Pre-scaling graphics for HD nonlinear editing
May 1, 2005 12:00 PM, By Mike Nann
 Leitch’s VelocityHD supports both uncompressed and full-raster compressed formats in real-time. |
At first glance, the creation and scaling of graphics to the correct size for incorporation with high-definition video in a nonlinear editing system may seem straightforward. However, the adage “looks can be deceiving” comes to mind.
Graphics-related issues such as color space conversions, color sampling and pixel aspect ratios have been covered in detail before, and the full raster sizes for HD video frames are well-known — 1920×1080 and 1280×720. Simply create your graphics to these sizes, and you're ready to import them into your NLE — or so it seems.
One size doesn't fit all
While this simple process is usable for bringing HD graphics into modern editing systems, there often are internal factors within an NLE that can make this workflow less than ideal. For instance, pre-scaling full-size HD graphics to sizes other than the full raster may be advantageous both in terms of quality and productivity. Otherwise, some NLEs may perform some unexpected automatic re-scaling of the graphics that editors would otherwise prefer to control. And, on some NLE systems, pre-scaling the images may result in improved real-time layering and effects performance. Counter-intuitively, in many cases, the best results in both quality and performance may be achieved by pre-scaling graphics to lower than full raster size, before bringing them into an NLE.
The key factor that can make pre-scaling HD graphics desirable relates to the frame size (in pixels) of the graphics, versus the actual frame size at which the NLE processes the HD video. For example, not all 1080i is created equally. While NLE operators may think they're mixing graphics and video of the same frame size, that might not actually be the case.
 Figure 1. Common acquisition formats and their native frame sizes. Click here to see an enlarged diagram. |
This issue arises when full-resolution (1920×1080 or 1280×720) graphics are mixed with HD video that is being processed natively in a compression format that has been sub-sampled from the full HD raster. While formats such as Panasonic's D-5 HD and Sony's HDCAM-SR support the full raster (as does the HDV format in its 720p variant), many other common HD formats do not. For instance, HDV, as well as Sony's HDCAM, uses 1440 luma samples per line, for a 1440×1080 recorded frame size. DVCPRO HD sub-samples to 1280×1080 in its 1080-line mode, or 960×720 for 720p.
If the nonlinear system works with media in any of these acquisition formats natively, the addition of a full-size graphic in the editing process may result in the mixing of a 1920×1080 graphic with video at a 1280×1080 or 1440×1080 frame size. Various formats and their native frame sizes are shown in Figure 1.
Similarly, as an alternative to processing HD media natively in its acquisition format, some NLE manufacturers offer their own compression schemes, optimized for post-production. While some of these codecs support the full HD raster, others are sub-sampled similarly to the acquisition codecs mentioned above.
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