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judder-free content movement

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judder-free content movement

Postby marlei » 02 Oct 2018, 04:30

Hi everybody,

I want to display some pretty simple content (white squares on black background). The squares move with various movement velocities from left to right. I use a projector with the ability to vary frame rates (50Hz, 100Hz, 200Hz, 400Hz).

I was wondering if there ares some formulas or tables or charts (... something to work with ...) that allow to calculate subjectively perceived judder-free content movement [PX/FRAME] (=target value) of the observer for various frame rates (=variable) in advance.

Can someone help me with this? How do professionals deal with this problem?

Kind regards

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Joined: 02 Oct 2018, 04:09

Re: judder-free content movement

Postby Chief Blur Buster » 03 Oct 2018, 10:22


This topic veers into Area51, so I've moved to Area51 as it's science/research.

I presume you are using a vision researcher projector such as Viewpixx or another projector, since those projectors are capable of up to 500 Hz.

I have written some popular science articles about behaviours of high Hz at Blur Busters Law And The Amazing Journey To Future 1000 Hz Displays which includes some simple math formulas for various things like multi-image effects on impulse-driven displays.

Take particular care to read the section that "stutters are the same thing as motion blur" when subjected to high frequencies, since a plucked guitar string -- slow vibrations is stutter -- and fast vibrations is motion blur. (The amplitude of the jump between the frames is mathematically the same for stutter & for sample-and-hold motion blur). Mainstreams users have noticed this happening with variable refresh rate displays (GSYNC and FreeSync) with framerate-ramping animations, and it's quite easy to prove that stutters & sample-hold motion blur

Now, we have to distinguish between stutter and judder
stutter -- the regular unsmoothness (e.g. perfect 20fps; a low framerate that looks stuttery)
judder -- the erratic unsmoothness (e.g. 3:2 pulldown because of 3 frames, then 2 frames, then 3 frames, then 2 frames)
So we have to make that distinction.

That said all harmonics (amplitude of 3 frame jump and amplitude of 2 frame jump) can still blend into motion blur at sufficiently high frequencies (e.g. 3:2 pulldown doing 200fps at 300Hz for example -- too high frequency to perceive but can create more motion blur).

And there may be low-harmonic-frequencies (beat frequency effects), e.g. 198fps at 200Hz = 2 stutters per second.

So stutters can show up as harmonics/beatfrequency effects between framerate versus refreshrate. And there can be multiple harmonics between framerate and refreshrate, like there can be between two audio frequencies. (e.g. multiple stutter frequencies overlapping) And some of those frequencies may still be low enough to perceive by human eye, while others are too high to be perceived.

Also, make sure that the use of solid white squares on black background isn't weakening human detection of certian things. Certain tests requires fine patterns. I've found detectability of stutter goes up with higher--resolution. That's the "Vicious Circle" effect that I mention in the 1000 Hz journey article -- defects with refresh rate is easier to detcdt with higher resolution. The higher the resolution and the finer-detailed the pattern is, the easier to detect limitations (motion blur, stutter, etc).

And as a rule of thumb, the higher the Hz, the faster the motion needs to be (in pixels per frame), to allow human detection of motion flaw (e.g. stutter or motion blur). Which requires higher resolution and/or wider display. Again, the "Vicious Circle" effect also explained at Blur Busters Law And The Amazing Journey To Future 1000 Hz Displays. So your display limitations can potentially artificially limit human detectability; make sure you list this in your "Untested Variables" section of any papers you write.

So you may also need to define terms
-- "Stutter"
-- "Judder"
-- "Motion blur" (sample-hold effect) can include stutter/judder that are so high-frequency that it just blends to blur (like a blurry vibrating guitar string that vibrates faster than the eye can see)

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