From: damiel@netcom.com (Paul Theodoropoulos)
Subject: Re: A1200/4000 ramblings...
X-Spook-Fodder: hero heroin tree shrub bush birth life death FBI CIA NSA
Organization: Netcom - Online Communication Services  (408 241-9760 guest) 
Date: Sun, 3 Jan 1993 07:37:13 GMT

In <C09L8v.Fru@fc.hp.com> koren@fc.hp.com (Steve Koren) writes:
>Steve Winokur (swinokur@cellar.org) wrote:
>> Close... but no cigar.

>No, it is perfectly correct.  Movies have 24 distinct frame changes per
>second, which is plenty enough to be perceived as smooth movement by
>almost everyone.  However, the entire image is projected "at once", not
>scanned with an electron beam as in a CRT.  This is what I was trying to
>get across, in response to the orginal poster's comment that 24 FPS must
>be a high enough _scan_ rate because it is high enough to be perceived
>as smooth movement in movies.

no, mr. Winokur is correct. The image is *not* projected "at once" - all 
points on the screen are *not* illuminated at the same time. the frame is
illuminated by a smoothly spinning blade that exposes the frame as the 
blade spins down past the gate. the frame begins to be illuminated at the 
bottom of the image, and is fully illuminated at the top. then the next 
blade spins down and covers the frame, bottom to top. the frame is 
illuminated once again in this fashion (most projectors use a triple blade 
system [looks like the _Radiation_ symbol] or double blade system, and 
expose each frame twice), then the pulldown mechanism moves another frame 
into the gate while the blade has the gate covered.

the main reason why 24/48 frames per second projected film has little 
noticeable flicker is because of the massively greater lumen output of
a projector compared with a video screen. an image that is easily five to
ten times brighter than a video screen enhances persistence of vision 
immeasurably.

>Your point about doubling each frame and projecting at 48 FPS is
>something else entirely and this used because there is, in reality, some
>time between frames.  If you could envision a perfect projection system
>which could put up one frame and then another immediately with no
>interval between them, 24 FPS would be perceived as both flicker free
>(because there is no scanning of the images or time between images) and
>smooth (because 24 images per second is enough to look that way).

well, yes the image would be flicker free but it would be subject to 
visible strobing, similar to what can be seen in some of the motion
compression schemes being applied to broadcast video.

24 fps is a barely adequate speed for image capture and reproduction -
on film. there is significant, easily visible blurring and flicker at that 
speed. 30fps is far better, and 60 is ideal. improved shuttering systems 
help immensely - strobe synced film at 30fps can look pretty gorgeous.

but that's another story.

[my qualifications: former member, IATSE Local 16 - former projectionist
 and audio technician at LucasArt's Skywalker Sound. experienced with 
 Tokina 35mm and Simplex 35/65mm projectors.]

-- 
paul theodoropoulos     damiel@netcom.com     (hooools@well.sf.ca.us)
       Cosysop - Threat Management Institute BBS 707 935 1713 
Newsgroups: comp.sys.amiga.hardware