Graphics has become a major portion of the branded identity of broadcasters. Glance at a logo and the network and broadcaster are instantly identified. Graphics production is a core process for all broadcast operations.
With the necessity to simulcast NTSC and DTV for the next few years, dual color space, pixel/raster formats and aspect ratios will require intelligent design and creation of GFX elements, infrastructure engineering and workflow processes. In addition the infrastructure must be agile enough to handle elements used for IPTV, Internet, cell phones and other emerging technology-based business models.
Designing GFX for SD and HD
The difficulties broadcasters face when creating graphics fall into two broad categories: presentation and color.
- Aspect ratio:16/9 HD and 4/3, SD (square pixel issue)
- Resolution: HD, SD and analog NTSC
- Display scaling (multiplatform, cell phone, IPTV)
- Color space: HD, SD and NTSC
- Color depth and range: 8-, 10-, 12-bits; ATSC 16 -235; CG 0-255 (both 8-bit)
- Color components. Y,U,V and R, G, B
By the numbers
Aspect ratio and resolution are dependent on each other (at least in the United States). HD will always be 16/9, SD 4/3. An efficient production process will create GFX once in such a way so that the elements can be used in both formats and look good. This must be true regardless of whether the GFX is overlayed on an HD source, center cut and downconverted to SD, or if it is used in an SD production and mixed in master control.
The difficulty becomes apparent when doing simple calculations for various formats on a pixel by pixel and line by line basis. Ignoring the interlaced/progressive issue, only a few pixel and line ratios are simple small numbers when comparing formats.
HD to NTSC
Vertical: 1080/480 = 2.25, 720/480 = 1.5: 9 to 8, 3 to 2
Horizontal: 1920/640 = 3, 1280 /640 = 2: 3 1920 or 2, 720 pixels are 1, 640 pixel
HD to HD
Vertical: 1080/720 = 1.5: 3, 1080 pixels are 2, 720 pixels
Horizontal: 1920/1280 = 1.5: 3 1080 pixels are 2, 720 pixels
HD to SD
Vertical: 1080/480 = 2.25, 720/480 = 1.5
Horizontal: 1920/720 = 2.666 …, 1280/720 = 1.7777, not small whole numbers
For the mathematically inclined:
Anti-aliasing of Images Using Line Sampling. Robert M. Kotredes under the direction of Ray Jones of Mitsubishi Electric Research Labs
Graphics created in either format must scale easily in whole numbers between formats, or edges will get soft due to interpolation. Anti-aliasing algorithms are used to correct this (see sidebar) and are applied to both alphanumeric characters and graphics.
864 x 486 canvas mapping
A compromise approach is to use an 864/486, 16/9 canvas to create GFX elements. GFX designers can feel comfortable because what they see on the display is what the element will look like geometrically in HD.
864/486 = 1920/1080 = 1280/720 = 16/9 = 1.777… square pixels
A center cut to NTSC will not cause visual distortion of the GFX element because NTSC has square pixels.
NTSC 640/480 = 4/3 = 1.333… square pixels
This canvas size also facilitates anamorphic conversions that can be layed to tape as SD on D-1 or DigiBeta. But care must be taken to use the correct settings in the graphics application to convert to non-square pixels.
864/486 = 16/9 = 1.777… square pixels
720/480 = 3/2 = 1.5 which on a 4/3 aspect ratio (1.333…) are not square pixels
Color space conversion
Computer GFX systems represent RGB color values in various bit depths (8, 10, 12). For 8-bits, computer color space runs the range 0-255 but for ATSC DTV the range is 16-235. Subsequently, if a GFX element is composited with video content later in the production process, non-legal colors may be produced.
Standards for color space are abound. For ATSC HD, SMPTE 274/ITU 709 while ITU 601 /SMPTE 259 is used for SD. NTSC color space should adhere to SMPTE 170 and SMPTE 240.
SMPTE EG 36-2000 details “Transformations Between Television Component Color Signals”. Heavy on the math, a glance at the color standards listing Appendix B reveals why there is so much confusion.
Color space legalizers, such as the Leitch DL-850HD color legalizer (a Broadcast Engineering NAB Pick Hit Award recipient) can ensure that any signal leaving the MCR occupies legal color space. However, there is no guarantee that the colors are aesthetically pleasing!
Most GFX creation applications have the capability to legalize color space to broadcast standards and store elements in RGB or convert to YUV.
With the need to repurpose content for various distribution channels and consumer devices, GFX elements cannot be indiscriminately piped though conversion resources and be expected to result in an acceptable display. It is obvious that fonts and GFX designed for HD will not play well on a 2.5in LCD no matter how they are anti-aliased.
An intelligently engineering infrastructure married with an efficient multiplatform workflow is the answer. In one scenario, appropriate for DTV or NTSC broadcasts, the GFXs and video are mixed in a PCR with conversion further downstream.
To use this identical content on a cell phone, a different workflow must be used. The original video must be converted from a clean feed to the appropriate format. The GFX must also be converted clean (key and fill) to a size (and font) appropriate for a cell phone display. So now there are two parallel production processes.
The efficiency of this multipath production and conversion process is that the GFX element is created only once and then converted to the necessary formats. An asset management system can facilitate the workflow by performing transparent folder-based format conversion.
GFX automated assembly
Automated graphics assembly is an efficient way to assemble GFX just in time to take to air. For sophisticated data dependent designs, the elimination of manual intervention and integration of database access is a powerful process and speeds up the time-to-air. This is particularly useful in a live production or news show.
First GFX templates are created. For example, consider a full screen, dual headshot with online voting results and previous results displayed in the lower third. The layout with placeholders is created in a GFX application. Headshots and previous results exist in a database. Online voting is updated in real time and is periodically updated in the template. An application will direct all this information into a template and push the completed GFX to a GFX playout server and is ready to air.
Who knows what tomorrow may bring?
The challenge is to engineer an infrastructure that supports all formats, while minimizing production time by integrating efficient GFX production workflow processes. This emphasizes the need for careful planning, requirements gathering and business integration. A brute force approach may work, but it will be expensive and stressful.
Fonts and graphics must be carefully designed to minimize aliasing when scaling between formats. Should they be produced once in highest resolution and then down converted? Or is original creation best in the final delivery format, going on the assumption that no conversion is a good conversion, i. e. that any conversion will degrade image quality. What will be the impact of cascaded transformations?
In any event, professional engineering practice dictates an analysis of GFX artifacts produced by conversion as they travel through the production process and distribution chain.
Creating sophisticated GFX, effects and animations requires high-end compute power, high-speed networks and HPHA storage. In the next Transition to Digital, IT resources, computer platforms, render farms, GFX cards, networks and storage used in the creative phase of the media lifecycle will be discussed.