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Field Order – Who’s on First?

Chris and Trish Meyer
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If you thought most NTSC video ran at 29.97 frames per second, that's only half the story – literally. It actually runs at a speed of 59.94 fields rather than 29.97 frames per second (fps), with pairs of fields “interlaced” to form a complete frame (see the illustration at left). When you shoot footage with your camcorder, it does not record whole video frames unless you are explicitly in a special mode known as “progressive scan.” Instead, most of the time it captures one field, then a second field, and lays these fields down in a linear fashion to tape.{C}

Likewise a television monitor plays back an interlaced video frame in two passes: It draws every other scan line on its way down, and when it's done, it returns to the top 1/59.94th of a second later and fills in the missing lines to complete one interlaced video frame.

Inside the computer, your video playback hardware determines the order of the fields when it encodes a full frame digital image into an analog signal during playback. It's therefore important to render the digital frames with the field order expected by your hardware. In Adobe After Effects, the options are referred to as Upper or Lower Field First. (Some systems also user “Odd” and “Even” field first, but this is dangerous method to rely on as some systems start counting at 0 – an even number – while others start counting at 1 – an odd number.) Generally:
 

If you are uncertain (or simply don't believe us), to determine your playback system's field order, run through this simple test: Animate a fast moving still image at the system's optimum resolution (i.e. 720 x 480), and render the comp using “Upper Field First” from the Field Render popup (an example from After Effects is shown below). Render the comp again as “Lower Field First” and play back both movies through your hardware. The version that looks noticeably smoother is using the correct field order for your hardware.


 

To output a field-rendered animation in After Effects, select the correct field order from the Field Render popup in the Render Settings dialog. Other programs have similar settings for the sequences or timelines.

Field Rendering in Progress

When you field render an animation in After Effects, you'll notice that it makes two render passes for each frame, before it compresses and writes an interlaced frame to disk. So when rendering an Lower Field First movie:
 

  1. After Effects starts at 00:00L (the L stands for Lower Field) and figures out what all the layers look like at this starting point (i.e., their position, scale, opacity, effects, etc.). This frame is then set aside for a moment in RAM.
  2. Next it advances the composition 1/59.94th of a second (i.e., a position between 00:00 and 00:01), and renders frame 00:00U (or Upper Field) after again figuring out what all the animated layers would look like at this point in time.

  3. Now, since you specified “Field Render: Lower Field First”, After Effects keeps all the odd lines from 00:00L (for Field 1), and all the even lines from 00:00U (for Field 2), interlaces them, and saves the image to disk as frame 00:00. The resulting interlaced movie would be “lower field first” since the lower field belongs to an earlier point in time than the upper field.

Degrees of Separation

When After Effects imports an interlaced movie, it may default to treating the video as if it were non-interlaced (i.e., the entire frame belongs to the same point in time). If you then use this source in a composition and scale or rotate the layer, you'll most likely mix information from two points in time. Even if you field-render on output, a mixture of two different fields from the source movie would end up on each field in the output movie. The result is a shuddering mess, often referred to by the highly technical term “field mush.”

To avoid field mush, every time you import an interlaced movie, make sure its fields are separated in the correct order. In After Effects, select the footage in the Project panel, open File > Main > Interpret Footage, and use the Separate Fields popup as show below. This tells After Effects to separate each frame into two separate fields and space them 1/59.94th of a second apart. Then it constructs a full frame out of each field which is displayed in the Comp window. (Later versions of After Effects sport a Preserve Edges option which does a better job of interpolating fields to reconstruct frames; we always enable this.)


 

Set the field order for interlaced source movies in the Interpret Footage dialog box. Provided you field render your final output, both fields will be sent to their respective fields in the output movie.

Now when you Field Render your animation, interlaced source footage will be processed correctly (i.e., information from the first temporal field of the source will end up on the first field of the output movie, and pixels from the second field of the source will be routed to the second field on output).

 

 

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

Postscript

This article was originally written in 1998, and was updated in 2008 (640×480 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:
 

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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