Newsgroups: sci.image.processing
Path: cantaloupe.srv.cs.cmu.edu!das-news2.harvard.edu!news2.near.net!news.mathworks.com!udel!gatech!newsxfer.itd.umich.edu!agate!library.ucla.edu!csulb.edu!csus.edu!netcom.com!perry
From: perry@netcom.com (Perry West)
Subject: Re: help! what's frame/field mode
Message-ID: <perryD2EpIq.Eox@netcom.com>
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
X-Newsreader: TIN [version 1.2 PL1]
References: <3f6ko8$jht@jadzia.CSOS.ORST.EDU>
Date: Sat, 14 Jan 1995 17:49:37 GMT
Lines: 47

A frame is the entire image comprising all scan lines.  A field is a 
half-image comprising either the odd or even numbered scan lines.

To understand why we have fields at all, you have to look at the history 
of broadcast television.  During the development of television, 
conserving broadcast bandwidth was a prime objective.  Transmitting the 
entire image 30 times a second caused human observers to notice a flicker 
between frames.  Transmitting the images twice as often, 60 times a 
second, avoided the perception of flicker, but required twice as much 
broadcast bandwidth for an image with the same amount of data.  However, 
if one-half of the image was transmitted in 60 seconds (say, the odd 
numbered scan lines), and the other half of the image (the even numbered 
scan lines) the eye did not percieve flicker.  Thus the whole image could 
be transmitted in one 30th of a second with no apparent flicker.

So, the relevant standards evolved (RS170, RS330, NTSC, CCITT, PAL, 
SECAM) using this interlaced scanning (the two fields).  Now most of the 
low cost cameras use it even though it is lousy for computer interface.

Most monitors can adapt their display to handle either interlaced or 
field-only images.  If you look closely and run some tests you will see 
some differences.  An interesting test is to image a ruler held 
vertically in the image.  See how far away you can get it and still see 
all the divisions.  As you move it further away, you will notice 
apparently coarser divisions appearing -- this is aliasing.  Try it with 
your camera in the frame and field mode.  The resolution should be higher 
(you can see the dividing marks further away) in the frame mode than the 
field mode.

Regarding the frame and field mode of your camera.  Some cameras allow 
you to switch the scanning from field to frame, others change the way the 
sensing elements are connected to sense the light, but still provide 
scanning that is like an interlaced signal.  You will have to look into 
your camera's information to see if the field mode is related to scanning 
or integration (exposure).

For strobing purposes, I suspect you will want to use it in the field 
mode.  Most cameras do not store the image for more than a field.  Also, 
you cannot reliably strobe the scene at any random time.  Some cameras, 
those with interline transfer or frame transfer sensors, require that you 
not strobe the scene for the brief period while the transfer is taking 
place.  Other cameras with CID or MOS sensors will require you not to 
strobe the scene while the scanning is taking place (which is most of the 
time).

Hope this helps.  Good luck.

