3dRender.com : Glossary : Fields, Interlaced Video, Fieldrate Rendering

Field - one half of a video frame, consisting of either all of the odd-numbered scanlines or all of the even-numbered scanlines in the frame.

Interlaced Video - Video systems in which each frame consists of two video fields.  Regular TV sets use interlaced video.

Fields in Interlaced Video
In interlaced video, the scanlines of one field and then the other are displayed on the TV screen to produce a full frame.

To produced interlaced video, television cameras actually shoot two separate exposures per frame, to capture the two video fields.  The odd numbered scanlines (lines 1, 3, 5, etc.) are shot first, then another field is shot to capture all the even numbered scanlines (coming back up to record lines 2, 4, 6, etc.)  When the two fields are interlaced to fit together on TV, a frame is created that takes advantage of all of the scanlines from both fields to reproduce the full-resolution picture.

The advantage of splitting the frame into two fields is that moving subjects are effectively sampled at twice the framerate, for a smoother, more accurate reproduction of motion.  Instead of having the shutter open only 25 or 30 times a second, it opens and captures a field 50 or 60 times a second.  This means that any moving subjects are scanned twice as frequently.  Also, television sets appear to flicker less because the beam passes through the full height of the screen twice as fast.

Fieldrate Rendering - Most 3D software has an option to render in video fields instead of full frames, simulating the exposure of a video camera.  (If your 3D software doesn't render in fields, or you don't like the quality of antialiasing in its fieldrate output, you can also render fieldrate video by rendering full frames at twice your normal framerate, then use a compositing program such as Adobe After Effects to produce fieldrate video.)

Fieldrate rendering gives smoother motion, and can even reduce or eliminate the need to render motion blur, which can save rendering time.  Fields are a staple of corporate video production and "flying logo" style animated graphic designs.  However, rendering on fields adds to the artificial perfection of computer animation, and makes your work look more like video and less like film.  Character animation generally shouldn't be rendered on fields, even when output to video.  Quality character animation depends on snap and strength of pose which are hurt, not helped, by fieldrate rendering.

Related Terms: Progressive Scan, Scanline, 3:2 Pull-Down.

Source: Digital Lighting & Rendering by Jeremy Birn, definitions and 3D images adapted from the book by permission.  This page © 2001 by Jeremy Birn.