We’re going through our book Creating Motion Graphics with After Effects 5th Edition (CMG5) and pulling out a few “hidden gems” from each chapter. These will include essential advice for new users, plus timesaving tips that experienced users may not be aware of.
There are a number of nonintuitive technical issues – including interlaced fields, frame rates, frame sizes, pixel aspect ratios, safe image areas, and color spaces – that differentiate video from images destined to be displayed on a computer. You can’t just ignore them; they must be handled properly to ensure your final work appears on television as you intended – otherwise your final image may be mangled (not to mention your relationship with your client). Chapter 41 of CMG5 contains an overview of many of those topic; we’ll share a few gems here.
Common Field Orders
Let’s dispel a common myth up front: All high definition footage you’re ever likely to see is either progressive scan (no interlacing), or upper field first. All too often I see users advise others that HD is lower field first, usually based on a mistaken rationale such as “DV is lower field first, so HDV must be lower field first.” It’s not. Once upon a time there was an experimental Japanese HD format that was lower field first, but that’s it; all other HD is either progressive or upper.
In the standard definition world, the picture is less clear (no pun intended). All DV is lower field first; all non-DV PAL is upper field first; NTSC is usually lower field first, but there are upper field NTSC sources out there – such as from the Aurora cards, the first D1 Avid systems, and some earlier uncompressed capture devices.
Of course, not all video footage is interlaced. Check to make sure you see the telltale signs of interlacing (such as in the image at right – footage courtesy Creative License) before separating fields. But if you do see interlacing, then in almost all cases you will be better off if you separate those interlaced fields in the After Effects Interpret Footage dialog.
Fixing Frame Rates
The most common frame rates for video in North America are 23.976 or 29.97 frames per second (fps). Don’t be tempted to round these numbers to 24 or 30 fps: The difference sounds tiny, but it adds up to a frame every 33.3 seconds (or a field every 16.7 seconds), which quickly becomes noticeable. Mismatches in frame rates between 30 and 29.97 can cause audio/video synchronization errors, as well as skipped or repeated images.
If you import footage, and the thumbnail along the top of the Project panel says its frame rate is 29.97 or 23.976, but you still encounter problems with frame being skipped or repeated, you may need to conform the clip’s frame rate to clean up internal timing errors in the file. This used to be a problem with Avid footage in particular. To do so, select the clip in the Project panel, click the Interpret Footage button at the bottom of the Project panel, and type the desired number into the Frame Rate >Conform field.
Pulling Out the Pulldown
Film usually runs at 24 fps. When film is transferred to NTSC video, it is slowed down by 0.1% to 23.976 fps, and every four frames of film are distributed across five frames (ten fields) of video through a process known as pulldown.
Some video cameras also shoot at this rate and add pulldown to simulate film-like motion. The result is a staggered set of “whole” (progressive) and “split” (interlaced) frames, as pictured at right.
You can treat the result as normal interlaced footage, but it is preferred to remove the pulldown sequence so that you are working with the original source frames inside a composition. In particular, you want to do this before tracking, stabilizing, masking, painting, or rotoscoping the footage.
Clean versus Production Aperture
The frame sizes defined for standard definition video have a quirk: They contain more pixels than are supposed to be used. When engineers designed these formats, they built in a few extra pixels along the edges to allow for artifacts caused by video processing algorithms and the like. The full captured frame is referred to as the Production Aperture; the smaller area inside which refers to the actual image is referred to as the Clean Aperture.
For many years, video hardware and software ignored this difference, using just the Production Aperture size for everything. A side effect of this choice was that they also used very slightly wrong pixel aspect ratios to make the math work – otherwise, the extra pixels in the Production Aperture made it look like the frames were wider than 4:3 or 16:9.
As of After Effects CS4, Adobe decided to rectify this and started using the technically correct pixel aspect ratios. The suggested square pixel sizes on the previous page encompass the entire Production Aperture; that’s why they have image aspect ratios wider than 4:3 or 16:9 (for example, 720 ÷ 534 = 1.348; not the expected 1.333).
What does this mean for you? Less than you might expect:
- None of the original, non-square-pixel frame sizes have changed. This means you can interchange files with other programs without worry.
- When you create imagery using the new, correct square pixel frame sizes, be aware that a few pixels on the left and right sides are extra. For example, the Clean Aperture area for 4:3 D1 NTSC is 712 x 486, meaning four pixels along each edge in your 720×486 pixel composition will be ignored when displayed.
- When using square pixel compositions or artwork created at the “old” sizes, scale the width to match the target non-square pixel comp, and allow those extra pixels on the top and bottom to be cropped off. This is shown below: The red outline is the full square pixel image; note a few pixels along the top and bottom extend onto the gray pasteboard.
- Know that from now on, your “perfectly round” circles will actually be perfectly round.
This issue does not affect any of the anamorphic HD formats; in all of their cases, Clean and Production Apertures are the same.
Chris wrote an exhaustive piece on the history of non-square pixels, should be you interested in overwhelming your friends and colleagues on the subject.
Converting Between DV and D1
Some incorrectly believe that an NTSC DV frame of 720×480 pixels has a different pixel aspect ratio than an NTSC D1 frame of 720×486 pixels. Actually, they both have the same pixel aspect ratio; DV is just a D1 frame with 6 lines cut off to save on compression when going to tape. (Remember, DV originally intended as a consumer format; not for real video production or broadcast.)
There are times when you need to convert between the two formats. The easiest approaches – scaling one to fit the other’s composition size, or centering one format in a different format’s comp – both happen to be the less-optimal way to do it. On CMG5’s DVD-ROM, we included a one-page bonus chapter on the best way to convert between these formats; you can click here to download that PDF file.
Trish and Chris Meyer share seventeen-plus years of real-world film and video production experience inside their now-classic book Creating Motion Graphics with After Effects (CMG).
The 5th edition has been thoroughly revised to reflect the new features introduced in both After Effects CS4 and CS5 (click here for free bonus videos of features introduced in CS5.5). New chapters cover the new Roto Brush feature, as well as mocha and mocha shape. The 3D section has been expanded to include working with 3D effects such as Digieffects FreeForm plus workflows including Adobe Repouss©, Vanishing Point Exchange, and 3D model import using Adobe Photoshop Extended. The print version is also accompanied by a DVD that contains project files (CS5-only) and source materials for all the techniques demonstrated in the book, as well as over 160 pages of bonus chapters on subjects such as expressions, scripting, and effects.
We will be pulling a few “hidden gems” out of each chapter to share on ProVideoCoalition.com roughly every week. These will give you a taste for the multitude of time-saving tips, not-obvious features, little gotchas, and other insider knowledge you will find in CMG.
FTC Disclosure: We receive software from Adobe to help us create our books, blogs, and videos. Aside from that, we have also used their software for nearly 20 years to make a living creating commercial work for clients – so the tips we share are based on real-world experiences, not as promotional material for Adobe.
The content contained in our books, videos, blogs, and articles for other sites are all copyright Crish Design, except where otherwise attributed.