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Filed under: CS4 •Motion Graphics •Post Production •

Unsafe Areas

Two of the more intriguing new features in After Effects CS4 (and other members of the Adobe Creative Suite 4 family) are updated pixel aspect ratios, and the ability to display 4:3 “center cut” safe area guides inside 16:9 widescreen compositions. I applaud both. But as I work with them more, I realize there are some slight errors in where the safe area guides are being drawn, particularly in light of the new pixel aspect ratios. First I’ll cover the relatively minor 4:3 case, and then move on to the far more egregious 16:9 case.

Safe Area Review

As you probably know, you’re not supposed to use the entire frame when composing a shot or graphics for video; you have to compensate for a TV’s bezel cropping off the outer edges, and for old CRTs distorting the image as you get close to that edge. (In reality, these problems are all but eliminated by digital video and flat panel displays, but unfortunately, video production remains constricted by legacy issues…) Thus, we have Action Safe and Title Safe zones:

• Action Safe is inset 10% from the outer edges of the frame (5% on each side). Don’t put anything you expect the viewer to see outside of this area.
• Title Safe is inset 20% from the outer edges of the frame (10% on each side). Don’t put anything that viewer has to read outside of this area.

In Preferences > Grids & Guides, After Effects defaults to drawing these guides at the 10 and 20% marks. It even compensates for non-square pixels.

The default safe area overlays in After Effects. The red zone is beyond Action Safe; the yellow zone is beyond Title Safe but inside Action Safe; the green zone is inside Title Safe.

4:3 Clean Aperture

(This section is only for those who like to be pedantically correct. The far bigger problem is the 16:9 center cut case; feel free to skip to the next page if you like.)

One of the biggest under-the-hood changes in After Effects CS4 was the introduction of new pixel aspect ratios for standard definition compositions and footage. They recognize that the 720 horizontal pixels in a NTSC or PAL D1 or DV video frame actually contains more information than the viewer is supposed to see. These 720 pixels are referred to as the Production Aperture - all of the pixels you’re supposed to move through the production chain. In 525 line “NTSC” systems, only 712.8 (rounded down to 712) of those pixels make up the Clean Aperture - the actual 4:3 image for viewing (minus the TV bezel etc.). In 625 line “PAL” systems, it’s 702.9 pixels, rounded down to 702. (If you want to make your head explode, I wrote a big article on this subject elsewhere on PVC.)

You could certainly argue that the safe areas should be based on the Clean Aperture (which contains the 4:3 image), not the Production Aperture (which contains additional pixels for technical reasons). But if you place a 712x486 image inside a 720x486 composition, you will find that the safe area guides no longer align:

The true safe areas for a Clean Aperture image (the colored boxes) are slightly inset on the left and right from the default guides in After Effects. The black strips down the sides are the difference between Safe and Production apertures. The differences are larger for PAL footage and compositions.

So, if you wanted to be pedantically correct, in the NTSC case, you would want to inset the Action Safe areas approximately 10.1% on the left and right (but still 10% on the top and bottom - it’s only pixels on the left and right that get cropped), and 20.2% on the left and right for Title Safe. In the PAL case, these numbers would be approximately 10.2 and 20.5.

As the differences are so small (not to mention how imprecise these areas actually are in practice, that it’s not possible to enter different vertical and horizontal safe area guides in After Effects, and that every other program on the planet currently ignores this issue), blissful ignorance is probably your best approach: leave After Effects at its defaults. But in the case of creating template files in Photoshop and the such, it would be nice to see Adobe update the guide placement to the pedantically correct locations in the future.

next page: problems with the 16:9 center cut case

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Thank your lucky stars you don’t live in the UK, where we have 16 by 9 anamorphic SD digital broadcasting, which is 14 by 9 centre cut, and letterboxed into a 4 by 3 image for analogue broadcast!  Fun stuff!

BBC guidelines on it can be found here for those who are interested:

http://www.bbc.co.uk/commissioning/tvbranding/picturesize.shtml

Posted by Nick Shaw  on  06/12  at  08:29 AM

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To follow up on Nick’s comment, I gave same examples of the 16:9, 15:9, and 14:9 crops in this article:
http://provideocoalition.com/index.php/cmg_keyframes/story/open_wide/P2/

Also, after a conversation with one use who has ran afoul of the BBC’s guidelines, it seems that the BBC is adding an additional 0.5% insert to the center cut safe areas - so to be extra safe, set the center cut guide values to 33.0 & 40.5%, respectively.

As Nick noted above, you can view the BBC’s very thorough picture size documents - including downloadable templates - at:

http://www.bbc.co.uk/commissioning/tvbranding/picturesize.shtml

Posted by Chris Meyer  on  06/19  at  10:11 PM

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Another note, re: Nick Shaw’s comment above: BBC and EBU rules might still bite you…NTSC producers who have material they want to distribute internationally (i.e. convert to PAL at some point) should be looking at the worst case scenario and planning accordingly. For example, if applying the current conventions specified by the BBC and EBU (protecting a 14:9 area for action and a 4:3 area for titles), you must consider that BBC has the strictest 4:3 protection area (65% cut-out of clean aperture for titles), and that additionally, PAL’s clean aperture is only 702 out of 720—meaning any material that might one day end up in PAL SD broadcast has to deal with an effective center cut of 63.3333% relative to production aperture.

Posted by .(JavaScript must be enabled to view this email address)  on  09/25  at  06:02 AM

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I made a chart noting these values:
http://pub.f11films.fastmail.fm/bbc+ebu-safeareas.html

Posted by .(JavaScript must be enabled to view this email address)  on  09/25  at  06:04 AM

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Sorry, bad link
http://pub.f11films.fastmail.fm/bbcebu-safeareas.html

Posted by .(JavaScript must be enabled to view this email address)  on  09/25  at  06:05 AM

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Just a quick note: The questionable defaults for widescreen center-cut safe areas in CS4 have been changed in CS5 to be the same as suggested in this article. (Still no tweak for production vs. clean aperture in those safe areas, but the difference is pretty small.)

I see a fair amount of material center-cut for 4:3 that has text chopped off on the sides; I suspect at least some of those designers may have used the questionable defaults in CS4.

- Chris

Posted by Chris Meyer  on  04/28  at  04:46 PM

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