Frame
per seconds
Human
vision
The temporal sensitivity and
resolution of human
vision varies depending on the type and characteristics of visual
stimulus, and it differs between individuals. The human visual system
can process 10 to 12 images per second and perceive them individually, while
higher rates are perceived as motion. Modulated light (such as a computer display)
is perceived as stable by the majority of participants in studies when the rate
is higher than 50 Hz through 90 Hz. This perception of modulated
light as steady is known as the flicker fusion threshold.
However, when the modulated light is non-uniform and contains an image, the
flicker fusion threshold can be much higher, in the hundreds of hertz. With
regard to image recognition,
people have been found to recognize a specific image in an unbroken series of
different images, each of which lasts as little as 13 milliseconds. Persistence of vision
sometimes accounts for very short single-millisecond visual stimulus having a
perceived duration of between 100 ms and 400 ms. Multiple stimuli
that are very short are sometimes perceived as a single stimulus, such as a
10 ms green flash of light immediately followed by a 10 ms red flash
of light perceived as a single yellow flash of light.
Film
and video
Silent films
Early silent films
had stated frame rates anywhere from 16 to 24 frames per second (fps), but
since the cameras were hand-cranked, the rate often changed during the scene to
fit the mood. Projectionists could also change the frame rate in the theater by
adjusting a rheostat
controlling the voltage powering the film-carrying mechanism in the projector.
Film companies often intended that theaters show their silent films at higher
frame rates than they were filmed at these frame rates were enough for the
sense of motion, but it was perceived as jerky motion. To minimize the
perceived flicker, projectors employed dual- and triple-blade shutters, so each frame was displayed two or
three times, increasing the flicker rate to 48 or 72 Hertz and reducing eye
strain. Thomas
Edison said that 46 frames per second was the minimum needed for
the eye to perceive motion: "Anything less will strain the eye. In the mid
to late 1920s, the frame rate for silent films increased to between 20 and
26 fps.
Sound films
When sound film
was introduced in 1926, variations in film speed were no longer tolerated, as
the human ear is more sensitive to changes in audio frequency. Many theaters
had shown silent films at 22 to 26 fps—which is why the industry chose
24 fps for sound as a compromise. From 1927 to 1930, as various studios updated equipment, the rate of
24 fps became standard for 35 mm sound film. At 24 fps, the film travels through the
projector at a rate of 456 millimeters (18.0 in) per second. This allowed
for simple two-blade shutters to give a projected series of
images at 48 per second, satisfying Edison's recommendation. Many modern
35 mm film projectors use three-blade shutters to give 72 images per
second—each frame is flashed on screen three times.
Modern video standards
Due to the mains frequency
of electric grids, analog television broadcast was developed with frame rates
of 50 Hz (most of the world) or 60 Hz (US, Japan, South Korea).
Hydroelectric generators, due to their massive size, developed enough
centrifugal force to make the power mains frequency extremely stable, so
circuits were developed for television cameras to lock onto that frequency as
their primary reference.
The introduction of Color Television
technology made it necessary to lower that 60 fps frequency by .1% to avoid
"dot crawl", an annoying display artifact appearing on legacy
black-and-white displays, showing up on highly-color-saturated surfaces. It was
found that by lowering the frame rate by .1%, that undesirable effect was
highly minimized.
Today's North America, Japan, and South Korea's video
transmission standards are still based on 60÷1.001 or ≈59.94 images per second.
Two sizes of images are typically used: 1920x540 (1080i) and 1280x720 (720p);
Confusingly, interlaced formats are customarily stated at 1/2 their
image rate, 29.97 fps, and double their image height, but these
statements are purely custom; in each format, 60 images per second are
produced. 1080i produces 59.94 1920x540 images, each squashed to half-height in
the photographic process, and stretched back to fill the screen on playback in
a television set. The 720p format produces 59.94 1280x720 images, not squeezed,
so that no expansion or squeezing of the image is necessary. This confusion was
industry-wide in the early days of digital video software, with much software
being written incorrectly -- the coders believing that only 29.97 images were
expected each second, which was incorrect. While it was true that each picture
element was polled and sent only 29.97 times per second, the pixel location
immediately below that one was polled 1/60th of a second later part of a completely separate image for the
next 1/60 second frame.
Film, at its native 24fps rate could
not be displayed without the necessary pulldown process, often leading to
"judder": To convert 24 frames per second into 60 frames per second,
every odd frame is repeated, playing twice; Every even frame is tripled. This
creates uneven motion, appearing stroboscopic. Other conversions have similar
uneven frame doubling. Newer video standards support 120, 240, or 300 frames
per second, so frames can be evenly multiplied for common frame rates such as
24 fps film and 30 fps video, as well as 25 and 50 fps video in the case of 300
fps displays. These standards also support video that's natively in higher
frame rates, and video with interpolated frames between its native frames.
Some modern films are experimenting with frame rates higher than 24 fps, such
as 48 and 60 fps.
Frame rate in electronic camera
specifications may refer to the maximum possible number of frames per second,
where, in practice, other settings (such as exposure time) may reduce the
frequency to a lower number
No comments:
Post a Comment