Explanation: Display Persistence is like a Camera Shutter

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Explanation: Display Persistence is like a Camera Shutter

Post by Chief Blur Buster » 25 Feb 2021, 21:48

(Another crosspost from another forum, but it's very "Area 51" educational display question.)

Why is Display Persistence Like a Camera Shutter?

Think of the sharpness difference between stationary images VERSUS moving images[/B]. That's where higher resolution amplifies frame rate limitations. Higher resolution of the same size displays, panning at the same physical motion velocity (feet-per-second or meters-per-second or inches-per-second, choose your favourite unit) -- means you have more pixels to motion blur over. That amplifies the difference between an increasingly sharp stationary image, versus a moving image.

Both 1080p, 4K and 8K has equally blurry images at the same MPRT number. 16ms MPRT means 1000 millimeter/second motion will have 16 millimeters of motion blur regardless of resolution. Motion blur can be on physical units, so 1/60th of the physical motion distance is motionblurred.

A sample and hold display has identical motion blur to a blurry camera pan:

Image

No matter how good your film is (8mm film, 35mm film, 70mm film, medium format film), panning motion blur will kill any stationary resolution.

- Motion blur is based on camera shutter (camera POV, aka 1/60sec shutter)
- Motion blur is based on frame visibility time (display POV, aka pixels displayed continuously for 1/60sec)

Whereas, for framerate=Hz motion, persistence is:

- For strobed display, eye-equivalent shutter time is the pixel flash time (CRT, plasma, strobed LCD)
- For sample-and-hold display, eye-equivalent shutter time is the refresh cycle visibility time.

Thus, higher refresh rates are more important for sample-and-hold displays, because of the irrevocable link between refresh rate and persistence motion blur, due to mandatory display-whole-refresh-for-whole-interval.

So a 1080p, 4K, and 8K sample-and-hold display is identically motion-blurry during motion of the same physical motion speed, for the same frame rate (i.e. 60fps) and same refresh rate.

For a given persistence (e.g. 60fps 60Hz), a higher-DPI will have more relative count of pixels of motion blur for a given physical inches-per-second motion.

For simplicity, let's use a physical unit of measurement. Any unit, as long as we're consistent. This time I will use millimeters for this educational exercise:

All of these displays have identical millimeters of motion blur for the same physical millimeters-per-second motion:
  • 1920x1080 display, 60fps 60Hz, that is 1000 millimeters wide (1080p)
  • 3840x2160 display, 60fps 60Hz, that is 1000 millimeters wide (4K)
  • 7680x4320 display, 60fps 60Hz, that is 1000 millimeters wide (8K)
...Imagine the display is 60 Hz (1/60sec = 16.7ms)
...Imagine motion that is going 1 meter/sec (one screen width per second).
...That is 1 millimeter in 1 millisecond
...That is 16.7 millimeter in 1 refresh cycle (16.7ms)
...Millimeters have more pixels at higher resolutions (higher pixel density) = more pixels to blur over
...Your eyes are analog. Your eyes continuously move in one smooth analog motion.
...You've tracked eyes 1mm in 1ms
...But frame rate is digital. Pixels are stationary for 16.7ms on 60Hz sample-hold displays
...The stationary pixel is "smeared" (blurred) across your analog-moving eyes for 16.7ms at 60Hz
...Knowing this also makes testufo.com/eyetracking scientifically easier to understand

This is true for all pixels on the entire display surface. And you've tracked more pixels at higher resolution at same physical motion speed. The surface area is unchanged (i.e. same surface area of motion blur trail) but more pixels are motionblurred, amplifying stationary-versus-motion resolution difference at higher resolutions.

Next, look at the below image. It is easy to realize that motion blur in the below motion test is caused by eye tracking. That's persistence (MPRT) blur. Any non-retina-framerate material will exhibit this additional blur above-and-beyond camera shutter blur -- even 120fps or 240fps.



Now, at a specific frame rate -- e.g. 60fps -- the display is switching through a series of multiple stationary images. Refresh cycles are stationary. Your eyes are in a new position at the end of a refresh cycle than at the beginning of a refresh cycle. 16.7ms is as bad as a 1/60sec SLR camera shutter while waving the SLR camera! That is the cause of persistence motion blur -- aka perceived (MPRT) display motion blur caused by the motions of eye tracking. This is a fault of the invention of using stationary images to emulate analog moving images.

Steps To Conclusion:

1. Motion resolution hasn't improved because frame rate is the same (sample and hold effect)
2. Static resolution is improved because of the higher resolution of display
Therefore, sharpness difference of moving images versus stationary images is bigger, the higher resolution you go

Conclusion:

- The motion blur of sample and hold keeps motion resolution unchanged even if static/stationary resolution goes up.
- Higher resolution display amplify frame rate limitations on sample-and-hold
- Higher refresh rates and higher frame rates become more important at higher resolutions.
Therefore, sharpness difference of moving images versus stationary images is bigger, the higher resolution you go

Related comment: 1000Hz would be practically useless for VGA, while 1000Hz will be very human-visible for future 8K.

Good additional reading: The Stroboscopic Effect of Finite Frame Rates as well as UltraHFR FAQ: Real Time 1000fps on Real 1000 Hz Display.
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