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Old December 14th, 2004, 08:16 AM   #1
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interlace

trying to get my head round interlaced frames on computers
when you import footage to a computer and render 'lower fields first', what happens to the interlaced frames? does it effectively double the frame rate, as a standard tv or video monitor would? if this is the case, does it also 'stretch' the individual scan lines vertically so that they continue to fill the frame?
its all very confusing.
please help me.
thanks,
alex
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Old December 14th, 2004, 08:27 AM   #2
 
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rendering lower field first does not de-interlace the footage. It interlaces the footage with the lower set of field lines being placed in the lower position. If this sounds confusing to you, take a look here:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/734

All interlaced footage created in DV is, by definition, lower field first. In PAL land, it's upper field first. Interlaced MPEG footage is upper field first. Taking interlaced footage and rendering with the wrong field first will result in a very toothed or combed looking image, very disagreeable.

De-interlaced or progressive footage combines the field lines in one of several ways. It will stretch a single set of field lines, as you described, or it will do a mathematical interpolation of both fields to create a third full frame field. As such, you will lose vertical resolution, since your image is based on 1/2 the original info.
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Old December 14th, 2004, 08:29 AM   #3
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No, the settings for lower fields, upper or odd/even tells the
computer in which order the fields are stored inside a file.

For example, you would have a frame like this:

scanline 1....
scanline 2....
scanline 3....
scanline 4....

and would be in interlaced:

scanline 1.... field 1....
scanline 2.... field 2....
scanline 3.... field 1....
scanline 4.... field 2....

now this might be stored as:

scanline 1.... field 1....
scanline 3.... field 1....
etc.

scanline 2.... field 2....
scanline 4.... field 2....
etc.

this would be upper field first since the first field is stored first,
then the second field. However:

scanline 2.... field 2....
scanline 4.... field 2....
etc.

scanline 1.... field 1....
scanline 3.... field 1....
etc.

which would be lower field first since the second field is stored
first etc.

It has nothing todo with what the program does after it has read
in the footage!

Bill: just to be sure, PAL DV is *ALSO* lower field first! Analog PAL
is indeed upper field first.... but I guess this is what you meant.
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Old December 14th, 2004, 08:39 AM   #4
 
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Rob...

OOPS, I wasn't aware of the PAL distinction. Thanx for clarifying.
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