How do we perceive lag? Part 1

by Ugo

Humans can perceive from 10 to 12 frames per second individually. Early silent film projections had frame rates varying from 16 to 24 FPS, and nowadays the standard rate for movies is at least 24 FPS. But how is this perceived when we interact with whatever is being showed to us? This series of posts will report a part of my research on investigating the lag perception while drawing.

What is lag?

The phenomenon behind human frame rate perception is called Flicker fusion, i.e. the point in which a flickering image appears as stationary. Early silent film projections had frame rates varying from 16 to 24 FPS, and nowadays the standard rate for movies is at least 24 FPS.

Flicker fusion at 15 frames per second

Within the research domain of human vision, there are works addressing a possible frame rate limit that human beings are able to perceive: it is estimated that humans can perceive from 10 to 12 frames per second individually, above this limit the illusion of motion would take place, and humans would not be able to perceive individual frames.

However, the considered limit applies when no interaction is required. So I’ve started to investigate if this limit can also be applied for the specific case of interacting with devices. More specifically to drawing on electronic devices. The paper is called “Assessing lag perception in electronic sketching” and its available here and here.

15 vs. 30 vs. 60 FPS

Can you tell the difference between the three rates below? Bo Allen have done a simple yet very interesting comparison between the frame rates:

[kml_flashembed publishmethod=”static” fversion=”8.0.0″ movie=”http://ugosan.org/wp-content/uploads/2013/07/fps-compare-15fps.swf” width=”150″ height=”200″ targetclass=”flashmovie”]
Get Adobe Flash player
[/kml_flashembed]
[kml_flashembed publishmethod=”static” fversion=”8.0.0″ movie=”http://ugosan.org/wp-content/uploads/2013/07/fps-compare-30fps.swf” width=”150″ height=”200″ targetclass=”flashmovie”]
Get Adobe Flash player
[/kml_flashembed]
[kml_flashembed publishmethod=”static” fversion=”8.0.0″ movie=”http://ugosan.org/wp-content/uploads/2013/07/fps-compare-60fps.swf” width=”150″ height=”200″ targetclass=”flashmovie”]
Get Adobe Flash player
[/kml_flashembed]

(source: http://boallen.com/fps-compare.html)

* update: There is another great (interactive) tool for comparing refresh rates: http://frames-per-second.appspot.com/

Interaction and Lag

One of my research interests is investigating sketching (human drawing) on electronic devices. In fact, electronic sketching was one of the first input modalities to be considered in computing, take a look at these Youtube videos where Ivan Sutherland is demonstrating his electronic sketching system.. in 1963! I’m also maintaining a playlist about the history of electronic sketching, with some interesting videos, check it out.

Before touchscreen devices became mainstream the only viable option people had to sketch was by using WACOM devices. Now we have different devices, with different performances and screens of many sizes. And we also have HTML5 which allows us to build a single system that runs on all those devices through a browser.

So I’ve started implementing a sketching system and started trying different upper limits for the rendering, as you can see on the video below. Note how the sketch lags behind the pen:

How would people perceive the different rates while sketching without (of course) knowing it in advance? I’ve setup an experiment and tested with 35 subjects, that’s on Part 2 of this series.

The experiment

That will be on Part 2, soon. =)