Understanding the human Visual System
The Phi Phenomenon (or Persistence of Vision)
Ok, so some of you have heard of ‘persistence of vision’, a physiological theory that states that the human retina RETAINS an image for a certain amount of time, and if that image is replaced quickly enough then we perceive motion. Turns out this is false (well, not false, just way more complicated, and mostly psychological NOT physiological).
In fact, it is more to do with a physchological phenomenon called the Phi Phenomenon, that boils down to about the same thing as ‘persistence of vision’. It goes along the lines of:
If the amount of time between a series of brief still images shown to a human is very small (approx. 1/20 to 1/5 of a second after the previous image) the human mind perceives the image (assuming it is related… not a dog replaced by a monkey or something!) as a moving image.
In 1978 Anderson and Fisher wrote an article called ‘The Myth of Persistence of Vision’ in order to stop film makers and film critics referring to it as persistence of vision.
How about a quick demo?
View this link http://www.privatelessons.net/2d/sample/m01_03.html (ignore the persistence of vision stuff) This link (once loaded) shows two animations, one at 5 fps and one at 12fps. Compare the difference in smoothness, particularly the 12fps one. This is due to the ‘phi phenomenon’. That is, your brain perceives fluid (ish) movement in the 12fps stick figure due to it being quickly replaced by a slightly different moving image.
Ok, so the simple definition of the framerate of a film is:
the number of still frames displayed per second = frames per second (fps)
The fps per second of a given medium depends on a number of things eg, the history and development of the medium.
Current Motion Pictures (at the cinema): 24 fps
Super 8 film: 18 fps
NTSC tv: 29.9 fps
PAL tv: 25 fps
Standard digital video 15 fps
High quality digital video 30 fps
(note: I have not considered the implications of the interlacing of tv, such that two half fields are refreshed per second. ie every second scan line…)
No need to go into the history of why motion pictures are set at 24fps (combo of ease, mechanics, research at the time etc etc) but that is where it sits.
You have probably heard of professional stop motion animators ‘shooting on twos’ or ‘shooting on ones’. That is where they decide to take 2 frames at once (no movement) or 1 frame at a time.
Shooting on twos straight onto film results in a frame rate of 12fps (1/2 of 24fps) (or 9 fps if using super 8 film…)
“So what frame rate do I film at?”, I hear you ask…
Okay, so what this all boils down to is that between 9 and 12 fps seems to be enough to fool the brain into thinking movement is occurring (the phi phenomenon), BUT essentially, the higher the frame rate the better (up to a point, ie too high and you won’t notice anyway).
The point is to find a happy medium:
24 fps: If you are practicing for a professional future, by all means film at 24fps, but take into account the the increments you need to move arms etc is smaller and the time it takes to make your film will be heaps longer.
9-12 fps: Ok, 9-12 fps is great for a quick little film and to practice your animation skills, but it still looks a bit jerky. Yep, even 12 fps can look a little bit… unprofessional?
Greater than 24 fps: Time consuming with not much payoff… except if animating direct, at high motion for PAL (25fps) or NTSC tv (29.9 fps).
15 fps: This (to me, nick/hali) seems to be the happiest medium for a number of reasons:
1. It does give good results for the perception of smooth motion (in general)
2. It is not as time consuming at 24fps, but for only 3 more frames a second than 12fps it looks considerably better.
3. You’ll notice that quite a number of codecs (and computers) seem to be able to display animation filmed at 15fps very well. It is just enough re-inforcement for me to choose 15fps due to this. It is also why I don’t try 16 or 17fps because I know that for most cases that will be resampled back to 15fps for computer animation files.
4. I’m used to it, and thus I have got good at judging how far to move an arm or leg to get it to move at that speed (probably means I’m too used to 15fps if I ever got a job doing this full time… cough cough)
In speaking to wandrer2 about this article he reminded me that “A Christmas Carol in Bricks” was filmed at 12fps…
What does this show us? Well, it shows that an experienced animator, with a good feel for movement and timing can make 12 fps look very good indeed.
In addition ACCIB didn’t really have any ‘high motion’ scenes, so with careful animation we don’t notice the difference between 12 & 15 when taking motion into account.
It just goes to show that choice of framerate can also depend on your own animation skill. The better you get, you can probably get away with a slightly lower framerate because you are able to ‘inbetween’ more efficiently and realistically.