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Date:	Wed, 30 Jun 2004 04:47:46 PDT7
From:	Peter Skye <pskye@peterskye.com >
Reply-To:	scoug-help@scoug.com
To:	scoug-help@scoug.com
Subject:	SCOUG-Help: Re: Supposed OS/2 Port of FFMPEG (?)

Content Type: text/plain

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> Nominally, the eye will not perceive changes above a 30Hz rate,
> although there will always be variations among individuals.

30 Hz causes flickering for most people. Our television system uses a
60 Hz rate and our movies are 48 Hz (or 72 Hz with a three-leaf
shutter). Hewlett Packard's research on eye flicker for illuminating
with LEDs found a 100 Hz rate acceptable. Fluorescent tubes flicker at
120 Hz (in North America).

Home movies shot at 16 fps and projected at 32 flashes per second
"flickered" too much; that's why Kodak switched to 18 fps (36 flashes
per second) which helped.

Old silent movies were shot at all kinds of speeds (there were no
standards for a long time) and the rates were as low as 10 fps. No
wonder they were called "the flickers". (Youthful lurkers: The cameras
were hand-cranked and so were some of the projectors. The motorized
projectors had variable speeds. The projectionist was responsible for
setting the speed so the film "looked right".)

Many people don't realize that television (NTSC) is shot at 60 images
per second. That's why something shot direct-to-videotape looks so much
cleaner than film even though film has far superior resolution.

A really simple persistence of vision measurement test is a spinning
disk with spokes drawn on it -- just speed up the disk until you can't
tell there's a pattern, then calculate the "flicker" rate. As I recall,
the human eye's persistence of vision is color-dependent.

Note that the eye can "see" extremely short "changes". A photographic
strobe light might be "on" for just 1/1000th of a second and yet we
still see it.

- Peter

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