HD graphics
Aug 1, 2006 12:00 PM, BY NIR GOSHEN
Resolution
The resolution of the video standard is relevant when working with pixel-based graphics such as stills and video clips. Obviously, an HD image has a greater resolution than one created in SD. The 1080i HD format has almost six times more pixels than that of an NTSC SD signal. Other graphics elements will scale gracefully between resolutions when using a vector-based graphics system. When presenting pixel-based graphics in higher resolutions, you may find that the result will look blurred and diffuse if the pixel graphics are not of sufficient resolution.
If you have a small stock of high-resolution stills, now is the time to begin building a library with a resolution sufficient for HD. The library of HD-ready stills can be used for SD viewers as well, because the software will automatically downsample stills to the appropriate resolution.
That being said, downsampling might also show some artifacts if the graphics device does not support mipmap. Mipmap is a method where once an image is loaded, several versions of the image are automatically created in lower resolutions. Mipmap blends between the resolutions as per the total area that the image takes on-screen, creating an average-based resolution. (See Figure 1.)
Aspect ratio
Designers should carefully consider their image's displayed aspect ratio when the image needs to cover specific portions of the screen. Consider a lower-third banner graphic designed for SD that extends across the whole width of the screen. What portions of an HD widescreen would you want the graphic to cover? This is comparable to the difficulties encountered when adapting widescreen moves for 4:3 broadcast, except the width and high dimensions are reversed.
Figure 1. The mipmap of an image at 1/2, 1/4, 1/8 and 1/16 of the initial image resolution
What do you do for your HD audience when you don't have enough 16:9 graphics for broadcast? There are four main strategies for dealing with aspect ratio issues:
- padding the sides or the use of ears;
- trimming the height;
- stretching the height; and
- changing the layout of individual structural elements.
The first three are essentially stop gap measures. They provide methods of outputting HD graphics, which will keep your HDTV audience satisfied until you can start sending them true HD, 16:9-friendly graphics.
Padding the sides of the graphics will retain the aspect ratio of the graphics, but will leave blank areas on either side of the graphics. This strategy requires no effort because it can be completely handled by the graphics engine. However, graphics that previously stretched across the full width of the screen may look strange when they no longer reach the side screen's edges. This solution works well for graphics installations that only display side panels on either the left or right edge of the screen, because you can easily keep the panel aligned with its screen edge.
Because the market is still in transition for both the consumers and the content provider, it is likely that we will continue to see the HD ears for some time. The blank HD ear space can be used for sponsor logo, schedule information, game scores, etc. (See Figure 2.)
Figure 2. The center cut for 4:3 with extra ears for 16:9
When designing new graphics or redesigning existing graphics in a system that uses this strategy, the artist can accommodate both SD and HD aspects by extending background plates and other nonessential graphics to the edges of HD, while keeping important graphical elements within the safe area of SD.
In this case, the whole production is completed in HD and then downsampled to create the SD version. This approach is less expensive than producing both SD and HD graphics in parallel. Plus, only one HD graphics device is required, as long as it can handle both SD and HD.
Stretching the height of an SD signal to span across the height of an HD signal can be done without cropping or changing the aspect ratio of graphics as long as the graphics don't use the full height of the screen. This also requires no effort. The downside is that the proportion of the screen height used by the graphics will increase. The lower-third banner graphics designed for SD would cover only 4/9 of the height on an HD screen — not a desirable result.
Stretching the whole graphics horizontally to fill the HD widescreen also requires no effort. The downside is that it changes the aspect ratio of the graphics. Stretching will change perfect circles into ovals. However, unlike converting an SD signal to an HD signal, there will be no blurring, because the graphics system will render it with the appropriate resolution. Many graphics look fine after being stretched. Although, certain graphical elements such as circles, squares, natural photos and some text fonts will look odd after being stretched. Also, sponsors tend to dislike their logos being stretched or changed.
The best results are achieved by changing the layout of the individual structural elements of graphics designed for SD before broadcasting those same graphics in HD. While this requires more effort, it surpasses all other strategies on an aesthetic level.
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