# October 2014

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## 60 Hz, 120 Hz, 240 Hz…

Update: first, take this 60 vs. 30 FPS test (sadly, now gone! Too much traffic, is my guess). I’ll assume it’s legit (I’ll be pretty entertained if it isn’t). If you get 11/11 consistently, what are you looking for?

A topic that came up in the Udacity forum for my graphics MOOC is 240 Hz displays. Yes, there are 240 Hz displays, such as the Eizo Foris FG2421 monitor. My understanding is that 60 Hz is truly the limit of human perception. To quote Principles of Digital Image Synthesis (which you can now download for free):

The effect of temporal smoothing leads to the way we perceive light
that blinks, or flickers. When the blinking is slow, we perceive the
individual flashes of light. Above a certain rate, called the critical
flicker frequency (or CFF), the flashes fuse together into a single
continuous image. Far below that rate we see simply a series of still
images, without an objectionable sense of near-continuity.

Under the best conditions, the CFF for a human is around 60 Hz [389].

Reference 389 is:

Robert Sekuler and Randolph Blake. Perception. Alfred A. Knopf, New York, 1985.

This book has been updated since 1985, the latest edition is from 2005. Wikipedia confirms this number of 60 Hz, with the special-case exception of the “phantom array effect”.

The monitor review’s “Response Time and Gaming” section notes:

Eizo can drive the LCD panel at 240 Hz by either showing each frame twice or by inserting black frames between the pictures, which is known to significantly reduce blurring on LCD panels.

This is interesting: the 240 Hz is not that high because the eye can actually perceive 240 Hz. Rather, it is used to compensate for response problems with LCD panels. The very fact that an entirely black frame can be inserted every other frame means that our CFF is clearly way below 240 Hz.

So, my naive conclusions are that (a) 240 Hz could indeed be meaningful to the monitor, in that it can use a few frames that, combined by the visual system itself, give a better image. This Hz value of the monitor should not be confused with the Hz value of what the eye can perceive. You won’t have a faster reaction time with a 120 Hz monitor.

The thing you evidently can get out of a high-Hertz monitor is better overall image quality. I can imagine that, on some perfect monitor (assume no LCD response problem), if you have a game generating frames at 240 FPS you’re getting rendered 4 frames blended per “frame” your eye received. Essentially it’s a very expensive form of motion blur; cheaper would be to generate 60 FPS with good motion blurring. Christer Ericsson long ago informally noted how a motion-blurred 30 FPS looks better to more people than 60 FPS unblurred (and recall that most films are 24 FPS, though of course we don’t care about reaction time for films). What was interesting about the Eizo Foris review is that the reviewer wants all motion blur removed:

You probably already own a 120 Hz monitor if you are a gamer, but your monitor most likely does not have the black frame insertion technology, which means that motion blurring can still occur (even though there is not [sic] stuttering because of 120 Hz). These two factors are certainly not independent, but 120 Hz does not ensure zero motion blurring either, as some would have you believe.

The type of motion blur they describe here is an artifact, blending a bit of the previous frame with the current frame. This sort of blur I can imagine is objectionable, objects leaving (very short lived) trails behind them. True (or computed) motion blurring happens within the frame itself, simulating the camera’s frame exposure length, not with some leftover from the previous frame. I’d like to know if gamers would prefer 60 FPS unblurred vs. 60 FPS “truly” blurred. If “unblurred” is in fact the answer, we can cross off a whole area of active research for interactive rendering. Kidding, researchers, kidding! There would still be other reasons to use motion blur, such as the desire to give a scene a cinematic feel.

For 30 vs. 60 FPS there is a “reaction time” argument, that with 60 FPS you get the information faster and can react more quickly. 60 vs. 120 vs. 240, no – you won’t react faster with 240 Hz, or even 120 Hz, as 60 Hz is essentially our perceptual maximum. My main concern as this monitor refresh speed metric increases is that it will be a marketing tool, the equivalent of Monster cables to audiophiles. Yes, there’s possibly a benefit to image quality. But statements such as “there is not [sic] stuttering because of 120 Hz” make it sound as if our perceptual system’s CFF is well above 60 Hz – it isn’t. The image quality may be higher at 120 or 240 Hz, and may even indirectly cause some sort of stuttering effect, but let’s talk about it in those terms, rather than the “this faster monitor will give you that split-second advantage to let you get off the shot faster than your opponent” discussion I sometimes run across.

That said, I’m no perception expert (but can read research by those who are), nor a hard-core gamer. If you have hard data to add to the discussion, please do! I’m happy to add edits to this post with any rigorous or even semi-rigorous results you cite. “I like my expensive monitor” doesn’t count.

p.s. I got 4/11 on the test, mainly because I couldn’t tell a darn bit of difference.

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