method for seeing fields

FCP7 is fallible with field dominance. Even the FCP7 User Guide can't keep it straight. On one page it says:

Quote

If a format uses Upper (Odd) field dominance, the first field scanned (called field 1) is made up of the odd lines in the video frame. Lower (Even) field dominance scans the even lines first.

On another page it says:

Quote

In Final Cut Pro, there are two options for field dominance:
Upper (field 2 is dominant, so the second field is drawn first);
Lower (field 1 is dominant, so the first field is drawn first).

Is the second quote insane or am I missing something?

Interlaced video "frames" consist of two consecutive fields. It is only because fields are associated with frames that "Upper" and "Lower" dominance has meaning. "Dominance" here means coming earlier. Upper dominance means lines 1, 3, 5, etc. comprise the earlier of the two fields. Lower dominance means lines 2, 4, 6, etc. comprise the earlier of the two fields. It is presumed that the frame has an even number of lines. Otherwise "Lower" is a bit of a misnomer.

I'm probably not alone in having moments of panic: "is this interlaced clip, as rendered in FCP, really Upper or really Lower dominant?" I've always wished FCP allowed field-to-field stepping, as Avid does, but FCP won't. So I've worked out a method for determining within FCP whether an interlaced clip, as rendered, is Upper or Lower dominant. The method uses a frame in which the lines 1, 3, 5, etc. are magenta and the lines 2, 4, 6, etc. are green. A 1920x1080 version of this frame is downloadable here; 720x576 version here. The frame is placed on the track over the questionable clip, and stretched as desired. Next Modify > Composite Mode > Multiply is used to apply the strong magenta filter to the odd lines and the strong green filter to the even lines of the frames. Next a render is necessary. Set the canvas to 100% scale. Now all motions are colorized with green and magenta areas.

It takes some care to interpret the green and magenta areas. When a light thing is moving in front of a dark background, the magenta area shows locations of the thing in the odd lines, and the green area shows locations of the thing in the even lines. But when a dark thing is moving in front of a light background this is reversed. For example, suppose a backlit hand is waving in front of the sky and you see magenta at the earlier position of the hand, green at the later position of the hand. The clip is Lower dominant.

For a moving thing, earlier time corresponds to the trailing edge; later time corresponds to the leading edge. Therefore the interpretation rules are:

• dark moving thing with magenta leading edge -> Upper dominant
• dark moving thing with green leading edge -> Lower dominant
• light moving thing with magenta leading edge -> Lower dominant
• light moving thing with green leading edge -> Upper dominant

There is a second way to use the composite that avoids all this reasoning about light and dark. Put a magenta filter over one of your eyes and a green filter over your other eye. (I used Wratten #32 and #58; use what you have.*) Now look at a frame on the monitor with each eye separately. Your magenta filtered eye is seeing just lines 1, 3, 5, etc. Your green filtered eye is seeing just lines 2, 4, 6, etc. It is easy to stop on a frame with some action and then decide which field is earlier in time based on what the two eyes see.

There is also a way to use the test frame without compositing, somewhat like a ruler. When a frame has motions in a vertical direction, the test frame can be slid horizontally (by adjusting the basic motion anchor point's first value) so its edge lies in the motion. Then using a weak magnifier you can see directly whether the earlier part of the motion corresponds to magenta lines (meaning Upper dominance) or to green lines (meaning Lower dominance).

*Red/cyan is a better choice than magenta/green for people who have old 3D glasses around.

Dennis Couzin
Berlin, Germany

Fiendishly clever!!

Tip- Hold the H key for a second to pop either Canvas or Viewer to 100%.

(And while we're doing "hold" keys, hold Z to fit active image to window.)

Not only Avid parks on fields-- Premiere Pro 6 offers it too!

- Loren

Today's FCP 7 keytip:
Advance to next/previous keyframes in a clip with Shift/Option-K !

Your Final Cut Studio KeyGuide? Power Pack
with FCP7 KeyGuide --
now available at KeyGuide Central.
www.neotrondesign.com

Loren, thanks for the word on Premiere and fields. When I eventually leave FCP7, probably for Premiere, that will be some consolation.

Dennis Couzin
Berlin, Germany

With very very few changes, I see what I wanted in FCP 8 in PP6.

Choice is good.

- Loren

Today's FCP 7 keytip:
Advance to next/previous keyframes in a clip with Shift/Option-K !

Your Final Cut Studio KeyGuide? Power Pack
with FCP7 KeyGuide --
now available at KeyGuide Central.
www.neotrondesign.com