How many flicks per frame?

Wednesday, 24 January 2018

Facebook’s Oculus division have defined a new unit of time says BBC News:

The flick has been designed to help developers keep video effects in sync, according to a description on the code-sharing site GitHub.

A flick, derived from "frame-tick", is 1/705,600,000 of a second - the next unit of time after a nanosecond.

A researcher at Oxford University said the flick wouldn't have much general impact but may help create better virtual reality experiences.

Although most people are now aiming making VR hardware that refreshes its display 90 times a second, video is available at many different frame rates. It is hard to make sure all the frame updates happen at the same time and at the right time. The small monitors inside a head-mounted display must update more often than the frame rate of the source video in order for the video to follow the speed of normal head movement. The flat frames of video being sent to the viewer’s eyes are excerpts from a larger sphere of video.

If you have spherical footage that is designed to update every 59.94th of a second on a VR headset that is being refreshed 90 times a second, the mathematics gets complicated, and errors can creep into the tens of thousands of calculations that must be done during a VR experience. This is partially because true frame rates cannot be completely captured using multiple decimal place values. The frame rate for US TV is described as 29.97 frames per second for example. The true definition of this frame rate is a division calculation: 30÷100.1×100 = 29.970029970029970029970029970029970029970029970029 and on into uncountable infinity.

The flick trick is tom come up with a small enough unit of time that goes into all common video frame rates and refresh rates without any decimal places left over. This makes the calculations much simpler. Adding and subtracting is faster than dividing. It is also more accurate - as the duration of each video frame or VR refresh update can defined as a whole number of flicks.

Here is a table of how many flicks correspond to popular video frame rates. Final Cut Pro can edit audio clips and keyframes at ‘subframe’ resolution which is 1/80th of the project frame rate. 

Flicks per
frame

fps

1 frame
in seconds

flicks
per frame

flicks per
fcpx subframe

US Film for TV

23.976024

0.04170833

29,429,400

367,867.5

 

24

0.04166667

29,400,000

367,500

Worldwide TV

25

0.04

28,224,000

352,800

US TV

29.9700299

0.03336667

23,543,520

294,294

 

30

0.03333333

23,520,000

294,000

 

59.9400599

0.01668333

11,771,760

147,147

 

 

 

 

 

VR headset refresh

90

0.01111111

7,840,000

 

PS: The highest commonly used ‘frame rate’ is that used in high-end audio: 192KHz which defines samples of audio at 192,000 fps - which is 3,675 flicks per sample.

PPS: The Facebook conversation that prompted the creation of flicks.