Graphics Reference
In-Depth Information
We note that for dynamics, the rendering rate may be independent of the simu-
lation rate. Simulating at low rates and then interpolating between simulation steps
when rendering amortizes the cost of a simulation step. Simulating at high rates
and then subsampling simulation steps when rendering can increase accuracy and
stability. We return to these issues later in this chapter.
Most LCD monitors refresh at around 60-65 Hz, although some displays
refresh at 120 or 240 Hz for shuttered stereo viewing of 60 Hz or 120 Hz images.
Note that there are two standards for film: 24 Hz in the United States and
Japan and 25 Hz for the PAL/SECAM standard used in Europe and the rest of
Asia. Interestingly, projected films are typically not adapted to the display's frame
rate when moving between PAL and NTSC standards. Thus, 25 Hz European films
are screened in the United States at 24 Hz. The audio makes the corresponding 4%
speed change to remain in synchrony. The net result is that European films appear
slow and low-pitched while American films appear fast and high-pitched, when
viewed in the opposite projection mode.
Many television broadcast and storage formats are
interlaced.
In an interlaced
format, each frame contains the full horizontal resolution of the final image but
only half the vertical resolution. These half-resolution frames are called
fields.
Even and odd fields are offset by one pixel (or historically, one scan line). To
display a complete image, two sequential fields must be combined by interlacing
their
rasters
(rows of pixels).
Because each image merges pixels from different time slices, no single image
is consistent. However, fast motion can be represented at comparatively low band-
width. The artifacts of interlacing were historically hidden by the decay time of
CRT phosphors, which took longer to change intensity than the frame period.
Some contemporary displays can change images rapidly and thus reveal the inter-
lacing pattern. This can be observed when pausing playback on an LCD mon-
itor, although some displays attempt to interpolate between adjacent frames to
reconstruct a progressive signal from an interlaced one.
At the time of this writing, most broadcast television and archived television
shows remain in interlaced formats. With the advent of high-definition digital dis-
plays, new content is increasingly moving to
progressive
formats. Progressive is
what you would expect: Each frame contains a complete image.
The progressive rate for PAL/SECAM television is 25 Hz and the interlaced
rate is 50 Hz. This means that normal European television broadcasts send one
field every 1
25 s a complete image has been transmitted.
Console games displayed on televisions may elect to render in either progres-
sive or interlaced formats. The advantage of an interlaced format is that there are
half as many pixels to render per frame, yet the viewer rarely perceives a 50%
reduction in quality.
/
50 s such that every 1
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35.3.3.1 Telecine
The PAL/SECAM formats allow European films to be broadcast unmodified on
European television because they both are driven at 25 Hz. To interlace such a
film, simply drop half the rasters each frame.
In the United States, the process is not as simple. NTSC television requires
approximately 30 Hz, but U.S. films are at 24 Hz. These only align once every six
