Pyro Industries API-1394PCI Manual do Utilizador descarregar pdf (Página 220)

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■ STILL GRAPHICS

video with the upper field first. If we were to specify the field ordering as lower field

first, Mr. Bledsoe’s arm would begin forward, jerk back, jump forward, jump slightly

back, and then do the same thing over and over. In other words, the image would jitter

backward a lot, but ultimately it would make progress.

A video image that is 24p or 25p has been created so that both fields are combined

in a progressive frame. When you import these video files, there is no field ordering.

Images that are created in Photoshop and other still graphic applications will not

have field order, so any of these files should also be imported as noninterlaced.

Now that we’ve learned field ordering, how do we detect whether a video file is

upper or lower field first? The easiest way is to talk to the person who originally cre-

ated the file. If the file was created but the creator doesn’t know about field ordering,

you can learn which order is the default order used by whatever application it was cre-

ated in. When in doubt, read the manual!

If there’s no manual around or if the creator of the video is clueless, go ahead

and try one of the field orders or, if it’s a long piece, use just a sample, and study the

result. If the image seems to jitter as it progresses, go back and import it the other way

and observe. One of them is going to look much better, assuming that there is some

kind of motion in the frame.

Field Order versus Field Dominance

Field ordering is commonly confused with field dominance. Although the two terms

sound similar, they are very different.

Field dominance is the determination of a cutting point. When we record video,

synchronize it, and add timecode with the picture, it takes on certain characteristics,

including two fields for each frame. The two fields, when combined, make up a single

frame of video and one frame of assigned timecode. Therefore the first field of video

that is recorded for a specific timecode is field 1, the second recorded field is field 2,

and then the timecode number advances. Field dominance is the determination of

which field is first temporally in the edit frame and where each cut will occur. Avid

systems always cut before the first field. Therefore it is field-1 dominant. If it were cut

on the second field, it would be field-2 dominant.

Note: Which brings me to a question:If progressive frames are so great,why is it that each interlaced

field offers motion updated 60 times per second instead of 30 as in progressive frames? Wouldn’t that be bet-

ter motion? The answer is yes and no.Yes,an interlaced frame “spreads out”the motion during a given period

of a second while the progressive frame contains it in 30 full frames.That is, unless you play it more slowly,in

which case you’ll also detect the lack of resolution of single fields.

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Pyro Industries API-1394PCI Manual do Utilizador Página 220