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Tuesday, July 24, 2001

Urban Legends of Video

A trio of common myths and misconceptions that arise when working with video.

Like urban legends, there are a few pieces of “conventional wisdom” that float around the motion graphics and 3D communities about how to handle video. They are oft-repeated, but several are simply not true. Some are based on wishful thinking; some on a germ of truth; some from articles or manuals which are incorrect. Yes, you probably already know all of these – but they certainly have caught out colleagues of ours.

Not surprisingly, many of these legends are based around the subject of frame rates and interlaced fields. Fields in particular are an area where traditional video diverges perhaps the most from the computers we’re creating our video on, and for that reason are easiest to misunderstand.

Legend #1: Field Order and Field Dominance are the same thing

To this day, a large number of software manuals and dialog boxes refer to the “field dominance” of footage that has either been captured into the computer or which you’re about to render. But the concept of field dominance does not exist inside the computer; what they probably mean is field order. This is not just a simple naming mistake – there are times when the two can be at odds with each other.

First, a refresher course on what fields are. A video frame is not drawn on the screen from top to bottom. Legend has it that early television displays had a problem where the phosphors in the screen could not hold their luminance long enough. This meant that by the time the video raster was drawing the bottom of the image, the top was already fading. When the raster went back to the top to draw the next frame, the sudden change in brightness was noticeable.

The workaround that was to skip every other line as the image was drawn, reaching the bottom twice as fast. This meant the raster could get back to the top of the screen in half the time, where it could then go about drawing the lines it had to skip the first time through. The result was better averaging of the overall luminance on the screen, resulting in less noticeable flicker.

These two half-drawings of a frame are referred to as fields. Each field contains every other line of the final image, offset in order to mesh with each other: one field starts at the very top and paints in all the odd numbered lines; the other starts one line down, and paints in all the even numbered ones. As a result, they are often referred to as being interlaced. This is illustrated in the figure here.

When video is recorded to tape, it is not recorded a whole frame at a time. One field is recorded, and then the other follows it later. This fits in with the order that they get drawn on screen. Indeed, when interlaced video is captured by a camera, it actually captures each field a half of a frame later in time than the field before. Understanding that these two fields represent different points in time is pivotal to understanding fields and interlaced video.

If all you have on tape is a stream of fields, which two make up a frame? The video signal actually has a signature in it that unambiguously labels fields as being either “field 1” or “field 2”. The term Field Dominance refers to where you decide a frame starts – before field 1 (resulting in Field 1 Dominant), or before field 2 (resulting in Field 2 Dominant). This is especially important when you edit two different video streams together, as you can’t have two of the same types of fields appear back to back before you draw one of the others. A large number of systems – including all computer editing systems I’m familiar with – are Field 1 Dominant. But this is not a rule, and some editing systems even allow you to switch their dominance.

Fields Dominance describes where in the field order on videotape an editing system places its cuts. In this example, cutting before Field 1 would make it Field 1 Dominant. This is not necessarily the same thing as field order inside a computer.

Editing • Motion Graphics • Post Production •

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Staggering Mistakes: Reversing Field Order

Chris Meyer | 03/03- 12:23 PM

PMW-EX1 - use regen timecode

Adam Wilt | 02/21- 09:32 PM

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