Why Does Higher Resolutions Make Display Motion Blur Easier To See?

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Why Does Higher Resolutions Make Display Motion Blur Easier To See?

Post by Chief Blur Buster » 03 Mar 2021, 01:23

This is a frequently asked question at Blur Busters, so I am crossposting a reply of mine from a different discussion forum), since it is an excellent Area51 question.

From the Vicious Cycle section of Blur Busters Law: The Amazing Journey To 1000 Hz Displays -- higher resolutions can amplify visibility of display flaws (refresh rate / motion blur / stutter / etc). Here, I will explain from another angle:

Why Does Higher Resolutions Make Display Motion Blur Easier To See?

Think of the sharpness difference between stationary images VERSUS moving images. 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:

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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 www.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.

The display is flipbooking at the same frame per second (60 frames per second). Remember 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.

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:

- Higher resolution display amplify frame rate limitations.
- Higher refresh rates and higher frame rates become more important at higher resolutions.

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
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