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Filed under: Motion Graphics •Post Production •

Field Order - Who’s on First?

Before you can Separate Fields of course, you’ll need to know whether the source is Upper Field First or Lower Field First . If you’re not sure of the field order, guess Upper Field First and then open the movie by holding Option on Mac (Alt on Windows) and double-clicking it in the Project panel; this will open it in a special Footage panel that shows you the clip after it has been processed by the Interpret Footage dialog. Here, After Effects will display each field (interpolated to a full frame).

If you guessed the field order wrong, motion will seem to stagger two steps forward/one step back as you move through the fields - look at the rider in relation to the audience to follow the progression of motion. If you see this type of staggered motion in other footage, it means someone processed the footage with the fields reversed.

If you set the field order correctly, motion will progress smoothly as you step through the fields. Footage courtesy Artbeats.

Press Page Down to step through the fields. If the guess was correct, as you play the movie a moving object would make steady progress; if you guessed wrong, the object will appear to stagger back and forth as the fields play out of order. (You can be fooled by checking for field order in the Comp panel, as only one field from each frame is displayed in a 29.97 fps Comp.)

Tips for the Battle Field

• When treating multiple clips, save time by Separating Fields for the first movie only. In After Effects, select this clip in the Project panel, select File > Interpret Footage > Remember Interpretation, marquee all the other clips and use File > Interpret Footage > Apply Interpretation.
• When using a video frame grab as a still image, in Photoshop run the Video > De-interlace filter, which permanently throws away one of the fields and replaces the now-missing pixels with an interpolation of the other field. If you don’t de-interlace and there is prominent interlacing in the frame grab, the image will appear to flicker as both fields play back and forth.
• Don’t apply the Separate Fields option to photos, text, and non-interlaced movies.
• If interlaced video footage is intended for playback on a computer monitor (for multimedia or the web), you’ll need to de-interlace the source. To do this in After Effects, Separate Fields as usual (see above), but when you render set Field Render > Off. This will take only the first field from each frame of the source movie, and interpolate the single field out to a full frame (as with Photoshop’s De-interlace filter).
• For the highest quality, when converting a 720x480 interlaced movie to 320x240 non-interlaced for the web, don’t just render a 720x480 comp at Half Resolution. Instead, after Separating the Fields, either scale the movie to fit a 320x240 comp, or render at 720x480 and scale the output to 320x240 in the Output Module (as shown below).
  
• Because field rendering requires a full-size frame, you should not field render movies that are less than full frame. Even if you use hardware zoom to make 320x240 fill the screen on playback, each “field” would be zoomed up to occupy two scan lines.

• Most 3D programs are unable to field-render an animation, but you can get the same effect by rendering the 3D animation at 60 frames per second. (However, unless objects are moving quite quickly, it may not be worth doubling the 3D rendering time.) When you import this 60 fps movie, Conform the sequence to 59.94 fps in the Interpret Footage dialog, place in a 29.97 fps Comp and field render on output. After Effects will retrieve one 3D frame per video field.
• Importing a 29.97 or 30 fps animation and processing it through After Effects with field rendering turned on will not give you an “interlaced” movie. If After Effects doesn’t have the necessary data to work with (i.e., one distinct image per field), the result will be no different from the source.<.li>

• When prerendering layers for reuse in After Effects, render full frame animations with fields, and Separate Fields on re-import. If the layers are small and will later be moved around (as in a looping sprite), for the highest quality render at 59.94 fps and no fields, so that on re-import you have one clean image per field.
• For best results when reversing the field order of an interlaced QuickTime movie: import the movie into After Effects but don’t Separate Fields; center the clip in a comp that’s the same size and duration; move it down one pixel and render again at 29.97 fps, with no field rendering. This moves each field down one scan line, so the odd field become even, and the even field becomes odd.

Postscript

This article was originally written in 1998, and was updated in 2008 (640x480 upper-field-first capture cards were the norm back then; today we’re using DV, D1, and HDV which have different frame sizes and field orders).

Between these two dates, we’ve created other articles and videos that also go over fields and interlacing in detail:

• We’ve written a free article for Artbeats.com on managing interlaced footage (download here - 542k PDF).
• Our books After Effects Apprentice (Chapter 5 plus the Appendix) and Creating Motion Graphics 4th Edition (Chapter 39) also discuss handling interlaced footage. Click here for more detail on these books.
• We’ve created a ~50 minute video training module that goes over numerous permutations of working with interlaced and progressive scan source material. It is available through Lynda.com’s Online Training Library. If you aren’t already a Lynda.com subscriber, click here to get a free 7-day pass on us.
• Before we started writing books, we created a 4+ hour video that went over universal gotchas such as dealing with alpha channels, fields, frame rates, pulldown, aspect ratios (including anamorphic widescreen), and 3D/video workflow issues. Most of the information is still highly relevant, as the video standards haven’t changed - and neither have the misunderstandings that surround them! Click here to learn more about VideoSyncrasies: The Motion Graphics Problem Solver.

The content contained in our books, videos, blogs, and articles for other sites are all copyright Crish Design, except where otherwise attributed.

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