Background Information and Techniques - Computer Animation: Algorithms and Techniques - page 496

digitally sampled, the YUV signals are referred to as YCrCb. A typical sampling scheme is 4:2:2, in
which the color difference signals (Cr, Cb) are sampled at half the sampling rate of luminance ( Y ) in the
horizontal direction. 4:1:1 sampling means that the color difference signals are also sampled at half the
rate in the vertical direction, resulting in one-quarter of the luminance sampling.
The D1 standard was developed when the broadcast television industry thought it would make the
composite-analog-to-component-digital transition in one fell swoop. But that did not happen because
the cost was prohibitive. D1 uses YUV coding, so-called 4:2:2, which means that the U and V com-
ponents are horizontally subsampled 2:1. Luminance is sampled at 13.5 MHz, resulting in 720 samples
per picture width. There is no compression other than undersampling the chroma information. Aggre-
gate data rate is roughly 27 megabytes per second (MB/s). The D2 standard was developed as a low-
cost alternative to D1. D2 is a composite NTSC digital format (i.e., digitized NTSC). The composite
signal is sampled at four-times-color-subcarrier, about 14.318 MHz at one byte per sample (aggregate
data rate, of course, 14.318 MB/s). It has all the impairment of NTSC but the reliability and perfor-
mance of digital. It uses the same 3/4 00 cassette as D1. As with D1, D2 uses no compression other than
undersampling the chroma information. Other uncompressed digital formats include D3, D5, and D6.
D3 is a composite format that uses 1/2 00 tape. D5 is a component format that uses 1/2 00 tape. D6 is a
component HDTV format.
Common compressed DTV formats include Digital Betacam, Ampex DCT, and Digital8 [ 9 ] . Dig-
ital Betacam uses 1/2 00 tapes similar to the Betacam SP format with 2:1 compression based on DCT.
Ampex DCT is a proprietary format; the DCT in its name stands for Digital Component Technology
and not the compression scheme. The trio of DV, DVCam, and DVCPRO are similar formats using
DCT compression. Depending on image content, the encoder decides whether to compress two fields
separately or as a unit. Digital8 is a consumer-grade version of the DV format but uses cheaper Hi8
tapes. Newer formats include W-VHS, Digital S, Betacam SX, Sony HDD-1000, and D-VHS.
High-definition television and wide-screen format
In the United States the Grand Alliance was created by the FCC to develop the American HDTV spec-
ification. It is most commonly based on the MPEG-2 codec [ 8 ] . The basic idea behind high-definition
television (HDTV) is to increase the percentage of the visual field occupied by the image [ 12 ]. HDTV
is a type of DTV. Currently, there are two popular wide-screen HDTV formats: 720p and 1080i.
The number refers to how many scanlines there are in an image. The “p” in 720p refers to progressive
scan; the “i” in 1080i refers to interlaced scan. NTSC-format television has an aspect ratio (ratio of
width to height) of 4:3, whereas the widescreen format more closely matches that found in movie
theaters and is 16:9.
B.11 Camera calibration
For digitally capturing motion from a camera image, one must have a transformation from the image
coordinate system to the global coordinate system. This requires knowledge of the camera's intrinsic
parameters (focal length and aspect ratio) and extrinsic parameters (position and orientation in space).
In the capture of an image, a point in global space is projected onto the camera's local coordinate sys-
tem and then mapped to pixel coordinates. To establish the camera's parameters, one uses several
points whose coordinates are known in the global coordinate system and whose corresponding pixel