Re: Apple ][gs VGA Card

[blakjak@panix.com (Noah Mittman)]

Herbert Fung (herbertf@ersys.edmonton.ab.ca) screamed from comp.sys.apple2:
: >such big ugly blocky pixels?). But, if you were to send a different
: >picture on the even and odd rows, then you can double the vertical
: >resolution without too  much effort. Thus, the difference between
: >640x200 and 640x400 is that in the x200 mode, each row is displayed
: >twice, but only once in x400.
:  
: I see what you are saying, but I can't understand how we perceive it as
: higher resolution. Perhaps it's just an optical illusion.

It's the phenomenon we call persistance of vision. The best way of
describing it the fact that if you had a light source that cut off
instantly, your perception of that light would be a very quick fade to
darkness. You simply can't perceive such a rapid change. So imagine that
your monitor is a really elaborate strobe light which is flashing between
odd lines and even lines before you can see them fade away.

Now, like Nat was saying, instead of using two scan lines for one line
of horizontal resolution, we use one for one. Honestly, I've never heard
of interlacing used like this, but I don't know enough of _computer_ video
to argue. Had I known we'd been wasting scan lines like that...
For NTSC video, OTOH, interlacing was done to induce persistance
of vision, since we were timing to 60Hz current, and 30 frames would not
have been enough to make any flicker unnoticable. So the 2 interlaced
fields per frame came to be, basically.
[and don't let film's 24 frames/sec throw you off... film projectors
show each frame twice, rasing the effective viewing rate to 48, just over
the "flicker border", per se.]

hope that helps everybody

-- 
noah [blakjak@panix.com] | <tankPolice!> needs a break from editing
"but if the answer isn't violence / neither is your silence" - pwei
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