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The Hi-Def Checklist

Questions to ask and issues to consider when you tackle a high-definition graphics job.

By Chris and Trish Meyer | September 16, 2005

The darker areas to the left and right of these images show what happens when a 16:9 image is "center cut" to create a 4:3 version. Make sure information such as lower thirds survive this cut. Image courtesy Belief and HGTV.

Many motion graphics artists are tackling their first high-definition jobs. In some respects, hi-def is just like normal video; only larger. However, hi-def also comes with a number of issues which can throw some major curves at you. As with all problems in waiting, it's best to solve them before you start, rather than when you think you're almost finished. Here are a series of questions you need to ask, and what the implications are - both technical and artistic - of the answers you may get.

Frame Rate Issues



When working with standard definition video, frame rate is dictated by the format, such as 29.97 frames per second (fps) for NTSC and 25 fps for PAL. The video is also probably interlaced, which means there are two fields - captured at different points in time - per frame. If the footage originated on film at 24 fps but is to be played at 29.97 fps, chances are that is slowed down to 23.976 fps and then has had 3:2 pulldown applied using a pattern of "split" (interlaced) and "whole" (non-interlaced, or progressive scan) frames to spread every 4 film frames across 10 video fields.

However, the Advanced Television Standards Committee (ATSC) - who set the hi-def standards - allow frame rates of 23.976, 24, 29.97, 30, 59.94, and 60 fps progressive scan; the 29.97 and 30 fps variants may also be interlaced or progressive! Therefore, the first question to ask the client is: What frame rate should I use for the final animation I hand you? That's the rate you should use when building animations (in other words, what to enter for the composition's or sequence's frame rate). If you are to deliver an interlaced file, hi-def is always upper field first, in contrast to the lower field first order of DV.

The second question you need to ask is: What frame rate is the footage you're giving me? This is more devilish than you may think. Quite often, a studio will not deliver footage as a QuickTime or AVI movie with the frame rate already embedded; it will come as a sequence of TIFF, SGI, or even Cineon DPX frames, with no inherent frame rate attached. It will be up to you to then assign the correct frame rate when you import the footage into a program such as Adobe After Effects (see below).


Adobe After Effects has a preference to set the default frame rate of sequences you import.




If you forget to set this correctly, you can fix it later in the file's Interpret Footage dialog.



Even if you receive footage as a QuickTime or AVI, you cannot necessarily trust the frame rate embedded in the file. HD decks can not be relied upon to automatically detect the frame rate a tape was shot at, and might play it at a different speed. Verify that the frame rate was transcribed from the tape - and hopefully, from the shooting notes - themselves. (And if you are the shooter, please remember to mark these details on your tapes.)

In most cases, the answers to these first two questions will probably be 23.976 fps, progressive scan. However, it is worth double-checking, as some cameras can shoot at either 24 or 23.976 fps. The difference can cause subtle audio synchronization issues that become noticeable within a minute, and which need to be corrected by speeding up or slowing down the audio track. For example, if the audio track was meant to go along with 24 fps footage, but you are conforming all of your footage to 23.976 fps for a final delivery, you need to slow down or stretch the audio track by 100.1% (some software thinks in terms of speed rather than stretch; in that case, set the speed to 99.9%).

Beware of complacency! On one recent job, virtually all of the dozens of clips we received (which were delivered as SGI sequences) were at 23.976 fps, except for one which was at 29.97 fps with 3:2 pulldown added. If you have footage that is supposed to be progressive scan - such as all 23.976 or 24 fps footage - but you see the telltale "comb teeth" of interlacing on moving objects (see below), you know something is wrong. Set the field order to upper field first, and ask your software to detect the pulldown sequence. And don't automatically trust what it says: Manually step through the resulting footage to make sure you don't see interlacing artifacts. In After Effects, Option+double-click on Mac (Alt+double-click on Windows) on the footage item in the Project window to open it in a special Footage window, and use the Page Up and Page Down keys (above the normal cursor arrows) to step through several frames to make sure you don't see those artifacts. If you do, go back and try different pulldown phases until those artifacts disappear.

Look for the tell-tale "comb teeth" look around moving objects to see if your hi-def sources are interlaced. When interlaced, hi-def footage is always upper field first. Footage courtesy Artbeats.



Another important implication of frame rate is the smoothness of motion. When objects move, you get to see their new positions only 40% as often at 23.976 fps than you would at 29.97 fps interlaced. That means formerly smooth motion can now take on a strobing appearance.

The easiest answer to this problem is to add motion blur. Hopefully, your program supports this; if it doesn't, or if you received sources that were not rendered with motion blur, you may need to add it using a plug-in such as RE:Vision Effects' ReelSmart Motion Blur. The downside of this added blur is that you will lose some clarity on items such as fast-moving text (see below). You may need to back off on the motion blur amount to find a compromise between smoothness and readability. Render tests, and run them by the client before delivering the final.


Motion-blurred text that is perfectly readable at 29.97 fps interlaced (top) can be much harder to read at 23.976 fps progressive, even with the same blur shutter angle (above). For fast-moving objects, adjust the motion blur amount to balance readability off against strobing.


next page: frame size issues

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Comments

ManyArms: | August, 18, 2010

Wow, I’m really surprised there’s not any comments on this. I’ve been needing some sort of relevance to a checklist like this for some time! Thank you..

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