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From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: comp.sys.amiga
Subject: Re: Interlace
Message-ID: <5160037@hplsla.HP.COM>
Date: 22 May 89 15:40:19 GMT
References: <1222@psueea.UUCP>
Organization: HP Lake Stevens, WA
Lines: 55

> papa@pollux.usc.edu (Marco Papa) writes:
>>In article <15500001@ux1.cso.uiuc.edu> phil@ux1.cso.uiuc.edu writes:
>>>Broadcast television does not suffer from flicker because the change from
>>>one line (scanned in one cycle) to the next (scanned in the other cycle)
>>>is very little.
>
>You must be joking! Get your facts, dude. Broadcast television suffers the
>SAME flicker problems that the Amiga shows with images that have 1 scanline
>tall horizontal lines. This is simply due to the NTSC standard. The flicker
>on TV is very noticeable during the sports and weather reports.  Images
>that have a lot of color variations don't flicker (on either a broadcast
>TV or the Amiga) because the eye "averages" the colors.

Well, "dude", the last statement may indicate you need a bit of brushing
up on facts yourself.  If I build an image with "a lot of color variations"
by making the first field of a frame entirely different in intensity (as
well as hue and saturation) from the second field, I can about guarantee
you will be able to see flicker in it on most standard NTSC tv's.  The
least flicker is on images that are (1) very similar from one field to
the next (e.g., an image with no distinct lines but only gradual spatial
changes -- like most program images that are broadcast, except as noted
by previous posters.  And it probably helps that the images are usually
in motion.), and (2) dark.  Furthermore, because NTSC also interlaces the
color subcarrier with the horizontal (and therefore with the vertical)
rates, pictures with highly saturated colors, when displayed on a monitor
that doesn't try to play games with what it "thinks" should be luminance
and what should be chrominance, will show a lot of _color_ flicker.  This
is particularly noticable on sharp vertical edges between saturated colors.

There are perhaps some practical lessons in this for those who would like
to keep down flicker on interlaced (computer) screens:

 -- Since an RGB monitor doesn't use NTSC color encoding, you don't have
    to worry about the last problem.  Your eye may still perceive flicker,
    though, if one field has significantly more of one of your colors than
    the other(s).  So chose a set of colors for the screen that are not 
    too different in intensity.  There's an obvious tradeoff here with
    legibility of (text) information.
 -- Keep the intensity low.  This is easier to use if your room lighting
    is also fairly dim.  If you can't control the room lighting, you
    can achieve the same effect with dark glasses, leaving the screen
    itself fairly bright.
 -- Use text fonts that put the same number of "on" pixels on each field
    (that is, on alternate lines).  A test of this is to display a few
    lines of equal signs and/or minuses.  The minuses should be two lines
    high, and the equal signs should have two rows blank between the
    turned-on rows (or for larger fonts, something equivalent).
 -- If all else fails, de-interlace the screen (e.g., FlickerFixer).

Hope this is useful.
>
>-- Marco Papa 'Doc'

Tom Bruhns
tomb%hplsla@hplabs.hp.com