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Filed under: Editing •Motion Graphics •Post Production •

Urban Legends of Video

Legend #2: You can add fields later

This one pops up on discussion lists every few months, and usually from a 3D user: “If I didn’t field-render my 3D animation, can I add fields later by field-rendering it out of After Effects?” You can understand the desire for this to be true – 3D renders usually take a long time, and field-rendering usually takes twice as long. But as the song goes, you can’t get something for nothing…

As we’ve noted above, each field in an interlaced video frame actually represents two different instances in time. This is one of the visual characteristics of video: smooth motion, because that motion is actually being sampled at twice the frame rate, and then interlaced into a single frame. Programs like After Effects are then able to extract these individual fields and treat them separately when it field-renders them back out again.

What happens if you bring a frame-rendered file into After Effects, and then field-render the output? Less than you might hope. When After Effects renders the first field of a frame it is outputting, it will look at a frame of your 3D render. And when it goes to render the next field, it will look at that same exact frame – even if you tried to separate the non-existent fields of the source movie.

There are a couple of ways you can indeed create different visual information for those fields. One trick some try to use is to enable Frame Blending in After Effects. When this feature is enabled, After Effects will create a crossfade between adjacent frames if a unique frame (or field) is not available. However, this does not create new motion; it just creates echoes of the image. This helps a little if your frame rate is slower than normal video, or if you are time stretching the source, but is not really a satisfactory solution to the field rendering issue.

A better solution is to use software or a plug-in which performs motion interpolation, creating new pixel locations for in-between moments in time. This software studies the patterns of the pixels from frame to frame, looking for similar groups, and the tries to figure out the path along which they moved between frames of the source material. We personally use RE:Vision’s ReelSmart Twixtor plug-in for After Effects. Twixtor gives a few different parameters you can adjust to help it track different types of source material. As you would expect, better tracking usually takes longer to render. But it’s usually faster than a 3D render, and might help you out of bind when the only options are either strobing motion, or a very long re-render. A comparison between different techniques is illustrated below:

Above are side-by-side comparisons of no frame blending (upper left), the default frame blending Frame Mix mode (upper right), Pixel Motion mode (lower left), and RE:Vision Effects’ Twixtor (lower right). Look at the forearm on right to see the differences; notice the wrinkles between the arm and body in the Pixel Motion example in the lower left.

(Update Note: After Effects version 7 and later has added Pixel Motion and Timewarp to supplement its built-in Frame Blending. Like Twixtor, they create new intermediate frames, and are based on The Foundry’s Kronus technology from their Furnace plug-in set. We personally prefer Twixtor over Pixel Motion and Timewarp, but you may want to try these alternatives first as they are included free with After Effects 7 and later.)

Keep in mind that your renders don’t have to be field-rendered to work with video. Many render their 3D animations at 24 fps (the common film rate), and let their video applications add the equivalent of 3:2 pulldown to make the frame rates match. Rendering whole frames at 29.97 (NTSC) or 25 (PAL) frames per second is a perfectly acceptable compromise. I would just personally make sure I included a bit of motion blur in the render (or added it later with RE:Vision Effects’ ReelSmart Motion Blur plug-in for After Effects), to help cover any strobing that might result from not capturing the motion in your animation at every field.

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