[ This article was first published in the December, 2009, issue of
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Florian Gintenreiter writes:
I have been in the video-business for a while and I do know what FIELDS and FIELD ODER are, but sometimes things happen that make me doubt my “wisdom”.
For example: I have 1080i XDCam (natively LOWER FIELD FIRST) footage on a timeline that is 1080i ProRes (LOWER FIELD FIRST). its a TV commercial, so there is a couple of video-clips making up the first half of the spot and animated logos and graphics (PSD Files imported into FCP) making up the latter half. I render out a Quicktime-Movie of the finished film, expecting both halves of the film showing the typical jagged “lines” of interlaced video, but behold: I have those “lines” in the video-footage, but the animated graphics at the end don’t have them.
When I now put the file into compressor for downrezzing to 720×576 PAL and set the source video to LOWER (Which is the field order of XDCam and the FCP ProRes Sequence) and the OUTPUT field order to PROGRESSIVE (Frame Controls deinterlace ON) the VIDEO-part of the commercial gets deinterlaced properly, but I can see aliasing in the charts and graphics at the end, as if one of the fields just had been doubled.
When I set the source-video-field-order to NONE, the graphics are fine when downrezzed, but the video-part shows strange artifacts, where there should be smooth motion blur. Artifacts I would expect from video being downrezzed with wrong settings.
When I switch the sequence in FCP to field order NONE (from LOWER FIELD FIRST) it looks like the sequence has no fields anymore, but shows aliasing on the full HD preview monitor.
Can you maybe shine some light on how FCP deals with field order especially when using graphics and animating them within FCP?
Larry replies: NTSC, PAL, and some HD formats are interlaced. That means that they record every other line of your image, then, a fraction of a second later, they record the remaining lines.
There are several technical reasons why they do this, but when you create video for the web, this interlacing drives us all nuts.
NTSC and PAL are both lower-field (or even-field) dominant. This means they record all the even-numbered lines of your image first, then the odd-numbered lines a fraction of a second later.
Most HD formats are upper-field dominant, which means they record the odd-numbered lines first. However, as you point out, not all HD formats follow this convention.
When you create an animated logo in After Effects, or other graphics software, interlacing is optional. By default, computer graphics are progressive, which means all lines are recorded at once.
You can export an image as interlaced, but, most of the time, images are exported progressively. This means that when a clip is imported into a Final Cut sequence and interlacing is added, you won’t see it because all the lines were recorded at the same time.
This explains why you didn’t see interlacing when you first exported the file.
When you down-sample the video from HD to SD, the aliasing you see is not caused by interlacing but by the decrease in resolution between HD and SD. This is especially noticeable in computer graphics, which tend to contain fine lines and detail which the video camera ignores.
This is the answer to your second question.
Setting the field order to NONE creates video which is not optimized to the PAL standard, which requires interlacing, which is why your video looks as bad as you mention in your third question.
A few other things to keep in mind:
The world will be a better place when interlacing goes away. However, that won’t be anytime soon.
NOTE: By the way, here’s an article I wrote that describes how to see the interlacing in an image in Final Cut Pro.
And here’s an article I wrote that explains fields and frames in more detail.
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