Spatial Preprocessing - A Practical Guide to Video and Audio Compression - page 565

34.6.5 Local or Global Cropping Set-Up?

Be aware that Cleaner and other compression tools usually only give you a global setting
for any of the parameters. If you want to apply preprocessing in a key-framed way, you
must load your footage into another tool such as Adobe After Effects.
Scrub through the video to check that the framing of the crop is correct at all times.
All frames should fit your crop. If necessary, pull it in for the lowest common denomina-
tor. If the movement of the area of interest is more than you are prepared to accept, then
run the file through Adobe After Effects. Take advantage of the motion-tracking and key-
framing features to reposition errant video segments so they fit your crop window better.
The first lines at the top and possibly as many as 10 lines at the bottom of video that
has been sourced from a consumer format are usually badly synchronized. This is where
you get tears, luma levels going funny, and half lines. You occasionally see some digital
text data and what looks like one line of color-bar video. This is a mixture of Vertical
Interval Time Code (VITC) and Vertical Interval Test Signals (VITS). You don't want to
have any of this in your compressed video, and unless you introduce better-quality equip-
ment or some kind of time-base correction and VITC/VITS line blanking, you will have to
deal with it by cropping.
After Effects allows frame-accurate cropping and repositioning. You could track the
edges and correct gate weave and misalignment due to multiple sources being edited
together, but this is very time consuming.

34.7 Alpha-Channel Coding

In Chapter 39, there is a discussion about shape-coded outlines that enable the video to be
encoded in layers and composited with a non-rectangular outline. This is one of the capa-
bilities of MPEG-4 Part 2. So far, none of the commercially available tools supports this
capability, but it is likely that they will in the future. It remains to be seen whether this
capability is added to H.264 or will remain a differentiating feature between H.264 and
MPEG-4 Part 10. Companies are likely to be reluctant to commit to an implementation
until the appropriate standard is revised.
When this capability is implemented, it would seem appropriate to deal with it at the
same stage as scaling and cropping since it is essentially a cropping operation.

34.8 Catching Some Zs

No, it's not time for a nap yet. It is all about that Z-axis now. We've sorted out T, X, and Y
with the temporal and spatial corrections. It is all scaled and cropped the way we want it,
but maybe the colors aren't quite right or it has lost some of its sparkle. That's all Z-axis
stuff, and we'll find out how to fix it up in the next chapter.