Pre-scaling graphics for HD nonlinear editing
May 1, 2005 12:00 PM, By Mike Nann
When not to pre-scale
The above discussion outlines how pre-scaling can minimize or eliminate the problems associated with mixing full-resolution graphics with HD compressed video that has been sub-sampled from the full HD raster.
However, some NLE systems support compression formats that can handle the full HD raster. These systems also feature full-raster internal processing, and can easily combine full-size HD graphics with video content in these compressed formats without any internal re-scaling of video or graphics. These systems maintain the same frame size (1920×1080 or 1280×720) throughout the workflow.
Similarly, NLEs that support uncompressed HD editing will also handle the full HD raster when using uncompressed media. Full-size HD graphics can be mixed with uncompressed HD video clips without re-scaling. Working with uncompressed HD video creates other issues. For instance, many of the new affordable HD editing systems offer better real-time layering and effects performance with compressed media than with uncompressed (if they support uncompressed at all). And naturally, working with uncompressed HD video requires far more storage and higher bandwidth than when operating in the compressed domain.
The moral of the story is that it is important to thoroughly understand how an NLE internally processes both graphics and video. This will help operators make the best decisions as to whether it's beneficial to pre-scale graphics before ingesting them into the NLE.
If the NLE provides full-quality, real-time performance on uncompressed video or full-raster compressed formats, then pre-scaling may not be advantageous. If, however, you plan to work in the compressed domain with sub-sampled compression codecs (including native acquisition formats), it may be beneficial in terms of quality or performance to pre-scale first.
Protect your HD
With the enhanced pixel resolution of HD, there's a natural inclination to take advantage of it by using detailed and intricate graphics. However, be careful because the extra detail can end up working against you when the finished HD project is distributed.
As explained above, if the NLE system will be working at less than full raster, the graphics may be down-scaled or sub-sampled, resulting in lower horizontal resolution. It's also important to remember that, for the foreseeable future, a high percentage of HD content will be downconverted to SD for at least some of its distribution, which means a loss in both vertical and horizontal resolution.
Detailed graphics that look exceptional when created at full-raster HD resolution may lose considerable detail (making elements such as text all but unreadable) when converted to SD for playout.
Here are a couple of solutions. If all of the graphics will effectively be used as downstream keys in the NLE (superimposed over other video layers), it's often best to first downconvert a version of the finished HD project without the graphics in place. Then, add the graphical overlays (which have been specifically designed for SD) separately to this downconverted version. This process protects the HD version, while maintaining the best possible graphics quality and readability for the downconverted SD distribution.
This isn't always possible, of course, as graphics are often layered between other elements in the overall project. Even so, keep in mind the potential future downconversions that may occur when creating graphic elements, and you can avoid many of the associated pitfalls.
Mike Nann is the technical marketing manager, Professional Post Production, for Leitch Technology.
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