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> Field Ordering in Graphic Imports and Exports |
Field Ordering in Graphic Imports and Exports
Graphic images are composed of one or more image files, each of which contains a full frame. These frames contain fields (formed from the odd-numbered and even-numbered lines of the image frame) that have three basic arrangements:
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Progressive or still frame: The upper and lower fields in the frame
originated at the same instant of time, or are coherent with each other,
as shown in the following illustration.
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Upper field is first: The upper field in the frame (odd-numbered lines,
when the frame lines are numbered starting from 1) occurs temporally
before the lower field, as shown in the following illustration. This
arrangement can be termed "upper field first" (or "lower field
second").
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Lower field is first: The lower field in the frame (even-numbered
lines) occurs temporally first, as shown in the following illustration.
This arrangement is termed "lower field first" (or "upper field
second").
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Preventing a Spatial Field Mismatch on Import
The following table shows the proper spatial field position for each of the common video formats in Avid systems.
Recommended Field Settings for Two-Field Import and Export
AVR NTSC | 720 x 243 x 2 | Upper field is first. | Odd Field |
AVR PAL | 720 x 288 x 2 | Upper field is first. | Odd Field |
Meridien NTSC | 720 x 243 x 2 | Lower field is first. | Even Field |
Meridien PAL | 720 x 288 x 2 | Upper field is first. | Odd Field |
1Odd Field or Even Field specifies whether the temporally first field in the frame has the odd-numbered or even-numbered lines, starting from 1.
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Graphics utilities, such as Adobe After Effects, permit selection of either spatial relation for the fields when rendering a sequence of interlaced fields. If the rendering is performed as shown in the preceding table, then the import is correct.
If the spatial positions of the two fields are reversed (for example, the upper field should be a lower field), the import cannot be done without correcting the spatial relationship. In this spatial mismatch situation, the Avid system converts the upper field to a lower field by deleting the top line of the upper field and replicating the bottom line. The field is thus converted to a lower field relative to the other field. The import operation can now proceed.
Spatial Field Relationship on Export
The default export operation is automatically carried out as shown in the preceding table. The fields are properly interleaved in the export frames (one frame per file).
If an import comes in mismatched and the dropping of the top line and repeating of the bottom line is performed, you might choose to have the export operation performed to prevent a shift up or down by a line. The Export Settings dialog box permits you to select the spatial arrangement that is the opposite of what is otherwise recommended. This option should rarely be used.
24p and 25p Import and Export
The spatial field selection options do not apply for 24p and 25p projects because the frames in these projects are already in progressive or still-image form.
Field Dominance
Editing in Avid systems is frame based. All timecode is expressed in frame numbers, and all cuts are at frame boundaries. A raw video stream has no concept of frames (ignoring color framing) until the frame unit is defined. Defining the dominant field for the system defines the "frameness" of the video stream, as shown in the following figure.
Avid editing systems all use field 1 as the dominant field. This means that the first field temporally in the edit frame is always field 1. Field 2 is always the second frame in the edit frame. Cuts always precede field 1.
Fields in Video
In the video signal, fields have a temporal position that is unambiguously and uniquely tied to the details of the video signal, regardless of whether the signal is analog or digital (SDI). This means that working around a field spatial mismatch should be done by correcting the spatial relation between the two fields rather than the temporal position. In some cases, it is possible to modify the field dominance of the input image files, but this is cumbersome, results in the loss of two fields, and is more difficult to carry out than either the workaround provided in Avid systems or rerendering properly to the other spatial relationship.
