Bright line information

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  • Member since
    September 2003
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Posted by Overmod on Saturday, August 8, 2020 4:49 AM

Human vision has a latency in the frame rate that the brain can handle.

This is almost completely different from the 'backward wheel' artifact.  You'd far better ask yourself the more basic question 'why does seeing backward-turning wheels at speed annoy me?'

As I recall from psychophysics, image-formation latency in people is about 19ms.  Substantial image change faster than that results in a blur, or due to human 'image processing' the disappearance of the thing changing faster -- hence the reason you can see the brake detail behind fast-turning car wheel spokes.  

Film on the other hand is shot not only at a fixed frame rate but with a shutter speed fast enough to eliminate more unpleasant motion blur as well as give varied depth of field and a couple of other effects.  Movies are a trick, a way of fooling the human perceptive system into thinking it is seeing smooth motion, but the actual information coded into the individual frames is massively reduced, and one very significant consequence is that phase information is lost.  The actual thing captured on the film at 'wheels backward' speed is like stroboscopic vision with the pulse rate slightly slower than the advance between identical spokes -- this being indistinguishable from the same wheel turned slightly backward at much lower speed at the same lighting repetition rate.

Far more 'fun' and annoying are some of the visual effects from CCD cameras, which can have really fast effective 'shutter speed' but long image-processing time and hence low effective frame rate separate from nominal resolution.  Propellers shot with these have all sorts of weird distortions in addition to what can be wildly varying perception of rotational direction.

Meanwhile there is a perceptual 'quirk' that recognition of a processed image in the brain can 'freeze' certain images; the original determination of that 19ms. acquisition was done stroboscopically to see how long an item needed to be illuminated for a subject to 'remember' details of it... the conscious memory taking much longer to form.  When I first joined SMPTE I had the bright idea that a vastly enhanced perception of dizzying speed might be produced by combining the two effects of blur and periodic capture, by shooting most frames with corresponding average blur but interposing periodic frames with high stop-motion resolution, a bit like the research into recognizable subliminals.  This worked remarkably well... at recreating precisely the confusion in image recognition that makes people disoriented and sick watching roller coasters and the like without vestibular coupling.  It certainly worked to enhance the thrill of speed ... the thrill of helpless speed where you have no idea where you'll be pulled next.

  • Member since
    June 2009
  • From: Dallas, TX
  • 4,687 posts
Posted by CMStPnP on Monday, August 10, 2020 10:00 AM

Wow! Brightline ends agreement with Virgin Trains.    That was kind of sudden.   I guess they got sick of waiting for the capital pay in from Virigin.


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