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Date: Thu, 26 Oct 1995 13:38:55 +0100
From: [e-mail] (Frank Aalbers)
Subject: Re: GLOW and FIELD RENDERING QUESTION
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Robert Cohen (Robert Cohen <[e-mail]>) wrote:

 RC> But since these images are meant to view at 30 frames per second,

Oh no ! A full frame is made in 1/30 second but video needs 2 fields to
make this one frame . So it actually updates the screen 60 times per
second .

This is your screeen .

    uneven line   ------------------------------------ 1
    even line     ------------------------------------ 2
    uneven line   ------------------------------------ 1
    even line     ------------------------------------ 2
    uneven line   ------------------------------------ 1
    even line     ------------------------------------ 2
    uneven line   ------------------------------------ 1
    even line     ------------------------------------ 2
    uneven line   ------------------------------------ 1
    even line     ------------------------------------ 2
    uneven line   ------------------------------------ 1
    even line     ------------------------------------ 2
    uneven line   ------------------------------------ 1
    even line     ------------------------------------ 2
    uneven line   ------------------------------------ 1
    even line     ------------------------------------ 2
    uneven line   ------------------------------------ 1
    even line     ------------------------------------ 2
    ....


First the field of uneven lines (1) are drawn by the electron beam
on the screen . This happens in 1/60th of a second . The Frequentie
of the powernet . Then the even field (2) is drawn . This makes it
that a complete screen is made every 1/30 th of a second .

If you use Frame rendering the full screen will be used for every
frame that comes up every 1/30 of a second . You are using the even
and uneven lines to make this frame . This means you have a full
vertical resolution of 486 lines in NTSC D1 video .

Video ALLWAYS uses fields to make a full screen . So we can do a
little trick , and that is field rendering .

With field rendering you actualy put 2 visual frames into one rendered
frame .

How ?

Lets say fou are rendering an animation and are using FR . What is it
exactly doing ? If he is rendering lets say frame 10 the rendering of
this frame will be only for the uneven lines . The rendering of the
frame in the even lines will be rendered on an in between position of
frame 10 and 11 . Lets say frame 10.5 :-) . After rendering these 2
fields it will put it together in one rendered frame that will contain
both fields . Try this out with a fast moving white object on a black
background and look at the rendered file . You will clearly see the
difference between both fields .

If you now import these frames to lets say the PAR you will only
import 30 framefiles per second but each contain 2 frames . When the PAR
views the animation it will first draw the uneven and then the even of
each framefile you inported . This will give the illution of 2 frames
per full screen update and so 60 frames per second .

And of what you say we cant see difference that is beyond 30 fr per
sec . That is wrong . Maybe we can not se flikkering of lightintensity
like say the flikkering of 30 Hz lightbulb . But we CAN see difference
in motion beyond 30 f/s . This is why manny of the ride movies that are
projected in theme parks (specially the showscan rides) are projected
in 60 fr/s to make every detail as clear as possible even with very
fast movement . And fast movement is what it's all about in ride
movies :-)

I hope everybody finally understands how FR works and how you can use
it . It really makes the vertical resolution to the half of what you
think and it is only noticable with fast movements because of the
difference then between the fields . With static slow movement this
will be less visual because you dont have alot of difference between
the fields (or frames) .

One tip more ! When using Object dissolve to let objects appear or
vanisch on ONE frame time , remember that there, even if you make an
envelope of lets say fr10=100% and fr11=0% , there will be one frame
made in the even field that has the value of 50% . And this is quiet
visible when playing the animation .

Hope this explenation helped somebody because this FR thread was
becomming a bit long ... :-)

Frank Aalbers
GRID Productions
--
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