perceptual Processor is 100ms. It takes your Perceptual Processor about 100 ms to notice the data being streamed in from your eye. To put in terms of frame rates, simply take the inverse of the propagation tme. With our 100 ms value, that means you get 10 frames per second. That sure seems low!

Experimentally, 10 fps is a good lower-bound at which we tend to interpret moving objects as part of a continuous sequence of images. Notice that I said lower-bound. The upper-bound is about 20 frames per second. This discrepany arrives because you have a wide range of values for the propagation times of your Perceptual processor. The wide range of values is most associated with your age. The younger you are, the smaller your propagation time. Also, how tired you are, whether you are mentally distracted, etc. also factor into the propagation time. Overall, the margins for the Perceptual Processor’s propagation times vary between 50 ms and 200 ms. To account for the margin, studies divide people into three classes – fastman, middleman, and slowman.

Fastman has the smallest propagation time and is essentially capable of being very alert. Therefore, fastman’s propagation time for the Perceptual Processor is only about 50 ms. Slowman is around 100 ms. That means Fastman would be content with at least 20 fps, and Slowman would be content with about 10 frames per second. Middleman would fall somewhere in-between Fast- and Slow- man. Remember, contentness does not necessarily equal the best frame rate solution. Since having a faster frame rate does not impede fast, middle, or slowman’s enjoyment of the visual animation, most content played back is higher than 20 fps.

For further information on this, Prof. Rob Miller’s Lecture Notes on Human Capabilities should make for interesting dinner-time conversation.

Cheers!
Kirupa